Check for transparency, GraphicsMagick node.js - node.js

Question

I am making a code where users can upload a image. The image is converted with GraphicsMagick and uploaded to our cloud. But it will be best if non-transparent images was converted to JPG instead of PNG for transparent images. How can I check if the image contain a alpha channel in GraphicsMagick?

Answer 1

I am not sure you can achieve that using only GraphicsMagick, but it is possible in several other ways. For example with pngjs:
You can check PNG metadata:
const gm = require('gm');
const PNG = require('pngjs').PNG;
gm('/path/to/image')
.stream('png')
.pipe(new PNG({}))
.on('metadata', meta => {
if (meta.alpha) {
// image is transparent
} else {
// image is not transparent
}
});
Or iterate over pixels and decide if it's transparency valuable to you, or you can omit it:
...
.on('parsed', function() {
let isAlphaValuable = false;
for (var y = 0; y < this.height; y++) {
for (var x = 0; x < this.width; x++) {
var idx = (this.width * y + x) << 2;
// this.data[idx] - red channel
// this.data[idx + 1] - green channel
// this.data[idx + 2] - blue channel
// this.data[idx + 3] - alpha channel
// if there is at least one pixel
// which transparent for more than 30%
// then transparency valuable to us
isAlphaValuable |= (1 - this.data[idx + 3] / 255) > 0.3;
}
}
if (isAlphaValuable) {
// keep transparency
} else {
// ignore transparency
}
});

Answer 2

You may also give imagemagick a try
Snippet is in TypeScript and makes use of BPromise.promisify for better readability.
Note that this works for PNG, JPEG the expected way (returning string true/false), but for GIFs it will give you a concatenated 'true'|'false' string (e.g. 'truetruefalse', and apply the alpha check per frame).
I also recommend applying .trim() to the result to get rid of potential useless whitespace returned by imagemagick v0.x. every now and then.
import * as imagemagick from 'imagemagick';
import * as BPromise from 'bluebird';
...
const opaqueAsync: any = BPromise.promisify(imagemagick.identify, {context: imagemagick});
const isOpaqueReturnValue: string = await opaqueAsync(['-format', '%[opaque]', picturePath]);
const isPicTransparent: boolean = 'false' === isOpaqueReturnValue.trim();