A Digital Imaging Primer

Transcription

1 A Digital Imaging Primer Nick Dvoracek Media Services University of Wisconsin Oshkosh 2003

2

3 Contents Two ways that computers make pictures 1 The basic unit of digital images 2 Resolution 3 Types of images 4 Line copy 4 Graphics 4 Continuous tone 4 Full color 4 Color modes and pixel depth 5 Line art (1-bit) 5 Greyscale (8-bit) 5 RGB Color (24-bit) 6 Indexed Color (8-bit) 6 Other modes 6 Resolution for print 7 Line copy 7 Continuous tone and color 8 Resolution for images to be displayed on a monitor or projector 10 Using greyscale for line copy displayed on screen 10 Images for the world wide web 11 Images for presentations (Powerpoint) 11 Resize, Resample or Rescan? 13 Resizing 13 Resampling 13 Cropping 14 Improving an image 15 Brightness and contrast 15 Sharpening 15 Other improvements 16 Scanning 17 What kind of image? 17 What is the resolution of the finished image? 17 What do you want to scan? 18 What is the size of the final image? 18 Scanner interfaces 18 Hewlett Packard Precision Scan Pro 19 HP precision scan lite 20 Epson 21 Microtek 22 Nikon 23 Digital cameras 24 Transfering images from the camera 24 Resolution 24 Image orientation 24 Images from video sources 25 How applications store images 26 Import or copy and paste 26 Embedding or Linking 26 Embedding 26 Linking 26 The TIFF file format 28 Highest quality, largest files 28 1-bit black and white images for printing 28 The JPEG file format 29 Image type for JPEG files 29 File compression vs. Image quality 29 Deciding which level of compression to use 31 Examples saved from Adobe Photoshop 32 Examples saved from Microsoft Photoeditor 34 The GIF file format 36 Use GIF for graphic files 36 Transparency 37 Other file formats 38 Portable Network Graphics (.png) 38 Photoshop (.psd) 38 Adobe Acrobat (.pdf) 38 BMP (.bmp) 38 PICT (.pct) 38 Encapsulated Postscript (.eps) 39 PCX (.pcx) 39 Scanning halftone originals 40 Scan at higher than necessary resolution 40 Blur to eliminate the halftone 40 Resample the image to the resolution you need 41 Save the image 41 Table of general recommendations for color mode, resolution and file type 42 For print 43 For screen presentation 43 For web pages 43 i

4 The picture, divested of the ideas which accompany it, and considered only in its ultimate nature, is but a succession or variety of stronger lights thrown upon one part of the paper, and of deeper shadows on another. William Henry Fox-Talbot The Pencil of Nature 1844 ii

5 Preface This booklet is a revision and expansion of a pamphlet I did eight years ago. The use of pictures in computers has expanded exponentially since then, and digital picture making has nearly replaced traditional silver-based photography. There are two main themes basic to using digital pictures. 1. There is no universal scan. In order to achieve the best results, image files need to be made with a specific purpose in mind. 2. The goal in creating digital image files is to save as much data as necessary to optimize the display for your application while creating as small a file as possible. Getting the parameters of an image file a little off isn t fatal, there is some slack, but if you plan ahead and attempt to come close to the ideal, you ll get the best image quality for the least amount of disk space. Nick Dvoracek May 2003 iii

6 iv

7 Computers make images in one of two modes. Object oriented (aka vector, draw) graphics are made of lines and polygons whose shape, size, color, position and other attributes are remembered by the computer. This is the type of graphic used for charts, diagrams and for the most part, text. Although in the hands of an artist, this type of graphic can be quite realistic, it can never reproduce the exact realism of a camera or the character of a drawing rendered by hand. Two ways that computers make pictures To create realistic pictures, like photographs, paintings or drawings, which can be stored and displayed with a computer, an image is divided into a grid and each cell in the grid is given some value. In the simplest case, the value can be on or off. Pictures are then constructed of a pattern of dots, like a mosaic. Depending on how many dots you use, and how much information is conveyed by each dot, this scheme can vary from the rather crude images of the original MacPaint, to the incredible images produced by the Hubble Space Telescope. This type of computer image, which is the subject of this booklet, is known as bit map, raster, or a paint-type object. The programs which create and modify these types of images are known as image editors. Two of the most common examples are Adobe Photoshop, the industry standard, and Microsoft Photoeditor, a minimal utility included with Microsoft Office. In addition to the programs which create and modify them, digital images can be included, and sometimes modified, in documents created by word processing, page layout, graphics, presentation and web editing programs. Images created by object oriented software can sometimes be rendered as a bit map for specific uses, most commonly on web pages. Bit map (raster, paint) Object oriented (vector, draw) An image displayed as a bit map, and as an object oriented graphic. The illustration of the bit map is a little misleading. In order to illustrate the idea of a pattern of dots, I ve included the grid of lines to define each pixel. When displaying a bit mapped image, there are no grid lines, only black dots (which you can see) and white dots (which you can t). 1

8 The basic unit of digital images The basic unit of a digital image is the pixel, short for picture element. In the purest sense, the measure of the sharpness of a digital image is the number of pixels it contains. Below are four variations of the same image, displayed at the same size, but with different numbers of pixels. It s rather quickly apparent, that the more pixels you have, the clearer the image is, but the amount of information you have to remember is greater. Notice that the fourth picture, although only 10 times greater in resolution than the second picture, requires 100 times more information, since Area = (h x w). This geometric increase in file size is one reason it s important to create digital images specifically for an intended purpose. Another noticeable effect is that in the fourth picture, the image no longer seems to be made of square pixels, since now there are more dots in the image than the output device (in this case a laser printer) has to capacity to display A basic idea in digital imaging is that you want the image to have enough information to display well with the output device you intend to use the screen, a projector, a printer while taking up as little disk space as possible. 2 x 3 pixels (takes much less than 1K of memory) 20 x 30 pixels (takes 2K of memory) 40 x 60 pixels, (takes 8K of memory) 200 x 300 pixels (takes 170K of memory) 2

