Free and Open Source Software for the Manipulation of Digital Images

Transcription

1 Medical Physics and Informatics Computers in Radiology Solomon Free and Open Source Software for Digital Images Medical Physics and Informatics Computers in Radiology Robert W. Solomon 1,2 Solomon RW Keywords: digital images, free software, GIMP, image processing, open source software, Photoshop DOI: /JR Received December 1, 2008; accepted after revision January 5, Department of Radiology, Flushing Hospital and Medical Center, 4500 Parsons lvd., Flushing, NY ddress correspondence to R. W. Solomon (rob@robsolomon.name). 2 Department of Radiology, Jamaica Hospital and Medical Center, Jamaica, NY. WE This is a Web exclusive article. JR 2009; 192:W330 W X/09/1926 W330 merican Roentgen Ray Society Free and Open Source Software for the Manipulation of Digital Images OJECTIVE. Free and open source software is a type of software that is nearly as powerful as commercial software but is freely downloadable. This software can do almost everything that the expensive programs can. GIMP (gnu image manipulation program) is the free program that is comparable to Photoshop, and versions are available for Windows, Macintosh, and Linux platforms. This article briefly describes how GIMP can be installed and used to manipulate radiology images. CONCLUSION. It is no longer necessary to budget large amounts of money for highquality software to achieve the goals of image processing and document creation because free and open source software is available for the user to download at will. Q uite a few articles about how to prepare images for use in print journals or for lectures have been published [1 5]. These articles describe in detail how to use Photoshop (dobe Systems) and PowerPoint (Microsoft). lthough Photoshop and PowerPoint are useful and powerful programs, they entail significant costs. s of this writing, a new full copy of Photoshop CS4 costs $700. This puts it out of the range of most residents and can strain department budgets if multiple licenses are sought. Free and open source software is a type of software that is nearly as powerful as commercial software. s the name states, the source computer code for this type of program is open and available for inspection by anyone who is interested. Despite its zero cost, this software is excellent and can do almost everything that the expensive programs can. The program that is comparable to Photoshop is called GIMP. GIMP is an acronym that stands for gnu image manipulation program, and versions are available for Windows 2000 and above (Microsoft), Macintosh OS X (pple, Inc.), and Linux (multiple vendors) platforms. reasonable minimum amount of memory is 512 M, with 1 G recommended. Obviously, the more memory available for image processing, the better the performance will be. For those who are interested, the history of GIMP is available online at Wikipedia [6]. The examples in this article are for GIMP as installed on a Windows XP SP3 computer. s of this writing, GIMP for Windows version is the newest and can be obtained from gimp.org/downloads. The Help files are downloaded and installed separately from the program itself. detailed step-by-step guide titled Grokking the GIMP is also free and available online [7]. On starting GIMP, the layout is different from that of Photoshop, but very similar functions are available from the menus and toolbox. Figure 1 shows the GIMP windows. Notice that separate windows are opened for the toolbox; image workspace; and combined dialog boxes for Layers, Channels, Paths, and Undo. Several of my desktop icons are partially visible underneath these windows. However, these windows behave just as their counterparts in Photoshop do. s in Photoshop, when the mouse pointer hovers over a tool icon in the toolbox window, a Tooltip will pop up indicating the icon s function. In the following sections, I show how similar GIMP is to Photoshop by outlining how tasks are performed in GIMP that have already been described for Photoshop [8 10]. This article relies on a basic understanding of these Photoshop techniques. The following methods use either a menu item or toolbox tool to perform the needed function. It is worth noting that GIMP can use the Photoshop image format (.psd) used as the native W330 JR:192, June 2009

