How do I change the canvas size of a PNG with ImageMagick (GraphicsMagick)? (How to pad with transparency?)

Alternatively: How do I take a non-square PNG and "fill out" the "rest" of the image with transparency so that the resulting square image has the original image centered in the square?

ULTIMATELY, what I want is to take any image of any GM-supported format of any size, and create a scaled-down PNG (say, 40 pixels maximum for either dimension), with aspect ratio maintained, transparency-padded for non-square original images, AND with an already-prepared 40x40 PNG transparency mask applied.

I already know how to scale down and keep aspect ratio; I already have the command for applying my composite. My only missing piece is square-alizing non-square images (padding with transparency).

Single command preferred; multi-command chain acceptable.

(edit)

Extra info: Here's the composite command I'm using:

gm composite -compose copyopacity mask.png source-and-target.png source-and-target.png

where mask.png has white pixels for what I want to keep of source-and-target.png and transparent pixels for what I want to remove (and become transparent) of source-and-target.png.

Pistos

Posted 2012-04-23T22:44:44.280

Reputation: 337

Answers

This command will take any sized input file and fit it best to a 40x40 square and pad with transparency:

convert \
   original.png \
  -thumbnail '40x40>' \
  -background transparent \
  -gravity center \
  -extent 40x40 \
  -compose Copy_Opacity \
  -composite mask.png \
   original-resized.png

The gravity option ensures the image is centered in both directions, and transparent is used wherever there are no pixels. Then the compositing is done with the mask.png

Paul

Posted 2012-04-23T22:44:44.280

Reputation: 52 173

So we can't do it in one shot with composite? – Pistos – 2012-04-24T01:23:38.193

@Pistos I am not clear on what you are doing with the composite - it is just a transparency mask to hide/show portions of the thumbnail after resizing? Can you [edit] and add the command sequence you have so far, so we can see if it can be combined? – Paul – 2012-04-24T02:47:27.370

@Pistos I think the best approach is to do the compositing with -convert rather than the other way around. I have updated my answer above. – Paul – 2012-04-24T03:33:29.480

Close, but not quite. Your command didn't work as-is, and even with adjustments, it didn't work. It's okay, I'll do it in two steps. I'll update things here after I get the final, working CLI steps. Thanks again. – Pistos – 2012-04-24T03:53:16.660

I would add -filter Catrom before the thumbnail option because from my experience it generally produces higher quality images. – thdoan – 2013-12-04T02:40:37.870

One command to convert all PNGs from one folder:

mogrify \
 -resize 50x50 \
 -background transparent \
 -gravity center \
 -extent 50x50 \
 -format png \
 -path resized \
 *.png

mogrify is a command from ImageMagick package. You have to create output directory first.

uı6ʎɹnɯ ꞁəıuɐp

Posted 2012-04-23T22:44:44.280

Reputation: 151

Nice one. BTW, If you feel adventurous, you can omit -path resized to edit the files in place. – aaronk6 – 2016-05-19T10:55:04.727

Here's what I eventually went with. A two step process:

gm convert \
  -thumbnail '40x40>' \
  -background transparent \
  -gravity center \
  -extent 40x40 \
   original.png \
   intermediate.png

gm composite \
  -compose in \
   intermediate.png \
   mask.png \
   out.png

Where mask.png is white pixels for what I wanted to keep, and transparent pixels for what I wanted to mask out (discard).

Pistos

Posted 2012-04-23T22:44:44.280

Reputation: 337