When running a bitmap checkpoint, QuickTest compares the area that you are checking in the application with the bitmap stored in the checkpoint, pixel by pixel. By default, if any pixels are different, the checkpoint fails. The Bitmap Checkpoint Properties dialog box provides options for fine-tuning the bitmap comparison.
You can adjust the comparison to enable the checkpoint to pass even if the bitmaps are not identical by setting the RGB tolerance and Pixel tolerance options described below.
In addition, QuickTest enables you to use custom comparers for bitmap checkpoints. A custom comparer is a COM object that you or a third party developed to run the bitmap comparison in the checkpoint according to a more specific algorithm. If one or more custom comparers are installed and registered on the QuickTest computer, the Bitmap Checkpoint Properties dialog box includes a Comparer option. This option enables you to select the QuickTest default comparer or a custom comparer that performs the bitmap comparison according to your testing requirements.
If you select a custom comparer, some of the options in the Bitmap Checkpoint Properties dialog box are different.
Bitmap Checkpoint Tolerance Options
RGB tolerance: The RGB (Red, Green, Blue) tolerance determines the percent by which the RGB values of the pixels in the actual bitmap can differ from those of the expected bitmap and allow the checkpoint to pass. (The RGB tolerance option is limited to bitmaps with a color depth of 24 bits.)
For example, a bitmap checkpoint on identical bitmaps could fail if different display drivers are used when you create your checkpoint and when you run your test. Suppose one display driver displays the color white as RGB (255, 255, 255) and another driver displays the color white as RGB (231, 231, 231). The difference between these two values is about 9.4%. By setting the RGB tolerance to 10%, your checkpoint will pass when running your test with either of these drivers.
Note: QuickTest applies the RGB tolerance settings when comparing each pixel in the actual and expected bitmaps. The Red, Green, and Blue values for each pixel are compared separately. If any of the values differs more than the tolerance allows, the pixel fails the comparison.
Pixel tolerance: The pixel tolerance determines the number or percentage of pixels in the actual bitmap that can differ from those in the expected bitmap and allow the checkpoint to pass.
For example, suppose the expected bitmap has 4000 pixels. If you define the pixel tolerance to be 50 and select the Pixels radio button, up to 50 pixels in the actual bitmap can be different from those in the expected bitmap and the checkpoint passes. If you define the pixel tolerance to be 5 and select the Percent radio button, up to 200 pixels (5 percent of 4000) in the actual bitmap can be different from those in the expected bitmap and the checkpoint passes.
Using both RGB and Pixel Tolerances: If you define both RGB and pixel tolerances, the RGB tolerance is calculated first. The pixel tolerance then defines the maximum number of pixels that can fail the RGB criteria and allow the checkpoint to pass.
For example, suppose you define an RGB tolerance of 10 percent and a pixel tolerance of 5 percent for a bitmap that has 4000 pixels.
For the checkpoint to pass, each pixel in the actual bitmap must have RGB values that are no greater than or no less than 10 percent of the RGB values of the expected bitmap. If that criterion fails, QuickTest checks that the number of pixels that failed are less than 200. If that criterion passes, the checkpoint passes.
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