9 Often, particularly in the case of printing, you need to take into account the physical size of the image, and therefore the size of the pixels. You then need to refer to the measurements of the image and it s resolution in dots per inch, usually simply referred to as dpi. Resolution Here is the same 80 x 100 pixel image displayed at three different sizes. Since all three versions contain the same amount of information, the amount of memory involved is still the same. The size of the pixels is noticeably different. In the smallest version, the size of the pixels is smaller than the output device can display, and therefore the image appears smooth and continuous. Any additional resolution is unnecessary, and won t benefit the appearance of the image. In addition to occupying extra disk space, it would take longer to load, transmit over a network, or print. Most applications allow you to change the size of the image after you ve imported it. If you simply enlarge the image when using it in your final application, the resolution will be lower, the size of the pixels will increase, and the pixels become increasingly obvious. Another basic idea in digital imaging is that you have to take into account the properties of the output device the screen, a projector, a printer and the final size at which you will use the image. Following sections will deal with specific situations for different image types and output devices. 80 x 100 pixels at three different sizes.6 x.75 inches 133 dpi 1.5 x 1.8 inches 53 dpi 2 x 2.5 inches 40 dpi 3

10 Types of images Line copy example Graphic image There are several different types of images that have to be treated differently. They also had to be treated differently before computers. They will have differing requirements depending on whether they are printed or displayed on the screen. Line copy Some images consist of only solid black images and white space, with no intermediate colors or grey tones. These images are referred to as line copy. A typical example is the UW Oshkosh logo Graphics Some images have only flat areas of a few colors. Typical examples are cartoons, maps, and graphs. Traditionally, these were printed with each color printed separately on a different plate. The term spot color is a often applied to this type of printing. Computer printers and screens don t work this way, but there is a way the images with limited numbers of colors can be stored more efficiently than full color images and they should be treated as a separate class. Continuous tone Some images do have intermediate grey tones, but no color. Since printing presses can only put black ink on a page or not, but not various shades of grey, a technique was developed that broke the image up into a pattern of dots large densely packed dots where the image was dark, small thinly spaced dots where the image was light. When viewed from normal distances the eye and brain interpret them as continuous tones. Black and white laser printers continue to use this technique to render grey scale images, Monitors don t have to resort to this scheme since every pixel on a monitor can display it s own shade of grey. Full color Enlarged portion of an image with shades of grey simulated with halftones created by traditional optical methods Color images, like photographs, which contain full color require more information to describe those colors than greyscale, but otherwise have similar requirements. When printed, variations in tone are rendered as halftones, using four different colors of ink cyan, magenta, yellow and black which our eyes and brains reproduce as the full range of colors. Traditionally this is referred to as process color. Color printers still use variations of this scheme. On a monitor, as with greyscale, each pixel can display a different color, so no halftoning is necessary when the image is displayed on screen. 4

11 Each type of image can be classified into one of several categories based on the amount of color information necessary to display it. By choosing the appropriate mode, you can maximize the quality of the image, and minimize the amount of data needed to record it. With an image editing program, you can convert one mode to another, reducing a full color image to greyscale, for example. Color modes and pixel depth Line art (1-bit) Line art contains only black and white with no intermediate shades of grey. It is only necessary to remember whether that pixel is on or off. Since computers work with binary numbers, only one number (or bit) is required to remember how that pixel is displayed, on or off, 0 for off, 1 for on. This mode can be referred to as 1-bit, line art, black and white (Windows Imaging), Black and white drawing (HP Precision scan) Monochrome (Microsoft Photoeditor), or bit map (Photoshop). As noted earlier the term bit map can also refer to all digital images. There are some ways a range of tones can be simulated using only one bit per pixels by varying the density of dots. Called dithering, it was commonly used in the days of monochrome monitors and dot matrix printers, yet it still shows up in current scanning and image editing programs. There are several ways of doing it and it doesn t render a very realistic image. Because current printers and monitors almost universally support greyscale images, it is no longer necessary to use dithering. Line reduction of a continuous tone image. Everything darker than middle gray becomes black. Greyscale (8-bit) Photographs contain a continuous range of tones. By using more bits to describe each pixel, more shades of gray can be specified. Typically this is done using 8 bits per pixel, which yields 256 possible shades. (2 8 for the mathematically minded). This is about as many shades of gray that the human eye can distinguish. Even when scanning color pictures you may wish to use the greyscale mode if your method of reproducing the photograph is black and white, such as a laser printer, because the file size will be one third that of a color image. Diffusion dither used to create a 1-bit image that simulates shades of gray. Grey scale can also be useful with line copy type images when your intention is to display them on the screen, such as on a web page or Powerpoint presentation, because it allows a scheme known as anti-aliasing which appears to smooth out the jagged stair step edges line copy images exhibit at low resolutions (like a computer screen) The greyscale mode can also allow you to adjust the brightness to optimize a line copy image before converting it to 1-bit for your final application. RGB Color (24-bit) Color images require much more information to describe. Generally colors are remembered by computers by determining the relative intensity of their 5

12 red, blue, and green components (RGB) which can be combined to create a full range of colors. (Not all the colors the eye can perceive, but most of them). As with gray scale it is typical to specify 8 bits for each component. This boils down to 24 bits per pixel, which gives a possibility of over 16 million colors (2 24 ). Occasionally, you may encounter a 16-bit RGB image which yields only several thousands of colors. This was intended to reduce the amount of data needed to render an image, but as other schemes for reducing file size have been developed, the 16-bit mode is rarely used. Indexed Color (8-bit) It is possible to define a color image using only 8 bits per pixel, and therefore only one-third the amount of data of a full color image. The computer samples the image and sets up a table of the complete 24-bit descriptions of 256 colors that best match those in the image. Then each pixel is identified, or indexed, with a color in the table. Originally developed as a method for reducing file size of full color images, it has been replaced by JPEG compression for this purpose, but is still useful for some other purposes. Full 24-bit color This mode is ideal for graphic images that have only a few areas of a limited number of colors, such as cartoons, graphs, and maps. Indexed color mode is required for saving images in the GIF file format, which has other advantages for saving these types of images. The GIF file format also allows one of the 256 colors to be rendered as transparent so the background of a web page shows through. Reducing a full color image to 256 colors may have an effect on the quality of the image. Some banding might result where a color blends from a lighter to a darker shade. 8-bit indexed color Notice the areas where the transition from light to shadow is smoother in the 24-bit mode, but has changed to more definite flat areas of color in the 8-bit mode. Most programs that allow significant editing of an image require that image to be in the full color RGB mode. Converting from full color to indexed color usually results in loss of information. It s usually a good idea to keep an image in full color if you intend to modify it, then convert to the 8-bit mode before saving for your final use. Other modes There are several other modes that allow more colors and specialized treatments for special purposes, but that are not commonly needed by the average user. They include CMYK for specially calibrated color printing applications and modes with more bits per pixel which allows for wider latitude between light and dark and finer separation within a range of tones. 6

