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How to write an ArrayBuffer to a canvas

I’m playing with WebGPU. But it’s currently very bleeding-edge, and Chrome hasn’t implemented direct GPU integration with canvas yet. As a workaround to display stuff, WebGPU lets you read data as an ArrayBuffer, and we can write that ArrayBuffer to a canvas using the traditional 2d context. Here’s a canvas which I’ve written an ArrayBuffer to:

The ArrayBuffer is interpreted as pixel data as follows. Each pixel is four bytes: red, green, blue and alpha, in that order. Pixels are in “reading” order: top-to-bottom, left-to-right. The ArrayBuffer contains no image dimension metadata, or any other metadata. We provide these dimensions by wrapping the ArrayBuffer with an ImageData. We can then pass that to ctx.putImageData. To demonstrate, here’s the JavaScript that generated the canvas above:

const ctx = canvas.getContext('2d');

const WIDTH = 256;
const HEIGHT = 256;

const arrayBuffer = new ArrayBuffer(WIDTH * HEIGHT * 4);
const pixels = new Uint8ClampedArray(arrayBuffer);
for (let y = 0; y < HEIGHT; y++) {
  for (let x = 0; x < WIDTH; x++) {
    const i = (y*WIDTH + x) * 4;
    pixels[i  ] = x;   // red
    pixels[i+1] = y;   // green
    pixels[i+2] = 0;   // blue
    pixels[i+3] = 255; // alpha

const imageData = new ImageData(pixels, WIDTH, HEIGHT);
ctx.putImageData(imageData, 0, 0);

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Tagged #programming, #javascript, #web. All content copyright James Fisher 2020. This post is not associated with my employer. Found an error? Edit this page.