2 Free and Open Source Software for Digital Images image format for Photoshop, so it is possible to use both programs on the same image. Convert Image to Gray Scale Images that have been digitized by a scanner or a digital camera are in RG color. These images need to be converted to grayscale images (black and white) by selecting Image from the top menu, Mode, and finally Grayscale, as shown in Figure 2. Fig. 1 Screen shot shows GIMP (gnu image manipulation program) windows after program is started. Note that separate windows are opened for toolbox, workspace, and dialog boxes and that several of my desktop icons are partially visible underneath these windows. Crop Function Cropping an image is the quickest way to remove the patient s name and medical record number from an image. There are two ways to crop an image. One is to select the Crop tool from the toolbox in the third row and fourth column in the toolbox (Fig. 3). The Crop tool is shaped like a utility knife with a pointed razor blade as the tip. s in Photoshop, draw a rectangle around the part of the image to be kept and the remainder will be discarded. The second way to crop an image is to use the rectangle Select tool from the first row, first column of the toolbox to select the part of the image you want to keep, choose Image from the top menu, and then click Crop to Selection. Scale the Image Changing the size of an image is just as easy to do in GIMP as in Photoshop. In GIMP, resizing is called scaling. From the top menu, select Image and click on Scale Image. The pop-up dialog box (Fig. 4) allows altering the width and height of the image in whatever units desired. Pixels are recommended as the unit to be used in the selection. Image resolution can be changed by clicking in either the X or Y resolution boxes. I do not recommend changing both image resolution and size in the same operation. Note that there is a chain-link icon immediately to the right of the entry boxes for image size and resolution. Clicking on this icon toggles between opening and closing the chain link. When the links are closed, the respective dimensions are linked to prevent distortion. Fig. 2 How to convert images from RG format to gray-scale format using GIMP (gnu image manipulation program)., Selecting Image Mode shows top entry in Mode submenu is bulleted RG Color, which means that current image is in that format. Grayscale option is highlighted and will change image to gray scale when that option is clicked., Screen shot shows result of operation. JR:192, June 2009 W331

3 Solomon Fig. 3 Screen shot shows how to crop images using GIMP (gnu image manipulation program). rrow points to Crop tool in third row and fourth column of toolbox. Levels GIMP provides a method to change an image s dynamic range. From the top menu, selecting Colors Levels produces a dialog box that shows the histogram of the image s density values. The small triangle markers below the histogram can be moved manually. However, GIMP provides automatic and semiautomatic methods of optimizing image appearance. Clicking on the option button labeled uto will automatically set the levels for black, gray, and white. The semiautomatic method requires the user to select the Eyedropper button next to the uto button and then to select the black, gray, and white points on the image. GIMP will optimize the levels based on the user s selections (Fig. 5). Curves Selecting Colors from the top menu and then Curves shows a pop-up dialog box that is based on x- and y-axes that can be manipulated as in Photoshop. The y-axis signifies the density values present in the image, and the x-axis corresponds to how those values are to be output to the monitor or printer [3]. y default, there is a linear relationship between density values and their output. In gray-scale mode, darker pixels are on the upper part of the line. The purpose of this tool is to shape the line into a curve and thereby alter how the pixels are displayed. Unlike Photoshop, clicking on an individual pixel in the image with the Eyedropper tool will set the main inflection point for the curve. Further clicking on the image will move this inflection point and not add new reference points. This tool is one that requires more practice than the others to get good results. Fig. 4 Screen shot shows how to change size of images using GIMP (gnu image manipulation program). Scale Image dialog box appears when Image Scale Image is clicked. Clone Tool The Clone tool icon is located in the toolbox and looks like a rubber stamp. It essentially copies pixels from one part of the image to another. y choosing a small clean area that is similar to a small dirty part of the image, you can copy clean pixels from the clean area to replace the dirty ones. The sizes of the clean and dirty areas are determined by the size of the brush chosen. This tool works well for removing unwanted scratches or dirt from scanned images. tool with a similar function is the Healing tool; its icon looks like an X. Unsharp Mask Contrary to its name, Unsharp Mask is a filtering technique that is used to sharpen images. Used carelessly, this filter can enhance noise and make the quality of an image worse. Select this filter from the top menu by clicking on Filters Enhance Unsharp Mask, and the dialog box that pops up shows three sliders to individually set radius, amount, and threshold [4]. Good values for radius are 2 pixels; for amount, ; and for threshold, 1 2. Combination Example The techniques described in this article are actually used in combination to prepare an image for use. The final example consists of an old film that was digitized and cleaned up using GIMP. The results of each step are shown in the order needed to achieve the final manipulated image. Figure 6 shows a digital image obtained with a digital camera (Kodak DC MP) by photographing an old radiograph. Note that the film W332 JR:192, June 2009