13 Determining the resolution of images intended for print must take several criteria into account. First of all, an image on paper is more commonly thought of by the physical size it will appear on the page, so that size has to be specified in addition to the resolution in dots per inch, which is determined by the resolution of the printer that will be used. Resolution for print Line copy In order to utilize the highest resolution of the printer, line copy images should be used in the 1-bit, line art mode. The resolution printers are capable of is measured in several ways. The more common measure is the size of the smallest dot of a single color that the printer can make, generally described by how many can fit in an inch, hence dots per inch or dpi. This describes how sharp the printer can make a single color image in the color inks available. So, line copy images, which only have one color (probably black) should be scanned at the full resolution of the printer. Even the least expensive printer these days prints at least at 300 dots per inch and can go up to 1440 dots per inch. Most office laser printers are 600 dots per inch, and professional journals as high as 4800 dpi. In actual practice, it is difficult for the human eye to tell the difference of anything over 400 dots per inch, so no matter what the resolution of the printer, scanning line copy over 400 dots per inch yields diminishing returns. 100 dpi 200 dpi 400 dpi 800 dpi 7

14 Continuous tone and color In order to simulate grey tones or color, printers use a scheme known as halftoning, where clusters of the dots the printer is capable of are used to simulate shades of grey. In color printing the image is divided into it s cyan, magenta, yellow and black component and each is printed in a halftone pattern, which the eye and brain reconstruct into the full spectrum of colors. Highly enlarged halftone image showing printer dots clustered to make up halftone dots. You can download the facing page as a.pdf file at digitalimaging.htm to try it on your printer. These halftone patterns are measured in lines per inch. It is this halftone frequency that determines how sharp an image a given printer can produce. It is quite rare for this measure to be published in printer manuals, and modern color printers use variants of the scheme for which the halftone frequency is not as obvious as with a traditional halftone. The best scheme is to make an assumption of what the halftone frequency is based on the full resolution of the printer. New laser printers will have a halftone frequency about 85 lines per inch. Color inkjet printers printing at 720 dpi have a halftone frequency about 90 lines per inch and at 1440 dpi, about 130 lines per inch. Professional journals probably use 133 line halftones. The general recommendation for the correct resolution for an image is one and a half to two times the halftone frequency of the printer, depending on how critical you are about image quality. So for a black and white laser printer, with a halftone of 85 lines per inch, you would need to have between 120 and 170 dots per inch. As with any application, you want to have enough data to get a sharp image, but no more. The images on the facing page demonstrate the practical implications of this. They are scanned at the same size, but different resolutions. The top row of images range from the top end of the range, 180 dpi, a little more than twice the printer s halftone frequency, to 105 dpi, only about 125% of the halftone frequency. It s rather difficult to tell the difference between the images in the top row. On close inspection the higher resolution images seem sharper, but it s hard to identify specific details that contribute to that difference, and without the 180 dpi image to compare it to, it would be easy to assume the 105 dpi image was as good as it could be, and it s file size is almost half the 180 dpi image. In the lower row, the 80 dpi image at first glance looks pretty good, but on careful inspection, the curves of the lamp base and the outer edge of the cat s eye reveal the stair step pattern of the square pixels that make up the image. Her whiskers are not as sharp and well defined, and the image seems softer overall. In the 60 dpi image, the pixels can be seen throughout the image, and at 40 dpi, they dominate the picture. 8

15 180 dpi 140 dpi 105 dpi File size as medium JPEG:164K File size as medium JPEG:128K File size as medium JPEG: 104K 80 dpi 60 dpi 40 dpi File size as medium JPEG: 92K File size as medium JPEG: 84K File size as medium JPEG: 76K 9

16 Resolution for images to be displayed on a monitor or projector Computer monitors come in a variety of sizes and even for monitors of similar size, different resolutions can be selected by the user. The most common way to measure this is to define how many pixels in either dimension the monitor is capable of displaying. The most common sizes are 640 x 480, 800 x 600, and 1024 x 768. When trying to decide what resolution to use for applications where you want to present the image on a monitor or projector, it s more appropriate to ignore the physical size of the image in inches and the resolution in dots per inch, and just concentrate on the number of pixels wide and high the image is and how much of the screen that number will occupy. For example, if you were planning on using an image in a presentation that would be displayed on a 1024 x 768 projector, and you wanted the image to occupy about half the screen, you would scan it at about 500 pixels wide. The ratio of height to width must be taken into account when deciding the right number of pixels to use for a screen image Keep in mind that in addition to the limits of the screen there are limits by the ratio of height to width of the picture. If your picture is twice as tall as it is wide, if you created the width of the image to be half the screen, it would overfill the screen vertically. If you wanted the whole picture on the screen, you would have no choice but to limit it s height to 768 pixels, and have it occupy whatever width it s shape dictates. Since each pixel of a monitor or projector can display any color without resorting to halftoning, line, graphic, continuous tone and color can all be treated the same as far as resolution is concerned. If the image editing program you re using won t let you specify the size of an image in pixels, assume an average screen resolution of 72 dpi, and therefore a width of 1024 pixels would equal 14 inches. Using greyscale for line copy displayed on screen Enlarged portion of low resolution 1-bit image When acquiring line copy images for use in screen display, which is much lower resolution than print, use the grey scale mode. That allows a scheme called antialiasing to trick the eye into smoothing out the stair step, jaggy pattern associated with 1-bit images at low resolution, giving them a much better appearance on screen. Other types of images should be kept in their normal modes indexed color for graphic images with a limited number of colors, greyscale for continuous tone black and white, and full color for continuous tone color. Enlarged portion of low resolution line copy image in greyscale mode 1-bit line copy image at screen resolution Greyscale line copy image at screen resolution For comparison, 1-bit line copy image at printer resolution 10

