在浏览器中调整ImageData大小的快速方法? [英] Fast way to resize ImageData in browser?

问题描述

我有一个600 x 400像素的画布，它返回一个数据长度为960,000(600 * 400 * 4)的ImageData.有什么办法可以缩小宽度和宽度吗?高度说10倍，我想得到一个ImageData，其数据长度为9600(60 * 40 * 4).

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}

我需要生成的ImageData.data数组以进行进一步的操作. 该方法将被循环调用，因此如果速度快的话会很好.

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.

修改 这是建议的方法，我不确定它是否正确:

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)

编辑2 似乎没有用，没有调整小画布的大小，而只是裁切了 https://codepen.io/bobiblazeski/full/drrQoB

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

推荐答案

经过一番摆弄之后，我发现了使用此以回答为起点.基本上，您可以将较大的画布绘制为较小的画布，然后从中获取ImageData.

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);

这是一个 codepen ，以证明检索到的图像数据是按比例缩小的版本 原始画布，而不仅仅是从中裁剪出来.

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.

如果要在循环中调整大小，请在调用drawImage codepen 之前清除目标画布.

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);
});

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