4 Free and Open Source Software for Digital Images Fig. 5 How to change image s dynamic range using GIMP (gnu image manipulation program)., Digital image from venous angioplasty in patient on hemodialysis., Levels dialog box for image in shows histogram and available options. has a very noticeable blue tint. Figure 6 shows the gray-scale digital image of the film that resulted from selecting Image from the top menu Mode Grayscale operation. The original film has flasher information that needs to be removed by a crop operation. Figure 6C shows the digital image after the Crop tool from the toolbox has been used. The flasher is no longer in the image; however, near the bottom of the film there is a large dirt spot, shown by the arrow, that made its way onto the film over the years. Figure 6D shows the image after application of the Clone tool from the toolbox; the dirt spot has been removed from the image. Other dirt spots that have not been removed are shown for comparison. The major finding on the original film is that a Greenfield filter is malpositioned; subsequent imaging confirmed that the filter was inadvertently placed into the left renal vein. Fig. 6 Digitization of old film shown step by step; see text for more detail about how each of these steps is performed., This digital image was obtained by photographing original film using Kodak DC-290 camera., Digital image after color information has been discarded; digital image of film is now in gray-scale mode. C, Digital image after Crop tool from toolbox has been used to remove flasher information. rrow points to large dirt spot. C (Fig. 6 continues on next page) JR:192, June 2009 W333

5 Solomon Fig. 6 (continued) Digitization of old film shown step by step; see text for more detail about how each of these steps is performed. D, Digital image after Clone tool from toolbox has been used to remove large dirt spot that is near Greenfield filter. Other dirt spots are not altered to show difference. E, Digital image after Unsharp Mask filter has been used to restore sharpness of Greenfield filter seen on original image. However, the Greenfield filter in the digitized image is not as sharp as it is on the actual film. Figure 6E shows the image that results after application of Unsharp Mask, which was accessed from the top menu Filters Enhance Unsharp Mask. I used a radius of 2.0 pixels, amount of 1.0, and threshold of 2 on a selection made using the Rectangle Select tool from the toolbox to limit application of the Unsharp Mask filter to the area of the Greenfield filter. Finally, I used the Level menu operation, but this change actually made the image worse (not shown). Conclusion GIMP and Photoshop are both imageprocessing programs that are more similar to each other than different. lthough GIMP is not a full substitute for Photoshop, it provides most of the functionality and is freely available for Windows 2000 and later, Macintosh OS X, and Linux platforms. In addition to GIMP, free alternatives exist for vector drawing (Inkscape) and page layout (Scribus) software, as well as an office suite (OpenOffice). OpenOffice includes a presentation application called Impress that is similar in function to and compatible with PowerPoint. It is no longer necessary to budget large amounts of money for high-quality software to achieve the goals of image processing and document creation because there is software that is entirely free and open source available for the user to download at will. References 1. Taylor G. Initial steps in image preparation. JR 2002; 179: Taylor G. Removal of dust and scratches from D E electronic image files. JR 2003; 180: Taylor G. Color correction and automating repetitive tasks. JR 2003; 181: Stern EJ, Richardson ML. Preparation of digital images for presentation and publication. JR 2003; 180: Corl FM, Garland MR, Lawler LP, Fishman EK. five-step approach to digital image manipulation for the radiologist. RadioGraphics 2002; 22: Gimp entry. Wikipedia: the free encyclopedia Website. en.wikipedia.org/wiki/gimp. ccessed November 30, unks C. Grokking the GIMP. gimp-savvy.com/ OOK. ccessed November 30, Taylor G. Improving image contrast. JR 2003; 180: Taylor G. Sharpening the image. JR 2003; 181: Caruso RD, Postel GC. Image editing with dobe Photoshop 6.0. RadioGraphics 2002; 22: W334 JR:192, June 2009