Use imagequick to compare images

Question

I don\'t understand how the compare script of imagequick work.

I compare an image to the same image (a copy)

I do

compare -metric A

Answer 1

Here are some examples to help you understand how image comparison works.

The -metric AE tells you the Absolute Error, which is the number of pixels that differ - so it will be zero if all pixels are identical.

1. Compare two images that are exact copies of each other in every respect

convert -size 128x128 xc:red red.png                    # Make red image
cp red.png perfectCopy.png                              # Make perfect copy
compare -metric AE red.png perfectCopy.png result.png   # Count differing pixels
0                                                       # There are none - identical

According to standard Unix tools (md5, diff, tmp), files are binary identical and md5 checksums identical:

md5 red.png perfectCopy.png 
MD5 (red.png) = 39236e0e0dfb70da0e9bcbfbcf7b8181
MD5 (perfectCopy.png) = 39236e0e0dfb70da0e9bcbfbcf7b8181

ImageMagick hashes over pixels only (not including metadata) are identical:

identify -format "%#:%f\n" red.gif perfectCopy.png 
1157038985fec4573888bc7e74a5a728e3aa5cbc49e18253c934295162a9aeea:red.gif
1157038985fec4573888bc7e74a5a728e3aa5cbc49e18253c934295162a9aeea:perfectCopy.png

2. Compare two images with identical appearance but different metadata

convert -size 128x128 xc:red red.png                    # Make red image
sleep 2
convert -size 128x128 xc:red redDifferentDate.png       # Make red image with different date
compare -metric AE red.png redDifferentDate.png result.png
0                                                       # No difference

But, according to standard Unix tools (diff,md5,sum), the files are different - because the date is in there.

md5 red.png redDifferentDate.png 
MD5 (red.png) = 004088f6d275f431cedb74bc0209bbc5
MD5 (redDifferentDate.png) = d7d36f56e1940251f9804bd795ef4157

But ImageMagick knows images better, and its calculated hashes (checksum) over pixel data only (not including metadata) are the same:

identify -format "%#:%f\n" red.gif redDifferentDate.png 
1157038985fec4573888bc7e74a5a728e3aa5cbc49e18253c934295162a9aeea:red.gif
1157038985fec4573888bc7e74a5a728e3aa5cbc49e18253c934295162a9aeea:redDifferentDate.png

3. Compare two images with identical pixels, but totally different sizes and formats

convert -size 128x128 xc:red red.png                    # Make red PNG
convert -size 128x128 xc:red red.gif                    # Make red  GIF
compare -metric AE red.png red.gif result.png           # Count differing pixels
0                                                       # No difference

But, files and md5 hashes differ:

diff red.png red.gif
Binary files red.png and red.gif differ

md5 red.png red.gif 
MD5 (red.png) = aed0840c2c99425c25bd782e7b409022
MD5 (red.gif) = 5869df00d7b3cab3495a6c402ba61ec9

Again, ImageMagick knows better and the hashes over pixel data only (not including metadata) are still the same:

identify -format "%#:%f\n" red.gif red.png 
1157038985fec4573888bc7e74a5a728e3aa5cbc49e18253c934295162a9aeea:red.gif
1157038985fec4573888bc7e74a5a728e3aa5cbc49e18253c934295162a9aeea:red.png

4. Compare two grossly different files

Obviously if we create two grossly different files each full of random noise, everyone agrees they are different:

convert -size 128x128 xc:gray +noise random random1.png   # Make random image
convert -size 128x128 xc:gray +noise random random2.png   # Make random image
compare -metric AE random[12].png result.png              # Count differing pixels
16384                                                     # Yep, a fair few differ!

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There are other metrics available, such as MeanSquared, RootMeanSquared etc - you can list them using:

identify -list metric

Output

AE
Fuzz
MAE
MEPP
MSE
NCC
PAE
PHASH
PSNR
RMSE