Digital Imaging - Photoshop A digital image is a computer representation of a photograph. It is composed of a grid of tiny squares called pixels (picture elements). Each pixel has a position on the grid and a colour value. These pixels can be edited (changing the pixel colours) individually on in groups The illustration below shows an image of a car and a section of it zoomed to show the individual pixels. The shape of the pixels is clear as is their positions on the grid making up the digital photograph. The resolution of an image is the number of pixels it contains. This is normally given as dots (pixels) per inch (dpi) or dots per centimetre (dpcm). The resolution of an image effects its quality - higher resolution - better quality, but this comes at a price as an increase in resolution means an increase in file size. A doubling of the resolution produces a quadrupling of the file size. The chart below shows photographs taken at 150dpi and 300dpi. The resultant file sizes are shown when the photograph is taken as black and white, grayscale and colour. Size (CM) Resolution (dpi) Black and White Grayscale Colour 5 x 7.5 150 300 17 Kb 67 Kb 132 Kb 528 Kb 538 Kb 2.06 Mb 10 x 12.5 150 300 56 Kb 221 Kb 440 Kb 1.72 Mb 1.72 Mb 6.87 Mb 20 x 25 150 300 220 Kb 879 Kb 1.72 Mb 6.87 Mb 6.87 Mb 27.5 Mb Below is shown typical resolutions for a number of applications: Digital presentations: 72-96dpi Websites: 72-96dpi Magazines: 300dpi Most image editing software programs assume a resolution of 72dpi when a new image is being created. Page 1
Colour models Image editing software uses colour models to represent the production of colour on the screen and on paper. The main colour models in use are: The RGB Model: This model is used on computer monitors and digital projectors. Values ranging from 0 to 255 can be assigned to each of the primary colours red, green and blue. These colours are known as the additive primaries because if they are combined at full value (255), they produce white. If they are combined at values of 0, the resulting colour is black. All other colours are achieved by varying the values assigned to the three primaries. Colour Values RED WHITE BLACK MIX The CMYK Model: The CMYK model. This model is used for printing. It is based on the colours cyan, magenta and yellow. In theory, pure cyan (C), magenta (M), and yellow (Y) pigments should combine to produce black. For this reason these colours are called subtractive colours. Because all printing inks contain impurities, these three inks actually produce a dark brown and must be combined with black (K) ink to produce a true black (K - Keystone black). As it uses percentages for each colour, the values only go to 100 and not 255 as in RGB. The subtractive (CMY) and additive (RGB) colours are complementary colours. Each pair of subtractive colours creates an additive colour, and vice versa. Complimentary Colours: Additive Subtractive Page 2
Graphics formats Digital images come in two main types: Bitmap images: These are made up of a grid of pixels. Similar to a mosaic, each pixel is assigned an address and a colour value. As the number of pixels is fixed (resolution dependant), these images become jagged when zoomed. Also, because of the amount of information each pixel holds, these images tend to produce large files. Photographs are held as bitmaps because they can display the vast number of colours necessary for photo realism. Often called raster (pixel based) images, they can be edited at pixel level. Paint software produces bitmap graphics. Vector images. These images are basically a series of geometric objects such as lines and curves. Each geometric object will have a number of properties such as colour, width, size, fill and position. They are resolution independent and can be scaled without any loss of resolution. Photo realism is difficult to achieve with vector graphics and because of this they are rarely used for photographs. They tend to be used in logo creation and technical drawings. Drawing programs such as CAD produce vector images. Their file sizes are considerably smaller than bitmap image files. Page 3
Digital file formats Image editing software can open and save files in many file formats. Some of the file formats are universal and can be produced by any image editing program.. Others are proprietary which means they are produced by specific image editing programs (Photoshop, GIMP etc) The most common universal file formats are: BMP: This is the Microsoft graphics format. Images produced using MS Paint will be BMP TIFF: These are high quality images. A good digital camera will offer TIFF as it s best output. It produces images with a large file size and is used in magazines etc. JPEG: This is the most common file type created by digital cameras. JPEG images use lossy compression. This means that quality of the image is reduces to give a smaller file size. It is mainly used for Internet photographs. GIF: Gif (graphics Interchange Format) files can only display 256 colours in an image compared to 16 million in a TIFF image. Because of this, it is used mainly for logo s, drawings and diagrams. GIF