Compare: Output image based on colour distance

[sabrehagen]

Hi,

I'm trying to compare two images. One is a screenshot, and the other is the same screenshot but after being processed by h264. I am currently using compare to show me the pixels that have changed between the two images. The output shows (as expected) in red the differing pixels. I would like the intensity of this colour to be proportionate to the colour distance between the original and encoded pixels. How would I go about doing this?

Thanks for your assistance!

[fmw42]

I do not think you can do that directly with compare.

If I assume you have two grayscale images, then try in unix syntax

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convert image1 image2 \
\( -clone 0 -clone 1 -compose difference -composite \) \
-delete 1 \
\( -clone 0 -fill red -colorize 100 \) \
-swap 1,2 -compose over -composite result

If you have color images, you need to reduce the contrast before overlaying the color

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convert image1 image2 \
\( -clone 0 -clone 1 -compose difference -composite -colorspace gray \) \
-delete 1 \
\( -clone 0 -fill red -colorize 100 \) \
\( -clone 0 -function polynomial "0.5 0.5" \) \
-delete 0 -reverse -compose over -composite result

If on windows remove the \ on the parens and change the line ending \ to ^.

If this does not do it, then let us know what you want differently.

[sabrehagen]

Thanks Fred! That seems very close to what I need, except there seems to be a constant factor by which the comparison is off. I mocked up two images to test with which are linked below. One is a base of blue (RGB 0, 0, 255) and the other has progressively lighter blue blocks across it. As you can see in the attached results image, the regions that are both exactly blue in the same place in both images still show as different, or a lighter purpley blue. My goal is to have the regions of exactly the same colour as white.

By eye it seems that if the purpley blue in the result that is in the common to both areas was subtracted from the result image, this operation would do exactly what I need. How would I modify the command to achieve this?

Blue: http://i.imgur.com/HPc8eVx.png
Blocks: http://i.imgur.com/QOGGBQ9.png
Result after running your second command: http://i.imgur.com/2wdiEcG.png

[fmw42]

I am still unclear as to what you really want. I had toned down the first image so that I could overlay red onto it to show where the differences were with stronger red being more difference.

If you just want to see the difference where white is the same and black is totally different with shades of gray between, then try

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convert orig.png mod.png -compose difference -composite -colorspace gray -negate result.png

If this is not what you want, then explain further.