Fast way to resize ImageData in browser?

Question

I have a canvas of 600 x 400 pixels, which returns an ImageData with data length of 960,000 (600*400*4). Is there any way to downscale both width & height say 10 times, I would like to get as a result an ImageData whose data length is 9600 (60*40*4).

const canvas = document.getElementsByTagName("canvas")[0]
var ctx = canvas.getContext("2d");
const origImageData = ctx.getImageData(0, 0, canvas.width, canvas.height)
origImageData
// => ImageData {data: Uint8ClampedArray(960000), width: 600, height: 400}
const smallImageData = downscale(origImageData, 60, 40);
smallImageData
// => ImageData {data: Uint8ClampedArray(9600), width: 60, height: 40}

I need resulting ImageData.data array for further manipulation. This method would be called in a loop so it would be good if its fast.

Edit This is the suggested approach, which I'm not sure it's correct:

var canvas2 = document.createElement('canvas');
canvas2.width = canvas.width/10;
canvas2.height = canvas.height/10;
var ctx2 = canvas2.getContext('2d');
// Step that I found confusing
// Is the new image data being created?
ctx2.putImageData(origImageData, 0, 0);
// Which image data I'm getting here resized or part of original ?
ctx2.getImageData(0,0,  canvas2.width, canvas2.height)

Edit 2 It doesn't seem to be working, small canvas isn't resized, but only a cropped https://codepen.io/bobiblazeski/full/drrQoB

Have you done any research to try to find a solution yourself? — Sean, Mar 25 '19 at 15:13
Just use the resulted imagedata and paint it on a smaller canvas? https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/putImageData — iacobalin, Mar 25 '19 at 15:13
Isn't there a direct transformation without creating extra elements? — slobodan.blazeski, Mar 25 '19 at 15:16
@sean The solution usually suggest using off screen canvas, and I'm not sure if I'm getting the new ImageData or portion of the old image data. — slobodan.blazeski, Mar 25 '19 at 15:34

slobodan.blazeski · Accepted Answer · 2019-03-27T08:58:33.880

After some fiddling around I found a solution using this answer as a starting point. Basically you could just draw bigger canvas into a smaller canvas, and then get ImageData from it.

const bigCanvas = document.getElementById("big");
const bigContext = bigCanvas.getContext("2d");        
const smallContext = document.getElementById("small").getContext("2d");         
smallContext.scale(0.5, 0.5);
smallContext.drawImage(bigCanvas, 0, 0);        
const smallImageData = smallContext.getImageData(0, 0, bigCanvas.width, bigCanvas.height);

Here's a codepen as a proof that retrieved image data is a scaled down version of the original canvas, not just a crop from it.

If you want to resize in loop, clear the destination canvas before calling drawImage codepen.

const sleep = (ms) => new Promise(resolve => setTimeout(resolve, ms));

const bigCanvas = document.getElementById("big");
const bigContext = bigCanvas.getContext("2d");
const smallCanvas = document.getElementById("small");
const smallContext = smallCanvas.getContext("2d"); 
const otherCanvas = document.getElementById("other");
const otherContext = otherCanvas.getContext("2d");

function getImage(i) {
    bigContext.clearRect(0, 0, bigCanvas.width, bigCanvas.height);
    bigContext.fillRect(((i+0)%5)*100,   0, 100, 100);
    bigContext.fillRect(((i+1)%5)*100, 100, 100, 100);
    bigContext.fillRect(((i+2)%5)*100, 200, 100, 100);    
    bigContext.fillRect(((i+3)%5)*100, 100, 100, 100);
    bigContext.fillRect(((i+4)%5)*100,   0, 100, 100);
    bigContext.fillRect(((i+0)%5)*100, 200, 100, 100);    

    smallContext.clearRect(0, 0, smallCanvas.width, smallCanvas.height);
    smallContext.drawImage(bigCanvas, 0, 0, smallCanvas.width, smallCanvas.height);
    const smallImageData = smallContext.getImageData(0,0, 
        bigCanvas.width, bigCanvas.height);

    otherContext.putImageData(smallImageData, 0, 0);
};

window.addEventListener("DOMContentLoaded", function() {
    var i = 0;
    setInterval(() => {
        console.log(i);
        getImage(i++);
    }, 3000);
});

Haskely · Answer 2 · 2020-11-23T05:37:15.667

try this but it's slow...

/**
 * 
 * @param {ImageData} originalImageData 
 * @param {Int32} targetWidth 
 * @param {Int32} targetHeight 
 */
function scaleImageData(originalImageData, targetWidth, targetHeight) {
    const targetImageData = new ImageData(targetWidth, targetHeight);
    const h1 = originalImageData.height;
    const w1 = originalImageData.width;
    const h2 = targetImageData.height;
    const w2 = targetImageData.width;
    const kh = h1 / h2;
    const kw = w1 / w2;
    const cur_img1pixel_sum = new Int32Array(4);
    for (let i2 = 0; i2 < h2; i2 += 1) {
        for (let j2 = 0; j2 < w2; j2 += 1) {
            for (let i in cur_img1pixel_sum) cur_img1pixel_sum[i] = 0;
            let cur_img1pixel_n = 0;
            for (let i1 = Math.ceil(i2 * kh); i1 < (i2 + 1) * kh; i1 += 1) {
                for (let j1 = Math.ceil(j2 * kw); j1 < (j2 + 1) * kw; j1 += 1) {
                    const cur_p1 = (i1 * w1 + j1) * 4;
                    for (let k = 0; k < 4; k += 1) {
                        cur_img1pixel_sum[k] += originalImageData.data[cur_p1 + k];
                    };
                    cur_img1pixel_n += 1;
                };
            };
            const cur_p2 = (i2 * w2 + j2) * 4;
            for (let k = 0; k < 4; k += 1) {
                targetImageData.data[cur_p2 + k] = cur_img1pixel_sum[k] / cur_img1pixel_n;
            };
        };
    };
    return targetImageData;
};

Fast way to resize ImageData in browser?

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