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Imatest Documentation The Screen Patterns display, shown on the right, works best with large flat screen LCDs (it's not recommended with CRTs). Click on Screen Patterns on the right the Imatest main window, select Distortion grid or Squares (checkerboard) in the box at the bottom-center, then maximize the window. You can select the number of Horizontal and Vertical lines as well as the line width (in pixels). For best results the pattern should be square. A portion of the Test Charts input dialog box is shown on the right, with recommended settings: Pixels per inch (PPI). 100-300. High resolution isn't necessary because Distortion doesn't measure sharpness. Type (grid or checkerboard) selects chart type. Horizontal divisions. 10-15 should work for most situations. For extreme distortion, you will either need to use fewer divisions or select an ROI (i.e., crop the image). Vertical divisions. Square is recommended in most cases. Screen Pattern module for Distortion. Line width (pxls). (Note: with the currentlyrecommended checkerboard chart, line width is not an issue.) Only applies when Type is set to Grid. Typically in the range of .01*PPI to .025*PPI. The printed line should be clear (not pale), but not too thick. The relative line widths in the preview (to the right of the selection boxes) is not to scale. They are (relatively) thicker than in the final image. The line width should be thick enough to cover at least two pixels in the captured image.Narrower linesTest Charts dialog box for preparing may result in errors (Inconsistent number...). For example, a file to print the distortion grid. suppose your captured image were 640x480 pixels (VGA). The line width (as shown on the right) would be 3 pixels out of a chart width of 1476. In the captured chart, the line width would be 640*(3/1476) = 1.3, inadequate for this small image size, though this chart would be perfectly adequate for 1+ megapixel cameras. Gamma, and Contrast are turned off for the Grid pattern. Print the test chart file. Size isn't critical, but larger is better. (13x19 inches or larger is recommended if your printer can handle it). Matte paper is fine. Handmade charts are acceptable if they're carefully done. The print should be made from an image editor using the saved TIFF file, not the Matlab preview. Photograph The chart. It should be evenly lit (though lighting is less critical than with Light Falloff). Lighting instructions are found in Using SFR, Part 1. Be especially careful to avoid glare "hot spots" on chart images. Unlike SFR, where small regions can be selected to avoid hot spots, glare hot spots are likely to interrupt grid lines, which will terminate Distortion analysis. Glare can be especially troublesome with wide angle lenses. To minimize glare we recommend printing Distortion charts on matte surfaces. The chart should be clean. Distortion will terminate if it mistakes spots in the image for lines. If needed, spots can be manually removed using an image editor "clone" function. 348 of 451
Imatest Documentation Set exposure compensation to overexpose by one to two f-stops so white areas are light (not middle gray) and line contrast is sufficient. The camera should be pointed straight at the chart. (The optical axis of the lens should be normal to the chart surface.) Small amounts of tilt and perspective distortion (convergence of lines; not a lens aberration) are tolerated, but large amounts may cause Distortion to terminate. The camera should be set to a low ISO speed to minimize noise. This is especially important for compact digital cameras, where noise can become severe at high ISO speeds. Good image Horizontal and vertical lines should fit between lines in the image for the lines to be detected reliably. Bad image Horizontal line cannot fit between all lines in the image, compromising the reliability of line detection. This image is usable if it is cropped (i.e., a Region of interest is selected). Imatest versions 2.3.6+ (released May 2007) performs better with images of this type with ROI filtering set to weak (frequently without cropping), but careful orientation (as shown in the image on the left) is recommended for the most reliable results. Save the image. High quality JPEG format is preferred because it preserves EXIF data in a format Imatest can read. There is no need for RAW quality. Resize the image? Because Matlab uses double precision for most math operations, each 24-bit color pixel requires 24 bytes. A 6.3 megapixel image can take 151 megabytes of memory (ouch). This can slow down computations in computers that don't have gobs of memory. (It can even bog down computers with 512k of memory.) We are looking into solving this problem. In the meantime we recommend resizing images to about 1000 pixels wide in an image editor. (Any editor will do.) Resizing has no effect on the results. Launch Imatest. Click on Distortion . Very large files (height x width x colors over 40 MB) may cause memory overflow problems. Files over 40 or 80 MB can be automatically reduced 1/2x linearly (using 1/4 the memory). Click Settings, Options and Settings... (in the Imatest main window) and make the appropriate setting in LARGE FILES (Light Falloff, Distortion). Open the image file. Batch mode This module can operate in batch mode in Imatest Master, i.e., it can read multiple input files. All you need to do is select several files using the standard Windows techniques of shift-click or control-click. There are three requirements for the files. They should (1) be in the same folder, (2) have the same pixel size, and (3) be framed identically. The input dialogs (cropping (if applicable), settings, and save) are the same for the first run as for standard non-batch runs. Additional runs use the same settings as the first run. Since no user input is required they can run extremely fast. If the order of the files in a batch runs is different from the selection order, click Settings, Options and Settings... (in the Imatest main window) and change the setting in Batch run order. One caution: Imatest can slow dramatically on most computers when more than about twenty figures are open. For this reason we recommend checking the Close figures after save checkbox, and saving the results. This allows a large number of image files to be run in batch mode without danger of bogging down the computer. Crop the image (select the ROI) if needed using the usual clicking and dragging technique. Cropping may be helpful if horizontal or vertical lines are not entirely within the ROI (region of interest), though Imatest 2.3.6+ often works well in such cases. An example is shown below in the section on Severe distortion. Click outside the the image to select the entire image. You can select either a grid pattern or a single line, which is useful in patterns such as the ISO 12233 chart, which contains two lines suitable for Distortion: one nearly vertical and one nearly horizontal (both slightly tilted). One of them is illustrated below. Make sure to leave some breathing room around the line. In selecting the crop for grid patterns (or deciding not to crop) try to choose a crop where no lines cross the crop boundaries, i.e., are partly inside and partly outside. The horizontal (well... sort of) line at the 349 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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