How to search an image for subimages using linux console?

Question

I have to search for the occurrence of a smaller image in a larger one using the console. As result I want to receive it`s image coordinates. What solutions are possible?

I heard about ImageMagick, but not really understand how it works. If it`s enough, then I would appreciate an example command.

Thank you.

Answer 1

Here's a little example so you can see how it works...

First, our needle image

Now make a haystack, green and blue - very stylish :-)

convert -size 256x256 gradient:lime-blue haystack.png

Now hide two needles in the haystack, one at a time, nothing fancy:

convert haystack.png needle.png -geometry +30+5 -composite haystack.png 
convert haystack.png needle.png -geometry +100+150 -composite haystack.png 

Now search for the needles in the haystack, two output files will be produced, locations-0.png and locations-1.png

compare -metric RMSE -subimage-search haystack.png needle.png locations.png > /dev/null 2>&1

This is the second, more useful output file locations-1.png. It is black where IM is sure there is no match and progressively nearer to white the more certain ImageMagick is that there is a match.

Now look for locations where IM is 95+% certain there is a match and convert all pixels to text so we can search for the word white.

convert locations-1.png -threshold 95% txt: | grep white

The output is this, meaning ImageMagick has found the needles at 30,5 and 100,150 - exactly where we hid them! Told you it was Magic!

30,5: (255,255,255)  #FFFFFF  white
100,150: (255,255,255)  #FFFFFF  white

Here is the entire script so you can run it and play with it:

#!/bin/bash
convert -size 256x256 gradient:lime-blue haystack.png                      # make our haystack
convert haystack.png needle.png -geometry +30+5 -composite haystack.png    # hide our needle near top-left
convert haystack.png needle.png -geometry +100+150 -composite haystack.png # hide a second needle lower down

# Now search for the needles in the haystack...
# ... two output files will be produced, "locations-0.png" and "locations-1.png"
compare -metric RMSE -subimage-search haystack.png needle.png locations.png > /dev/null 2>&1

# Now look for locations where IM is 95% certain there is a match
convert locations-1.png -threshold 95% txt: | grep white