17 Images for the world wide web Web browsers display images using the number of pixels in the image alone. Any information which refers to a the physical size of the image or resolution in dots per inch is ignored. For example if you have scanned a 2 x 3 inch image at 144 dots per inch, and place that image on a web page, the web browser will think of it as a 288 x 432 pixel image, and since the resolution of the monitor is probably 72 dots per inch, the user would see it as being about 4 x 6 inches on the web page, twice as big as you intended. Programs which create web pages may give you the opportunity to change the display size of an image, but this is usually a bad idea. If you enlarge the image, you re just stretching out the existing pixels and the image quality degrades quickly. If you reduce the size of the image, the entire image file gets downloaded from the server and then the display size is reduced. It s easy to see that it s rather impolite to make a user wait extra time to view your web page with an image that s no better than if you had sized it correctly in the first place. Another consideration when creating images for the web is that not all users will be looking at the same size screen. Currently experts on web design recommend designing web pages for an 800 x 600 pixel screen, since that s the most common sized monitor in use today. When creating image files for use on the web, decide what portion of an 800 x 600 pixel screen you need. If it s easier for you to think in terms of the physical size an image will be on the screen, 72 dots per inch is the most common resolution for computer monitors. Images for presentations (Powerpoint) Like monitors, data projectors vary in the number of pixels they are capable of displaying. At the current time, it s best to assume that the projector you will be giving the presentation on will be 1024 x 768 pixels. At UW Oshkosh in 2003, about 90% of our projectors are 1024 x 768. If you happen to run into a lower resolution projector, your images will still look OK. One complicating factor with Powerpoint is that it deals with resolution as though you were printing the page, and therefore takes resolution into account although it is completely irrelevant when the image is projected. The standard dimensions Powerpoint gives a slide is 10 inches by 7.5 inches, and that is the size it prints. If you ve created your image files with a 1024 x 768 screen in mind, and when you place them on the slide, they appear to be the wrong size, you can click on one of the handles and drag to resize the picture to fill the screen, or to the portion of the screen you had planned to have it fit. 11

18 If you use a resolution of 100 dpi and think of the dimensions of the slide as 10 x 7.5 inches, your images should fill the portion of the slide you intended when you insert it into your presentation. Another issue to be aware of with Powerpoint is how it deals with low resolution images. Instead of making the individual square pixels obvious, the program attempts to disguise the low resolution nature of the image by rounding off the pixels. In some sense I suppose this improves the quality, but it really just makes the image blurry. You can insert much higher resolution images into a Powerpoint slide and resize them to fit the slide, but the entire amount of data from the original image gets incorporated into your file. You can end up with some whoppingly large Powerpoint files, without any benefit to the quality of the image. Powerpoint has internal tools to crop and modify the brightness and contrast of images. For more on using images in Powerpoint see: nick/usingppt.htm Image with resolution appropriate to the display Low resolution image as displayed in Powerpoint Low resolution image enlarged without Powerpoint s display modification 12

19 The distinction needs to be made between resizing an image, when the number of pixels remains the same, but its size on the page or screen changes, and resampling, when the number of pixels that makes up the image changes. Sometimes both these values can be changed at the same time. Resizing Resizing can be done in most applications in which an image file is placed., such as graphics, page layout, word processing and presentation software. It is commonly done by clicking and dragging on the handles which appear when the image is selected, but may also be done by entering measurements in a dialogue box. This only changes the size at which the image file is displayed, and therefore also the resolution. It does not change the number of pixels in an image file. Enlarging may change the image to less resolution than the output device the printer, monitor or projector is capable, the pixels will become obvious and image quality will be degraded. If reducing the size changes the image to more resolution than the output device is capable, the image will look fine, but you will be storing, processing and transmitting more data, without getting any benefit for it. Resize, Resample or Rescan? Image being resized in Microsoft Word In most cases small amounts of resizing in the application in which an image is used don t make much difference. If the change is rather large, it may be of benefit to resample change the file to a more appropriate number of pixels or to rescan to achieve a better match for your intended use without using excessive disk space. Resampling Resampling changes the number of pixels that make up the image. The term comes from the idea that the each pixel is created by sampling, or measuring the brightness, of a tiny spot of the original picture. Resampling is done in an image editing program like Photoshop, Microsoft Photoeditor, or Imaging for Windows. Most of these programs allow both resizing and resampling, sometimes in the same dialogue box. Resampling is necessary when you have an image acquired for a different purpose not appropriate for your use. For example, the smallest image most digital cameras take is 640 x 480 pixels, over half the size of the typical users screen, probably much larger than you want on the typical web page. You may have originally taken a picture at very high resolution, intending it for a high quality print and later you want to place it in a Powerpoint presentation. Just inserting the original file may increase the file size of the Powerpoint file by much more than if the image was appropriately sized. Images provided on clip art CD collections may also be larger than you need for a screen application. If you wish to retain the original file, you can make a file copy and resample that to fit your new use. Remember to change the file name so you don t replace your original file. 13

20 You may have an image that is too small for your intended use. A common example is a small image from the web that you want to use to fill a Powerpoint slide. An important consideration in resampling is that you can always reduce the amount of data to a more appropriate level without compromising image quality, but you can t increase the amount of data to improve a low resolution picture. You can t create detail that isn t there. Resampling a low resolution picture to a higher resolution just subdivides the existing pixels and you get the same poor image quality with a larger file size. A small low resolution image enlarged in the final application - effective resolution is about 25 dpi Resampled to 125 dpi. Pixels are no longer apparent, but the quality of the image is about as bad, and the amount of data is 25 times larger. In the event where your image file is too low resolution for your intended use, you have to rescan or rephotograph at a more appropriate resolution, or live with the low quality of the image if you don t have the option to change it. In Adobe Photoshop and it s variants, such as Photodeluxe and Photoshop Elements resampling and resizing can be done in the same dialogue box, found under the Image menu. A check box at the bottom determines whether the image is simply resized, using the same number of pixels at a different resolution, or resampled to the new dimensions, possibly at a new resolution. In Microsoft Photoeditor, the Resize command under the Image menu actually does resampling. It allows you to change the image s dimensions, either in pixels, or physical dimensions. Since the resolution remains the same, smaller physical dimensions translate into fewer pixels. Photoeditor also allows changing the resolution without affecting the number of pixels in the Properties dialogue box found under the File menu Cropping Most image editing programs allow you to crop the image to remove unwanted parts of the image. When cropping an image in an image editing program, keep in mind that the number of pixels is being reduced and the resulting file will be lower resolution than the original image. 14

21 Sometimes an image you want to use is not of the optimum quality. Image editing programs can improve the quality of an image file. The two most common are adjusting the brightness and contrast of an image, and sharpening. Improving an image Brightness and contrast An image you are scanning may be old and faded, or when photographing a scene with a digital camera, the image may be underexposed. There are limits, but by adjusting the brightness and contrast of a digital image, you can bring many of these types of images back to almost optimum appearance. Generally both parameters, brightness and contrast, need to be adjusted. Increasing the brightness of an under exposed image will leave the shadows too light. Changing the contrast increases the range of bright and dark, which can bring the shadows back to their correct tone. In Photoshop this adjustment is found under the Adjust...Brightness/Contrast command under the Image menu. This illustration contains the adjustments that were made to the accompanying photograph. Photoshop includes several other schemes with more subtle control to improve the tonal quality of images. In Microsoft Photoeditor, these controls are found under the Image menu with the Balance command. Also included on this dialogue box is an adjustment to Gamma, which is similar to Contrast, but affects primarily the shadow areas. The highlights of this photo have dimmed as the paper has aged and the image itself has faded The same image after brightness and contrast have been adjusted. Sharpening Sometimes, after scanning an image, or after resampling an image to reduce the resolution to a more appropriate size, the image will seem softer than the original. Most image editors contain a command to sharpen the image which can seem to restore the image quality. Remember this is an artificial manipulation to the image data, and not an actual addition of detail Not all images benefit from sharpening. Sometimes, sharpening will just make flaws more obvious. Excessive use of sharpening can introduce unattractive visual artifacts. A single application of the sharpening effect will usually provide as much benefit as this technique can provide. Under the edit menu in almost all programs is an undo command that will allow you to switch back and forth between the image as it was before 15

