Algorithm to detect overlapping rows of two images

Question

Let\'s say I have 2 images A and B as below.

Notice that the bottom of

Answer 1

I had a little play at doing this with ImageMagick. Here is the animation of what I did, and the explanation and code follow.

First I grabbed a couple of StackOverflow pages, using webkit2png, calling them a.png and b.png.

Then I cropped a rectangle out of the top-left of b.png and a column the same width, but the full height out of a.png

That gave me this:

and this

I now overlay the smaller rectangle from the second page onto the bottom of the strip from the first page. I then calculate the difference between the two images by subtracting one from the other and note that when the difference is zero, the pictures must be the same, and the output image will be black, so I have found the point at which they overlap.

Here is the code:

#!/bin/bash
# Grab page 2 as "A" and page 3 as "B"
# webkit2png -F -o A http://stackoverflow.com/questions?page=2&sort=newest
# webkit2png -F -o B http://stackoverflow.com/questions?page=3&sort=newest

BLOBH=256  # blob height
BLOBW=256  # blob width

# Get height of x.png
XHEIGHT=$(identify -format "%h" x.png)

# Crop a column 256 pixels out of a.png that doesn't contain adverts or junk, into x.png
convert a.png -crop ${BLOBW}x+0+0 x.png

# Crop a rectangle 256x256 pixels out of top left corner of b.png, into y.png
convert b.png -crop ${BLOBW}x${BLOBH}+0+0 y.png

# Now slide y.png up across x.png, starting at the bottom of x.png
# ... differencing the two images as we go
# ... stop when the difference is nothing, i.e. they are the same and difference is black image
lines=0
while :; do
   OFFSET=$((XHEIGHT-BLOBH-1-lines))
   if [ $OFFSET -lt 0 ]; then exit; fi
   FN=$(printf "out-%04d.png" $lines)
   diff=$(convert x.png -crop ${BLOBW}x${BLOBH}+0+${OFFSET} +repage \
           y.png \
           -fuzz 5% -compose difference -composite +write $FN \
           \( +clone -evaluate set 0 \) -metric AE -compare -format "%[distortion]" info:)
   echo $diff:$lines
   ((lines++))
done
n=$((BLOBH+lines))