10.12. Verify Correctness?

After we have an optimization that has significantly reduced the filter's runtime, it is necessary to verify that the output image it produces is the same for both the optimized and the unoptimized filter. After loading up the original reference image and comparing it to the newly generated image, I used GIMP to take the difference of the two images. If the reference and optimized image are identical, all the pixels should be zero (black). However, the different image was not perfectly black. Visually, it looked black, but upon closer inspection (using the GIMP color picker), some of the pixels were nonzero. This means that the reference and optimized images are different.

This would normally be a cause for concern, because this might indicate that optimization changed the behavior of the filter. However, a closer examination of the filter's source code showed several places where random noise was used to slightly jitter the image before the filter was run. Any two runs of the filter would be different, so the optimization was likely not to blame. Because the differences between the two images were so visually small, we can assume that the optimization did not introduce any problems.