22 the application of the effect and after to allow you to decide if it actually improved the image. In Photoshop, Sharpening is found under the Filters command, and there is also a more advanced option called Unsharp Mask. In Microsoft Photoeditor, it is found under the Effects menu. The amount of the effect can be adjusted. Original image after it was downsampled from a much larger file. Other improvements Photoshop and other image editors include many other tools to retouch flaws, add or remove image elements, combine images, brighten or darken just parts of the image, distort the image, and to manipulate the image with a variety of effects to simulate painting or printing techniques. After a single application of Photoshop s Sharpening filter After four applications of Photoshop s Sharpening filter 16

23 Just about every scanner manufacturer has a different interface but they all include four decisions you have to make, based on the nature of your image and what your intended use of the image is. Scanning What kind of image? Most scanner interfaces offer several choices depending on the image type and the mode you want the image to be saved in.. Full color (aka color photograph, RGB, 24-bit, or millions of colors) - the best choice for natural color images such as photographs or paintings. Indexed color (aka color drawing, 8-bit or 256 colors) - a good choice for images that have a limited number of colors such as graphs, maps, or diagrams. Greyscale (aka continuous tone, black and white photograph) - Use when your image is a black and white photograph or drawing, when scanning a color image that will be only reproduced in black and white, or when putting line copy on the web or in a Powerpoint presentation Line copy (aka black and white drawing, 1-bit, bit map) Use when your image has only solid black on a white background with no intermediate shades of grey, such as text on paper or the UW Oshkosh logo, and your intended use is to print the image. (Sometimes if you want to adjust the exposure of a line copy image, you might scan it as greyscale so you can adjust the brightness, and then convert it to 1-bit before actually saving it.) Type - some scanner interfaces include a setting that scans a page of text, performs optical character recognition of the page, and places a text file in a word processing document. This doesn t result in a digital image. What is the resolution you want the final scan to be? How many dots per inch do you want the final scan to be? This will depend on your intended use. It s most critical when printing and depends on the printer. For color or grey scale images it can vary from 125 dots per inch for the average laser printer to 300 dots per inch for magazine quality output. For line copy for printing use the full resolution of the printer, up to a limit of 400 dpi. Remember that file size increases dramatically with increased resolution, growing by 4 times for every doubling of resolution. Another thing to consider is that you can always decrease the resolution without changing the image size later on, but you can t increase it. It also might not be relevant at all. When scanning for the web or for a presentation, it s more direct to think in terms of the image s dimensions in pixels since that s the most common way computer screens and projectors are described. Most interfaces will allow you to change the units of measure to pixels. 17

24 What do you want to scan? The interface will give you some way to preview what s been put on the scanner s glass. Some scanners automatically preview when you select the scanner with the import command, others include a preview or prescan button. You can then select what part you want to scan by clicking and dragging. Some interfaces will automatically select everything on the glass, and some will automatically select a small image you put on the glass. For example, if you put a 3 x 5 snapshot on the glass, it will select just that and not the entire area. This selected area can be changed by placing the cursor on the edge of the area the cursor will usually change to a two headed arrow and dragging it to adjust the area selected. You can usually move the area, and if you click outside the area, you can select a new area. Most interfaces will display the measurements of the area selected. What is the size of the final image? It s quite important to specify at what size you intend to use the image. You may be intending to enlarge or reduce the original image. Image size and resolution are related. If you scan a 1 inch by 1 inch area at 150 dots per inch, and then enlarge that image to 3 by 3 inches in your final application, the effective resolution will then be 50 dots per inch. It s best to do the enlarging when you scan the picture, specifying the size and resolution you need for your ultimate use of the image. You usually have the choice to specify the final image size in pixels instead of inches. In this case, resolution in dots per inch is irrelevant. When trying to determine how much of a computer screen or projector you want to fill, it s easier to think of how much of a 1024 x 768 pixel screen you want to fill than to figure out what the size in inches is at the screen s resolution. Scanner interfaces Most scanners are controlled by plug-in interfaces that are accessed from an Import command under the File menu in the application you re using. This is usually an image editor like Adobe Photoshop or Microsoft Photoeditor, but many other programs allow you to scan directly into your word processing, page layout, graphics or presentation documents. Some are also accessed by a stand-alone application, but it usually looks and functions exactly like the plug-in. The next five pages show examples of scanner interfaces noting how the four decisions specific to your use are made. There also may be other tools available that allow you to adjust the brightness, contrast or other settings of the scanner. 18

25 Hewlett Packard Precision Scan Pro HP s interface consists of several floating windows that can be displayed or not by selecting them under the tools menu. The default is to preview what s on the scanner when you call up the scanner control, but there is a command to preview under the scan menu if you move what you ve put on the scanner.. To actually complete the scan, there is a Return to... command with the name of the application you started from under the file menu. 1. Select the mode appropriate for your image type and intended use. 4. Specify the final size you want the scan to be. Note that you can change the unit of measurement, to pixels, if appropriate 2 Select the resolution appropriate to your use. 3. Click and drag to specify the area you wish to scan 19

26 HP precision scan lite The lite version of HP s software sets defaults based on the computers automatic analysis of the image and what you tell it about your scan, but these can be overridden by calling up dialogue boxes. 1. Select the mode appropriate for your image type and intended use. 2. Select the resolution appropriate to your use. 4. Specify the final size you want the scan to be. Note that here the unit of measurement has been changed to pixels. 3. Click and drag to specify the area you wish to scan 20

27 Epson 1. Select the mode appropriate for your image type and intended use. 2 Select the resolution appropriate to your use. 3. Click and drag to specify the area you wish to scan 4. Specify the final size you want the scan to be. Note that the unit of measurement has been changed to pixels. 21

