Print pixel values where images differ with imagemagick? - imagemagick

Question

Is there an easy way to print the pixel value where two images differ using imagemagick?
To be clear, I want to know what the value of that pixel is, as well as its coordinate. It doesn't matter from which image, since I can simply swap them to get the right one.

Answer 1

Let's make two images, both 3px wide and 1px tall:
convert xc:red xc:lime xc:blue +append 1.png
convert 1.png -flop 2.png
If we do the following, we can make any pixels that are identical in the two images become transparent:
convert {1,2}.png -compose changemask -composite mask.png # Note that {1,2}.png is just bash shorthand for "1.png" "2.png"
And if we re-order the input images:
convert {2,1}.png -compose changemask -composite mask.png # Note that {2,1}.png is just bash shorthand for "2.png" "1.png"
So, I assume you want the above, but in text format with the transparent pixels suppressed:
convert {1,2}.png -compose changemask -composite txt: | grep -v ",0)"
Output
# ImageMagick pixel enumeration: 3,1,65535,srgba
0,0: (65535,0,0,65535) #FF0000FF red
2,0: (0,0,65535,65535) #0000FFFF blue
Note that if you want to permit a small difference between the images, you can add some "fuzz-factor". So, if you want rgb(0,100,200) to be considered near enough equal to rgb(3,96,205), you could add -fuzz 5 at the start of the command.

Answer 2

In Imagemagick 6, you can do the following to list the coordinates where the two images differ:
convert image1 image2 -compose difference -composite -threshold 0 txt: | tail -n +2 | grep "white" | awk '{print $1}' | sed 's/://g'
If using Imagemagick 7, change convert to magick
ADDITION:
If you want both the coordinates and the color in one of the two images, then assuming the image has no perfect black pixels, you can do the following:
convert image1 image2 \
\( -clone 0,1 -compose difference -composite -threshold 0 \) \
-delete 1 \
-compose multiply -composite txt: |\
tail -n +2 | grep -v "black" | awk '{print $1,$4}'
For example, I take the lena image and put a blue square in the top left corner to make a second image.
Input:
convert lena.png \( -clone 0 -size 5x5 xc:blue -composite \) \
\( -clone 0,1 -compose difference -composite -threshold 0 \) \
-delete 1 \
-compose multiply -composite txt: |\
tail -n +2 | grep -v "black" | awk '{print $1,$4}'
Results:
0,0: srgb(226,137,124)
1,0: srgb(224,137,130)
2,0: srgb(225,135,121)
3,0: srgb(228,134,121)
4,0: srgb(227,138,125)
0,1: srgb(226,137,124)
1,1: srgb(224,137,131)
2,1: srgb(225,135,121)
3,1: srgb(228,134,121)
4,1: srgb(227,138,126)
0,2: srgb(226,138,124)
1,2: srgb(224,136,127)
2,2: srgb(225,135,120)
3,2: srgb(228,134,121)
4,2: srgb(227,137,121)
0,3: srgb(228,137,122)
1,3: srgb(225,134,114)
2,3: srgb(225,134,118)
3,3: srgb(229,132,112)
4,3: srgb(227,133,113)
0,4: srgb(224,130,109)
1,4: srgb(223,132,110)
2,4: srgb(224,132,116)
3,4: srgb(226,131,112)
4,4: srgb(226,134,117)
If you do have black and the images have no transparency, then you can do:
convert image1 image2 \
\( -clone 0,1 -compose difference -composite -threshold 0 \) \
-delete 1 \
-alpha off -compose copy_opacity -composite \
-background black -alpha background txt: |\
tail -n +2 | grep -v "none" | awk '{print $1,$4}'
For example:
convert lena.png \( -clone 0 -size 5x5 xc:blue -composite \) \
\( -clone 0,1 -compose difference -composite -threshold 0 \) \
-delete 1 \
-alpha off -compose copy_opacity -composite \
-background black -alpha background txt: |\
tail -n +2 | grep -v "none" | awk '{print $1,$4}'
Results:
0,0: srgba(226,137,124,1)
1,0: srgba(224,137,130,1)
2,0: srgba(225,135,121,1)
3,0: srgba(228,134,121,1)
4,0: srgba(227,138,125,1)
0,1: srgba(226,137,124,1)
1,1: srgba(224,137,131,1)
2,1: srgba(225,135,121,1)
3,1: srgba(228,134,121,1)
4,1: srgba(227,138,126,1)
0,2: srgba(226,138,124,1)
1,2: srgba(224,136,127,1)
2,2: srgba(225,135,120,1)
3,2: srgba(228,134,121,1)
4,2: srgba(227,137,121,1)
0,3: srgba(228,137,122,1)
1,3: srgba(225,134,114,1)
2,3: srgba(225,134,118,1)
3,3: srgba(229,132,112,1)
4,3: srgba(227,133,113,1)
0,4: srgba(224,130,109,1)
1,4: srgba(223,132,110,1)
2,4: srgba(224,132,116,1)
3,4: srgba(226,131,112,1)
4,4: srgba(226,134,117,1)