images can have transparency and this is useful when placing an image on a coloured background. PNG: This file format (Portable Network Graphic) was designed as a replacement for GIF. It has the transparency advantage of GIF and can hold a lot more colours making it suitable for photographs. TIFF GIF The most common proprietary file formats are: PSD: These files are produced in Photoshop. PSP: These files are produced in Paint Shop Pro. XCF: These files are produced in GIMP. CPT: These files are produced in Corel Photo Paint. As these file types are proprietary, they will have special features not available in universal file types. For this reason they can usually only be opened in their own programs. They must be converted into a universal file format to be used in wordprocessing etc. Page 4
Photoshop Photoshop is the industry standard for image editing. It has a comprehensive range of tools and is used by professionals the world over. A cut-down version called Photoshop Elements is available at a fraction of the cost and is aimed the amateur photographer. The interfaces are almost identical and moving from one to the other is an easy transition. When Photoshop is opened, the Toolbox appears on the left of the screen. The Tool Options bar is found below the Main Menu at the top of the screen. The contents of this bar changes depending on the tool selected in the toolbox. The Palette area is found at the right of the screen. The palettes are grouped together and are used to monitor and modify images. They are fully editable and can be regrouped, hidden etc. Options Toolbox Sizes bar Palettes When Photoshop is started, the Work Area is blank. This is because the application is mainly used to edit images loaded from camera s, the Internet etc. Page 5
The Toolbox The full range of tools are available from the drop down menu, but the most common tools are found in the Toolbox. The tools can be broken into categories: Selection tools - There are used to select an area of an image for editing Paint tools - These are used to change colours in an image Type tool - This tool is used to all text to an image Path tools - These tools are used to create paths (shapes) Colour tool - This is used to sample a colour from an image and set it as the foreground colour Hand tool - This is used to pan around an image when the image is larger than the window containing it Zoom tool - This tool is used to zoom in and out of an image Selection Tools Painting Tools Text and Path Tools Marquee Tool Lasso Tool Crop Tool Healing Tool Rubber Stamp Eraser Blur/Sharpen Selection Tools Path Tools Notes Tool Hand Tool Foreground Default Colours Standard Mode Image Ready Move Tool Magic Wand Slice Tool Paintbrush History Brush Fill/Gradient Dodge/Burn Type Tool Shape Tools Colour Tool Zoom Tool Switch Colours Background Quick Mask Screen Modes The default painting colours in Photoshop are Black foreground and White background. The Toolbox allows these colours to be reversed, or returned to default mode by clicking the relevant controls. Notice that some of the tool icons have a tiny black triangle in the lower-right corner. This means that there are more tools of the same kind available on a pop-out menu. Point to any tool that has a triangle, click, and hold down the mouse button to see the other available tools. Page 6
Each of the tools in Photoshop has it s own set of options. These appear on a ribbon above the Toolbox when a tool is selected. Below are all the options when the Text tool is selected. Palettes Palettes help with the editing process. The main ones are shown below: Swatches: These are used to set the foreground and background colours. Foreground colour Reverse colours Return to default colours Background colour Histogram: This shows the spread of the pixels in the image (Levels). The pixels will be divided between the Highlights on the right of the graph and the Shadows on the left and the Gamma (mid-tones) in the centre of the graph. A perfect image will show as a bell curve. Navigator: The Navigator palette is used to control the image magnification and the viewing area. Dragging the slider at the bottom of the dialog zooms the image in and out. Dragging the box around the dialog controls the area of the image shown in the workspace. Clicking an area of the image within the dialog moves the rectangle to that area. Page 7
History: The Undo History palette keeps a record of all the editing processes carried out on the image. The huge advantage of this is this system is that you can undo any past edit at any time. Normal undo has to be done in the reverse order to the edits. Layers: The Layers palette shows all the layers in the image in stacking order. The Background layer is at the bottom. Layers are like a stack of transparent sheets. Images or paint can be placed on individual sheets and edited without effecting images on other sheets. Layers can be dragged up or down the stack. Layer display is controlled by toggling the eye icon to the left of each layer. To edit a layer it must be active (highlighted). Opening images Files in all the universal formats can be opened in the workspace. Depending on the amount of memory in the computer, several images can be open in the editor at the same time. To open an image file from the editor: Click the File menu and choose Open. The Open dialog box opens. Navigate to the folder containing the file Click the file to select it and click the Open button Page 8