28 Microtek 3. Click and drag to specify the area you wish to scan 1. Select the mode appropriate for your image type and intended use. 2 Select the resolution appropriate to your use. 4. Specify the final size you want the scan to be. Note that the unit of measurement can be changed. 22

29 Nikon The Nikon Super Coolscan is a scanner specifically for 35mm slides but the decisions you have to make about your scan are the same 1. Select the mode appropriate for your image type and intended use. RGB is another way of referring to 24-bit full color 2. Select the resolution appropriate to your use. 3. Click and drag to specify the area you wish to scan 4. Specify the final size you want the scan to be. Note that the unit of measurement can be changed. 23

30 Digital cameras Digital cameras have become a common way to acquire digital images. They are available in as great a variety of cost and quality as are film cameras, but they share some characteristics. Transferring images from the camera Some cameras store images on floppy disks, and some burn miniature CD s, but by far the most common way is to store them on a small memory chip that holds from 4MB to 512MB of data. The camera itself connects to the computer through the USB input, or the memory chip is removed, and inserted into a reader which connects to the USB input. The chip then mounts on the desktop and looks like any storage device, like a hard disk or a CD, and image files can be transferred like any other files. Some manufacturers include a program which automatically transfers the image files to a folder on your hard disk and catalogs them. Most save the images as JPEG files, but some higher end cameras have the option of saving as TIFF. Resolution Digital cameras can acquire images from 640 x 480 to 4200 x 3000 pixels. Most have several choices of what resolution to use, and can change from picture to picture. How much is necessary, as usual, depends on how you intend to use it. Bigger pictures printed on high quality printers will require high resolution. Even the lowest resolution, 640 x 480 may be too large for the average use on a web page. If the image is too large for your intended use, you can use your image editing program, such as Adobe Photoshop or Microsoft Photoeditor to resample the image to a more appropriate size. Image orientation No matter what the orientation of the camera when the picture was taken, the image is stored as a horizontal (landscape) image. Many, but not all, applications into which you would insert an image have the capability to rotate the image. There is no way to rotate an image when creating web pages. If necessary, you need to rotate the image in an image editing program, and resave it. 24

31 The image from a television camera or VCR can be digitized to yield a still image. It makes a difference if the camera is a normal TV camera or a Digital Video Camera. Images from video sources Specialized input devices are available that allow you to connect a normal video camera or VCR and capture images. Most recent ones connect via a USB or Firewire port. Like scanners, these can be controlled either by Photoshop type plug-ins or by standalone programs provided by the manufacturer. Analog television images are interlaced. The image is made up of about 525 lines. When the camera makes the image, every other line is scanned, and then the other set is scanned to complete the picture. Each pass takes a sixtieth of a second, for a total exposure time of a thirtieth of a second for the whole frame. The difference in the two segments can be somewhat different with a fast moving image. You usually have the choice of capturing the whole image, which will generally yield more information and better quality on a still image, or to capture only one of the interlaced scans, which will usually yield a better image of motion. In some cases the computer will fill in the missing lines based the difference between the lines above and below. If you ve worked with video cameras and VCR s much, you ve probably notice that the tape recorded signal is not as good as the original camera image. The same is true when the image is digitized. This is more noticeable in color than in gray scale, and for small image sizes might not be noticeable at all. Digital video cameras typically connect over a Firewire port and yield sharper pictures more like digital still cameras. Unlike scanning, the resolution of a digitized video image is fixed at a certain number of pixels, which limit the amount they can be enlarged. Typical sizes are 640 x 480 pixels for the both fields or 320 x 240 for one field. Digital video cameras normally yield about 750 x 600. Usually you are given the option to make lower resolution files if you wish. 25

32 How applications store images Import or copy and paste There are two ways to incorporate images into applications like word processing, page layout, graphics and presentation programs. You can copy them from the image editing program or web page, and paste them into your document. Or you can save the image file to disk and use the place, insert or import commands, commonly found under the File menu, Some applications, notable those in the Microsoft Office group, have a separate Insert menu. Saving the file and then importing is generally the safer route. Just pasting an image into a document occasionally causes odd problems. The most common is a dramatic increase in the file size of the new document because the copied image is interpreted by the new application as an uncompressed image even though the original was a compressed JPEG file. Other problems can occur, such as mysterious crashes when attempting certain types of exports and images disappearing when the file is copied. In order to save an image from a web page, right click in Windows, or just hold the mouse button down on a Macintosh, and a menu or dialogue box will appear with options to let you save that file to your computer. Embedding or Linking Programs which import images have two schemes for storing the information. Some programs give you the option of choosing either method. Embedding Embedding means that when a picture is inserted, a complete copy of the image file is incorporated into the new document. The advantage is that the new file is now self contained. It can be copied or moved without losing track of the original image file. The size of the new file grows by the size of the image file that was inserted. Any characteristic, such as JPEG compression, which affects file size will be incorporated. After you have inserted the file, you can discard the original image file if you have no other reason to keep it. However if you may want to edit it in the future or use it in another document, it would be better to save it. Some applications don t allow images to be exported as individual files once they re imported, although you may be able copy it and paste it into a new image editor file if you need to modify or reuse it in the future.. Powerpoint and Word fully embed images into their documents and have no special command to export an individual image back as a file, although Powerpoint can export whole slides as image files. Linking When an image file is linked to a page layout, graphics or presentation file, only a low resolution preview of the image is stored in your new document, and a record of the location of the complete image file. When the document 26

33 needs the image information to show a high resolution image or print the document, it retrieves it from the original image file. When using this scheme, it s important to put the document and the original image file in the same folder, or keep the same relative path between the two files. If a linked image file is not where another document expects it to be, most applications, on opening, will warn you and give you a chance to reestablish the link. A special case are web pages, which are HTML files, which only contain a link to the image file, and therefore, the original image file always needs to remain available in order to be displayed. Most graphics, page layout and presentation (including Powerpoint) packages allow you to use either option. One major advantage of a linked file is that you can modify or change the linked image file and it will automatically update in the document into which it was imported. Another scenario where linking is particularly useful is where you might use one image over and over in several files stored together, such as on a CD- ROM with multiple lessons. 27

