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Imagemagick convert to name tiles as row/column doesn't work as expected with -extent

I have an image, 5120  ×  4352 that I crop into 2048x2048 tiles. I want to name my cropped tiles like

  • tile_0_0.png
  • tile_0_1.png
  • tile_0_2.png
  • tile_1_0.png
  • tile_1_1.png
  • tile_1_2.png
  • ...

But this command:

convert image.png -crop 2048x2048 -gravity northwest \ 
-extent 2048x2048 -transparent white  \ 
-set 'filename:tile' '%[fx:page.x/2048]_%[fx:page.y/2048]'  \ 
+repage +adjoin 'tile_%[filename:tile].png'

Gives me this result:

  • tile_0_0.png
  • tile_0_1.png
  • tile_0_16.png
  • tile_1_0.png
  • tile_1_1.png
  • tile_1_16.png
  • tile_4_0.png
  • tile_4_1.png
  • tile_4_16.png

I suspect it has do with the tiles on the last row and column aren't fully 2048x2048, but the extent command makes the end result still 2048, but how can I use this with tiles and file names?

My current workaround is to first resize the original image like this, and then run the above command:

convert image.png -gravity northwest \
-extent 2048x2048 -transparent white bigger.png

But it would be nice to do it in one swoop :)

Solution

  • Using ImageMagick you could set a viewport that is just enough larger than the input image so it divides evenly by 2048. Then a no-op distort will enlarge the viewport to that size. That way the "-crop 2048x2048" will create pieces that are already 2048 square.

    Here's a sample command I worked up in Windows, and I'm pretty sure I translated it to work correctly as a *nix command.

    convert image.png \
   -set option:distort:viewport '%[fx:w-(w%2048)+2048]x%[fx:h-(h%2048)+2048]' \
   -virtual-pixel none -distort SRT 0 +repage -crop 2048x2048 \
   -set 'filename:tile' '%[fx:page.x/2048]_%[fx:page.y/2048]' \
   +repage +adjoin 'tile_%[filename:tile].png'

    The "-distort SRT" operation does nothing except expand the viewport to dimensions that divide evenly by 2048, with a result just like doing an "-extent" before the crop. And "-virtual-pixel none" will leave a transparent background in the overflow areas.

    Edited to add: The formula for extending the viewport in the above command will incorrectly add another 2048 pixels even if the dimension is already divisible by 2048. It also gives an incorrect result if the dimension is less than 2048. Consider using a formula like this for setting the viewport to handle those conditions...

    '%[fx:w+(w%2048?2048-w%2048:0)]x%[fx:h+(h%2048?2048-h%2048:0)]'