How to square an image and pad with transparency from the commandline (imagemagick)

Question

The section entitled Square Padding or Cropping describes a method to generate a square image--for a file whose dimensions are unknown--and pad the background with a color.

How do I perform the same operation, but create a transparent background.

Answer 1

Let's make a red off-square image first, that is 300x200:

convert -size 300x200 xc:red image.png

Now let's put make a square image of it, but using a yellow background so you can see it:

convert -background yellow -gravity center image.png -resize 400x400 -extent 400x400 result.png

Now we can do the same thing again, but make the background transparent:

convert -background none -gravity center image.png -resize 400x400 -extent 400x400 result.png

and, just check to make sure it has worked:

identify result.png
result.png PNG 400x400 400x400+0+0 8-bit sRGB 418B 0.000u 0:00.000

Answer 2

These modified methods from Anthony's examples both work for me:

  convert thumbnail.gif \
      \( +clone -rotate 90 +clone -mosaic +level-colors grey -transparent grey \) \
      +swap -gravity center -composite    square_padded.gif

convert thumbnail.gif  -virtual-pixel none -set option:distort:viewport \
 "%[fx:max(w,h)]x%[fx:max(w,h)]-%[fx:max((h-w)/2,0)]-%[fx:max((w-h)/2,0)]" \
 -filter point -distort SRT 0  +repage  square_external.gif