34 The TIFF file format In the early days of digital imaging, every application and computer platform had it s own way of saving files. TIFF, Tagged Image File Format, was developed to be able to exchange files between programs and computer platforms. The TIFF format can accommodate files of any size, in any color mode, and can also incorporate additional channels which can incorporate extra information about the image. Highest quality, largest files TIFF files retain all the information in the original image file without any modification of the image. For this reason they are preferred for applications where image quality is paramount, such as in high end publishing, digital art work, and archiving important images. Files in the TIFF format are usually larger than most other file formats, Other formats may incorporate schemes to reduce file size, but have varying degrees of effect on the quality of the image. The TIFF format can utilize a scheme called LZW compression which gives modest reduction in file size for color and grey scale images without any effect on image quality. Keep in mind that you can always make a file copy in another format if you wish to use it in a manner where TIFF s large file size would be a problem, such as in a presentation or in a web application. Line copy images to be printed While only moderately effective in reducing file sizes for greyscale and color images, LZW or CCDIT compression gives rather dramatic file compression with black and white only, 1-bit images, so this is a good format for saving line copy images to be printed. Scans of text pages that will be used in Adobe Acrobat files that you want at high resolution to print well on a laser printer should be saved in this format. If the pages include grey scale or color images of any kind, it would be more effective to save them at lower resolution as JPEG or GIF files. 28

35 One of the most noticeable characteristics of digital images is the large amounts of data required to describe even modest sized images. This has led to the development of several schemes which apply sophisticated mathematics to encode the image data when it is saved to occupy less disk space, and then decode it back to display the image when recalled from the disk. Contemporary computers are fast enough that this process occurs practically instantaneously. The JPEG file format The most common scheme, which can be utilized by almost all applications and is universally utilized on the internet is JPEG, recognized by the file extension.jpg. JPEG stands for Joint Photographic Experts Group, an international standards organization which develops the techniques and standards which make this file type exchangeable between almost all computer platforms and programs. Image type for JPEG files JPEG works best for images which have many colors and smooth variations between those colors, such as photographs and paintings. JPEG requires the image to be in the millions of colors (24-bit) or greyscale mode. Some programs, most notably Microsoft Photoeditor, will save other types of images (such as black and white only) as JPEG, converting them to greyscale or full color in the process. Graphic types of images, with sharply defined edges and large flat areas of color are more effectively saved as GIF files. File compression vs. image quality There are two related characteristics of JPEG that you should be aware of. First, it is lossy compression. When the image is encoded and then decoded, you don t get back all the quality of the original image. Secondly, you have the option of specifying an amount of compression, and therefore, it s associated loss of quality. The smaller the file that is created, the less the image quality. Following this section are four pages which contain examples of the same image file saved at five different levels of JPEG compression as saved by two different programs Adobe Photoshop, the industry standard image editing program and Microsoft Photoeditor, a minimal image utility included with Microsoft Office. Included for comparison is the original image saved as a TIFF file, which retains all the image quality, at the cost of much larger file size. The images are saved at a resolution of 125 dots per inch, in the range appropriate for the average desktop printer. Also included is a section of the image enlarged 300 per cent, which gives you a better look at the effect the file compression has on the quality of the image. On close inspection of the enlarged portion of the image, it is apparent that even at modest levels of compression, there is some change in the quality of the image but one almost has to search pixel by pixel to find the changes. The normal sized image retains it s quality fairly well even when file compression 29

36 is fairly large, and without the original to compare it to, the average viewer wouldn t immediately notice a problem with the quality until the file size had been reduced to about a fortieth of it s original size. The amount of JPEG compression can be described in several ways. Sometimes a number from 0 to 100 is used, sometimes a number from 0 to 12, and sometimes just the verbal description low, medium or high. In comparing the images created by these two programs, although they both use the 0 to 100 scheme, the effect on the quality of the image is more severe with Microsoft Photoeditor. The 30 quality image from Photoeditor is noticeably worse than the same level from Photoshop. Another way to gauge JPEG compression is the ratio of the compressed file size to the uncompressed file size. Looked at this way, you can see that a 30 quality file from Photoshop gives 26:1 reduction but from Photoeditor gives 39:1. Most programs don t refer to compression ratios because the effect of JPEG compression can vary considerably, depending on the content of the image. If you compare images with similar amounts of compression (The Photoshop 30 quality and the Photoeditor 60 quality have similar file sizes) the effect on the image quality is about the same. Another thing that becomes apparent is that the worst Photoshop will do yields a reasonable picture, but Photoeditor will let you continue to where the picture is nearly unrecognizable. The moral of the story is that you have to become familiar with what effect the program you re using has on the quality of the image and file size with the levels of JPEG compression you have to choose from. In order to see the effect of saving an image with JPEG compression, you normally will have to close the file and then reopen the JPEG version. Some programs do automatically switch to actually show JPEG images after the file is saved. Photoshop includes a utility to preview the effect of the compression. Deciding which level of compression to use When deciding what level of JPEG compression to choose for a particular image, as usual, it depends on what you want to do with the image. If you are storing images for the future, and have plenty of disk space if you are archiving them on CDs for example save them with the highest quality level. You can always create a file copy at a lower resolution or quality level if you will be using the image on a web page or in a presentation. Printing is usually a little higher resolution medium and flaws are more readily apparent. Also when printing, disk space is a little more readily available, so keeping the quality level fairly high is a more important consideration than the size of the file. Since there is very little effect on the image quality with high 30

37 quality JPEG, but a significant savings in file size it is usually worth utilizing the JPEG file format rather than TIFF. With presentations such as PowerPoint, image quality is a consideration, but at the same time, the audience won t get a chance to inspect the picture too closely. Also with a presentation, the file will likely have to be transported on a disk or moved over a network so reducing the file size can be an important consideration, so medium quality may be a better choice. On the web, file size, and therefore download times, are of paramount importance, so you usually want to get as much compression as possible, even at the expense of a little image quality. As always with digital images, you want to have acceptable image quality with the smallest file size, and you have to decide what is appropriate and acceptable for your particular use of an image. 31

38 Saved from Adobe Photoshop (451 x 313 pixels, 3.6 x 2.5 inches at 125 dots per inch TIFF - File size: 557k, with LZW compression, 414k (1.3:1 compression) 300% enlargement JPEG 80 quality (maximum) File size: 73k (7:1 compression) 300% enlargement JPEG 60 quality (high) File size: 42k (13:1 compression) 300% enlargement 32

39 JPEG 30 quality (medium) File size: 21k (26:1 compression) 300% enlargement JPEG 10 quality (low) File size: 13k (42:1 compression) 300% enlargement JPEG 0 quality (low) File size: 10k (55:1 compression) 300% enlargement 33

