JSFiddle - React, Tailwind, and code Playground

by bgmort

HTML

<canvas id="canvas" height="256" width="256"></canvas>

JavaScript

var canvas = document.getElementById('canvas');
var canvasWidth  = canvas.width;
var canvasHeight = canvas.height;
var ctx = canvas.getContext('2d');
var imageData = ctx.getImageData(0, 0, canvasWidth, canvasHeight);
    
var buf = new ArrayBuffer(imageData.data.length);
var buf8 = new Uint8ClampedArray(buf);
var data = new Uint32Array(buf);
    
// Determine whether Uint32 is little- or big-endian.
data[1] = 0x0a0b0c0d;
    
var isLittleEndian = true;
if (buf[4] === 0x0a && buf[5] === 0x0b && buf[6] === 0x0c &&
    buf[7] === 0x0d) {
    isLittleEndian = false;
}

/* isLittleEndian = !isLittleEndian; */

if (isLittleEndian) {
    for (var y = 0; y < canvasHeight; ++y) {
        for (var x = 0; x < canvasWidth; ++x) {
            var value = x * y & 0xff;

            data[y * canvasWidth + x] =
                (255   << 24) |    // alpha
                (value << 16) |    // blue
                (value <<  8) |    // green
                 value;            // red
        }
    }
} else {
    for (y = 0; y < canvasHeight; ++y) {
        for (x = 0; x < canvasWidth; ++x) {
            value = x * y & 0xff;

            data[y * canvasWidth + x] =
                (value << 24) |    // red
                (value << 16) |    // green
                (value <<  8) |    // blue
                 255;              // alpha
        }
    }
}

imageData.data.set(buf8);

ctx.putImageData(imageData, 0, 0);