Inputting images Scanning digitises an image so that it can be read, displayed, edited and printed by a computer. To scan an image in Photoshop: Click the File menu and choose Import and select the scanner. Scanning takes place in two stages - preview and then final scan. The preview scan allows you to select the exact area of the page or photograph you want to scan. To do a preview scan, Click the preview button. When the scan is finished, the right hand portion of the scan window will display the preview. Before performing the final scan, the software needs to know some information. Firstly, the exact area of the image to be scanned needs to be set. This is done by dragging the handles in the preview window. Secondly, the image type must be set. This can be colour, grayscale, black and white or custom settings. Custom settings allow you to set the image type and the required resolution - up to the maximum resolution capability of the scanner. Page 9
When all the settings are set, Click Scan. The image is scanned into Photoshop and is ready for editing. What resolution to use: The simple formula below can be used to set the scanning resolution: Scanning Resolution Required Resolution = x Original Image Size Required Image Size Example: An original photograph is 50mm x 50mm. It is to be printed at 300dpi. The photograph is to be scaled to 100mm x 100mm in the publication. What resolution should the scanner be set to. Using the above formula: Scanning = Resolution Required Resolution (300dpi) Original Image Size (50mm) x Required Image Size (100mm) Answer: (300/50) x 100 = 600dpi Page 10
A Digital Camera stores pictures electronically on a small internal memory card. The pictures can then be transferred from the card to the computer. When the camera is connected to the computer, it is seen as a removable disk drive and the Removable Disk window opens: Select Copy Pictures to a folder and click OK The Scanner and camera wizard opens. Click Next Page 11
Tick the required images or choose Select All Choose a folder for the images. Tick the Delete Pictures box if you want the pictures erased from the memory card. The images are copied to the folder. Page 12
Click Next in the following screen and Finish in the final screen Alternatively, you can choose Open folder to view files in the Removable Disk window to control the files as if they were of a disk drive. Page 13
Selecting Just as text to be edited in a word-processor must be selected, an area of an image or an entire image must be selected if it is to be edited. The main selection tools are: Marquee Tools Lasso Tools Magic Wand Marquee: The rectangular and elliptical tools select a defined shape. Select the tool and drag the required area. The area is surrounded by marching ants. Hold down the Shift key while dragging to constrain the shape to a perfect square or circle. Lasso: Use this tool to drag freehand selections. Select the tool and drag the freehand shape. Releasing the mouse joins the start and end points. A difficult tool to control for precise selections. Page 14
Polygonal Lasso: Click at intervals along the perimeter of the object to be selected. Press delete to undo the clicks one by one. Hover over the start point until a small circle appears and click to complete the selection. Magnetic Lasso: This tool selects highly contrasting edges by allowing the positioning of control nodes around the edges of the image. Click your way around the edges of the object to be selected and join the last point to the first to complete. Magic Wand Tool: This tool selects areas based on colour. Choose the tool and pick a pixel. Set the tolerance in the options. This is the variation in shade that will be included in the selection. Page 15
Path Tool: The path tool can be used to create selections. Select the tool in the Toolbox and set the option as below. The tool creates nodes around the perimeter of the object. A curve is generated between the nodes to create a perimeter. Joining the first and last points creates a path. The layers palette is used to convert the path to a selection. Click the Path tab and click the small arrow in the top-right corner to produce a menu. Select the Make Selection option and click OK in the dialog box to create the selection. The path is converted into a selection. Feather: Feathering is necessary when blending images together. Standard selections have very hard edges, and when placed in a collage, the sharp edges ruin the realism. Feathering a selection creates a soft edge for better blending. Feather values are measured in pixels. Below is shown the options bar for the rectangular marquee. The feather value is set to three pixels which means that a transition area of three pixels exists outside of the selection to blend the image with the area below. Page 16