ImageMagick: How to extract tiles in a grid from a single PNG? - imagemagick

Question

I know questions like this get asked a lot, but I haven't been able to find an answer to my specific use-case.
I frequently have to extract rectangular tile images that are laid out in a grid in a single PNG. The image contains a grid of M x N tiles. The upper-left corner of the upper-left tile in the grid is at offset (X, Y) in pixels. Each tile is WxH pixels in size. In addition, each tile in a row is DX pixels from upper-left corner to upper-left corner (so that DX >= W), and each column is DY pixels below the one above it (DY >= H).
In this picture, M=3 and N=2
Give all these variables, could someone please tell me what command to use to extract the six tiles into their own PNG files? I'm assuming it's using the 'convert' command.
Thanks.

Answer 1

There is no simple crop command in Imagemagick that will do the offset tile cropping. But you can do that with a combination of 3 crop commands.
Crop the larger area that you want from the image
Tile crop with no skip
Crop each tile to remove the skip area
Input:
Here I make an animation simply for demonstration. Use PNG output or JPG output for separate tiles.
convert -delay 50 lena.png \
-crop 200x200+20+20 +repage \
-crop 100x100 +repage \
-crop 90x90+0+0 +repage \
-loop 0 lena_crop.gif

Answer 2

I was able to get this working using the following script:
#!/bin/bash
COLS=8
ROWS=4
WIDTH=263
HEIGHT=500
XOFF=154
YOFF=176
DX=267
DY=523
IMAGE=image
mkdir -p ${IMAGE}
for M in `seq 0 $(($COLS - 1))`
do
for N in `seq 0 $(($ROWS - 1))`
do
X=$(($XOFF + $M * $DX))
Y=$(($YOFF + $N * $DY))
convert ${IMAGE}.png[${WIDTH}x${HEIGHT}+${X}+${Y}] ${IMAGE}/${IMAGE}-${M}-${N}.png
done
done
I'm not sure how elegant this is.