javascript/gwt: converting Uint8Array or ArrayBuffer to ImageData element

It would go something like this in pure javascript:

var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");    
var imgData = ctx.createImageData(100,100);

var buffer = new ArrayBuffer(4*100*100);
var ubuf = new Uint8Array(buffer);
for (var i=0;i < ubuf.length; i+=4) {
    imgData.data[i]   = ubuf[i];   //red
    imgData.data[i+1] = ubuf[i+1]; //green
    imgData.data[i+2] = ubuf[i+2]; //blue
    imgData.data[i+3] = ubuf[i+3]; //alpha
}

ctx.putImageData(imgData,0,0);

To do this in GWT you need to wrap this in a JSNI method

2021 answer

The ImageData constructor now supports an optional Uint8ClampedArray parameter in most browsers, so no need to copy the values in a for() loop as rzymek suggested back in 2013.

So based on his example:

const buffer = ArrayBuffer(4 * 100 * 100)
const ui8ca = new Uint8ClampedArray(buffer);
const imageData = new ImageData(ui8ca, 100, 100);

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
ctx.putImageData(imageData, 0, 0);

Works in all modern browsers except Internet Explorer and Android Webview.

See https://developer.mozilla.org/en-US/docs/Web/API/ImageData/ImageData