40 Saved from Microsoft Photoeditor (451 x 313 pixels, 3.6 x 2.5 inches at 125 dots per inch) TIFF - File size: 557k, with LZW compression, 414k (1.3:1 compression) 300% enlargement JPEG 80 quality - File size: 32k (17:1 compression) 300% enlargement JPEG 60 quality - File size: 22k (25:1 compression) 300% enlargement 34

41 JPEG 30 quality - File size: 14k (39:1 compression) 300% enlargement JPEG 10 quality - File size: 8k (70:1 compression) 300% enlargement JPEG 1 quality - File size: 1k (500:1 compression) 300% enlargement 35

42 The GIF file format The GIF (Graphics Interchange Format) file format was developed by CompuServe to deliver image files over low bandwidth networks. GIF is universally supported by web browsers and by almost all print and presentation applications. Use GIF for graphic files GIF requires the indexed color mode. One method for reducing file size in images is to reduce the color mode to the 256 color, 8-bit format. For graphic type files, which have fewer than 256 colors, indexed color has no effect on image quality. With full color photographs this can degrade the image quality, and the JPEG file format provides better file compression for full color images. The JPEG file format can add visual artifacts to flat areas of color, and sharply defined edges, exactly the kind of things you find in graphic images like diagrams, charts, graphs, maps and cartoons. For these graphic images, GIF will provide better image quality and better file compression. Unlike JPEG, GIF does not have variable levels of image quality and file compression. Because GIF was developed for on-screen applications, the resolution is always 72 dpi, so you may have to resize GIF files to your intended size after placing them in a print or presentation application. The same 300 pixel square image saved in indexed color as GIF and as full color as a highly compressed JPEG file, and an enlarged section from each. Both files are 10K. The JPEG file exhibits visual artifacts characteristic of JPEG compression on flat areas and sharp lines. With less aggressive JPEG compression these artifacts would be less noticeable, but the file size would be much larger. GIF JPEG 36

43 Transparency In some applications, such as web pages, it is desirable to have a background image or color show through a graphic. Another advantage of the use of indexed color in GIF is that it allows one of the colors (usually white) to be identified as transparent. In Photoshop, this is done under the Save for Web dialogue, and in Microsoft Photo Editor it s a separate tool under the Tools menu. Powerpoint has tools to specify a transparent color in any type of image file that has been inserted into a presentation. Image saved as GIF without transparency Image saved as GIF with white specified as transparent 37

44 Other file formats Although nowhere near as common as TIFF, JPEG, and GIF, there are many other file formats you may encounter. Most image editors can convert one file type to another. Portable Network Graphics (.png) Portable network graphics format has been developed as a replacement for GIF. It supports both transparency and 24 bit color with variable levels of compression. It is supported by most newer browsers but is not quite universal yet. Photoshop (.psd) Probably because of it s dominance of the image editing market, Adobe Photoshop s file format is sometimes encountered as an import or export format. This is the application specific file format for Photoshop and it s variants, Photodeluxe and Photoshop Elements. It doesn t include any file compression and can contain many of the application specific features of Photoshop, such as multiple layers of various types. When using such features the file must be saved as a Photoshop file, which is usually converted to a more common format for use in other applications. Adobe Acrobat (.pdf) PDF files are not only image files, but can contain text and object oriented elements as well, but are commonly used to package together scanned images of text and under Macintosh OS X, screen captures are saved in this format. Acrobat files may contain images with JPEG or other types of file compression. Many image editors can open PDF s, converting any text and object-oriented elements to bit map images in the process. If individual image files are packaged together in a PDF, the full version of Acrobat can export them as individual files in their original format. BMP (.bmp) BMP (short for Bit map) is the native file format for Windows. It is sometimes referred to as Device Independent Bit map. In some applications, when you copy an image and paste it into a new application, you re pasting a BMP file, even if the original image was from a JPEG file. Since BMP doesn t include much file compression, the resulting document can have much larger file size. That s one reason why it s a better idea to save files to disk and then import, place or insert into the them to the new document. PICT (.pct) PICT was the native format for display on the Macintosh. When you re copying and pasting images on a Mac under OS 9 or earlier, you re basically moving around a PICT file. This is no longer true under OS X. PICT could incorporate JPEG compression, and could also include object-oriented elements. 38

45 Encapsulated Postscript (.eps) A format for using image files in high end publishing applications. EPS could include specialized color variations and clipping paths for cropping the image into special shapes. PCX (.pcx) The application specific format for PC Paintbrush, an early image editing program that was widely supported by many DOS and early Windows programs. 39

46 Scanning halftone originals Sometimes, when you scan a picture out of a book or magazine, the picture has an unattractive pattern superimposed on it, referred to as a moiré pattern. It is caused by interference between the dot pattern in the halftone of the original and the dot pattern of the digital image. The obviousness of the effect can vary greatly with the frequency of the halftone screen, the resolution of the digital image and the magnification. Some scanners have a descreen option to avoid this problem. Scan at higher than necessary resolution The secret to eliminating it is to scan the image at about twice the resolution you need in order to accurately image the halftone dots in the image without the interference pattern. This can create some stunningly large files. If you need to save the file in order to move to a machine with an application that can accomplish the steps necessary to fix it, save the image as a JPEG with maximum quality. That will reduce all but the largest files to floppy disk size (1.4MB) Blur to eliminate the halftone Both Photoshop and Photoshop Elements can eliminate the pattern. Under the Filters menu, select Blur, then Gaussian Blur. In the dialogue box which appears, by clicking and dragging in the preview you can move around the image to find a place where the halftone is most obvious. Then drag the slider to the value to blur the halftone dots until they appear to merge. Photoshop remembers it s settings from the last use, so experiment with the slider to just eliminate the halftone and no more or you will unnecessarily blur the image The Gaussian Blur dialogue in Photoshop 40

47 In Microsoft Photoeditor there is a Soften effect which has a similar result, but not as much variability, and does not give a preview. The best results probably come from using the Strongest setting. Resample the image to the resolution you need In Photoshop under the Image menu select image size. In Photoshop Elements under the Image menu, select Resize and then Image Size At the bottom of the dialogue make sure Resample Image is checked. In the resolution window, type in the lower resolution you need for your image and click OK. Soften effect from Microsoft Photoeditor The displayed image will probably be smaller, because the actual pixels are still displayed but because there are now fewer pixels, the image you see on the screen is much smaller. In Microsoft Photoeditor, you would use the Resize dialogue, which actually resamples the image. Save the image Save the image in the appropriate file format for your intended use. Image size dialogue from Photoshop The final image may not be as good as one scanned from an photograph, that had no halftones, but it will probably look better than with the moiré superimposed on it. Resize dialogue from Microsoft Photoeditor 41