ImageMagick split image by background color - imagemagick

Question

I have big image https://buildyourboard.pro/userboards/board-img/data/clipart.png
how can I segmentate this image to small images by transparent (or another) background?
Images not in grid, so code should find position of every pic in image

Answer 1

If you run this command, it will scrunch all the pixels up into an image that is just one pixel wide and has the original height of your image. You will then see that all lines that have the word none correspond to all the locations where you can draw a horizontal line across the image without intersecting with any of your board backgrounds:
convert boards.png -resize 1x! txt: | more
# ImageMagick pixel enumeration: 1,4667,255,srgba
0,0: (0,0,0,0) #00000000 none
0,1: (0,0,0,0) #00000000 none
0,2: (0,0,0,0) #00000000 none
...
...
...
0,547: (0,0,0,0) #00000000 none
0,548: (0,0,0,0) #00000000 none
0,549: (0,0,0,0) #00000000 none
0,550: (0,0,0,0) #00000000 none
0,551: (0,0,0,0) #00000000 none
0,552: (0,0,0,0) #00000000 none
0,553: (0,0,0,0) #00000000 none
0,554: (0,0,0,0) #00000000 none
0,555: (39038,36633,35446,0) #988F8A00 srgba(152,143,138,0)
0,556: (38889,36248,34984,12) #978D8800 srgba(151,141,136,0.000183108)
0,557: (38348,35253,33873,75) #95898400 srgba(149,137,132,0.00114443)
0,558: (35061,31775,30664,98) #887C7700 srgba(136,124,119,0.00149538)
0,559: (33894,30508,29428,164) #84777301 srgba(132,119,115,0.00250248)
0,560: (34379,30968,29853,243) #86787401 srgba(134,120,116,0.00370794)
Now you can see that the first place where it goes from there being nothing in a horizontal line to there being something is at line 555. So basically, you want to detect where it changes from none to something.
That is readily done through awk like this:
convert boards.png -resize 1x! txt: | awk 'inside && /none/{inside=0;print;next} !inside && ! /none/{inside=1;print}'
# ImageMagick pixel enumeration: 1,4667,255,srgba
0,0: (0,0,0,0) #00000000 none
0,555: (39038,36633,35446,0) #988F8A00 srgba(152,143,138,0)
0,911: (0,0,0,0) #00000000 none
0,1082: (36701,34188,35001,0) #8F858800 srgba(143,133,136,0)
0,1433: (0,0,0,0) #00000000 none
0,1570: (33489,32153,32838,4) #827D8000 srgba(130,125,128,6.10361e-05)
0,1937: (0,0,0,0) #00000000 none
0,2135: (29884,28945,29339,4) #74717200 srgba(116,113,114,6.10361e-05)
0,2486: (0,0,0,0) #00000000 none
0,2621: (28668,27300,25241,5) #706A6200 srgba(112,106,98,7.62951e-05)
0,2972: (0,0,0,0) #00000000 none
0,3216: (35066,32529,28581,2) #887F6F00 srgba(136,127,111,3.0518e-05)
0,3612: (0,0,0,0) #00000000 none
So, now we know we can make horizontal cuts across the picture at lines 555, 911, 1082, 1433 etc.
I will draw the first 3 such lines in like this:
convert boards.png -stroke red -draw "line 0,555 12000,555" -draw "line 0,911 12000,911" -draw "line 0,1082 12000,1082" result.jpg
Now, you cut along those lines - like this:
convert boards.png -crop x911+0+0 row1.jpg
convert boards.png -crop x416+911+1082 row2.jpg
Then apply exactly the same procedure in the vertical direction, replacing -resize 1x! with -resize x1!.