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

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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 40×40 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.

Best Answer

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

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Using GraphicsMagick or ImageMagick, how to replace transparency with a fill colour

Using GraphicsMagick, -background color -extent 0x0 should be able to replace the first two steps in your process (for ImageMagick, it'd be -mosaic instead of -extent). I'm not sure about the third step, but it sounds like you're just running +matte on the image from the first two steps, so the final call would look something like this:

gm convert -background color -extent 0x0 +matte src.png dst.png

or for ImageMagick,

convert src.png -background color -mosaic +matte dst.png

Linux – Convert grayscale image into alpha channel image in unix shell

Ooookay. After plenty of googling, and reading for more of the ImageMagick manual than I actually care for, here's the answer. Given that you have a grayscale image called source.png, here are my commands.

To make the make the black pixels transparent and keeps the white pixels as they are, run this command:

 convert source.png -alpha copy -fx '#fff' result.png

To instead make the white pixels transparent while keeping the black as-is, use:

 convert source.png -alpha copy -channel alpha -negate +channel -fx '#000' result.png

Let's explain that last command a bit more thoroughly:

convert – Is the ImageMagic command (one of several)
source.png – The greyscale source image.
-alpha copy – Copy contents of the previous file into the alpha channel.
-channel alpha – Specify that following operators only should affect the alpha channel.
-negate – Invert the alpha channel (will, because of the previous -chanel alpha not affect any other part of the image).
+channel – Specify that following operators only should should affect the color channels, and no longer modify the alpha channel. (This is the default, and therefore we need not provide it in the first, simpler example.)
-fx '#000' – Replace color channel contents with black pixels. (Because of +channel the alpha channel will not be affected).

It is quite important to include that final -fx option, otherwise all semi-transparent pixels in generated image will retain colors. (Since these pixels are half-transparent, it might not be obvious, but the end result is not what one expect.)

I found the list of ImageMagick options quite useful.

Best Answer

Related Solutions

Using GraphicsMagick or ImageMagick, how to replace transparency with a fill colour

Linux – Convert grayscale image into alpha channel image in unix shell

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