writing about things, sometimes.

CSS sprites with awk and ImageMagick

Written by Omar Polo on 08 August 2019.

CSS sprites are a nice idea, stolen from the game developers I think, that consist to merge all the images you need in a single one and download only that. I will not prove that it's best than downloading n images, but it should clear that downloading a file, albeit a bit bigger, requires less TCP segments going back and forth.

The other day I needed a CSS sprite. Being the lazy person I am, right after The GIMP was open I thought that I didn't want to do the math.

After a bit of searching, I found a post where the author wrote a script to generate the sprites (and the relative css file) with image magick. I have lost that link, and the history of my browser doesn't help. I remember that the domain name contains "php", but nothing more. However, I rewritten the script, so it better fits my needs.

The idea is to join all the image using convert(1) and then generate a CSS file with the offsets using identify(1). Both programs should come along with imagemagick.

Here's the script


convert sprites/* -append img/sprites.png

for i in sprites/*; do
	identify -format '%W %H %f\n' $i
done | awk '
function fname(path) {
	sub (".png", "", path)
	return path

	y = 0

	print "%sprite-base {"
	print "	background-image: url(/img/sprites.png);"
	print "	background-repeat: no-repeat;"
	print "}"
	print ""

	width = $1
	height = $2
	class = fname($3)

	print "%sprite-" class " {"
	print "	@extend %sprite-base;"
	if (y == 0)
		print "	background-position: 0 0;"
		print "	background-position: 0 -" y "px;"
	print "}"
	print ""

	y += height;
' > scss/_sprites.scss

Assuming that the images are within sprites/ and all of them are png files, this script will generate the sprite in img/sprites.png and a SASS file in sass/_sprits.scss with one placeholder for every image (the naming scheme is %sprite-$NAMEFILE with the $NAMEFILE being the file name without extension).

It should be trivial to edit to handle pure CSS output, and eventually other image formats.