我想使用滑块对大型图像进行客户端实时交互式阈值处理。是否可以在javascript中对图像进行阈值处理以生成二进制图像，而无需在所有像素上使用for循环？如果是这样，速度会更快吗？
这可以在两个阶段中仅使用globalCompositeOperations完成。
定义三个画布，一个在屏幕上，一个在屏幕外保留灰度图像，另一个在屏幕外“工作”画布。
//--- on-screen canvas
var onScreenCanvas=document.getElementById("canvasTest");
var ctxOnScreen=onScreenCanvas.getContext("2d");
//--- off-screen working canvas
var drawingCanvas = document.createElement('canvas');
var ctx=drawingCanvas.getContext("2d");
//--- off-screen canvas to store the greyscale image
var greyscaleImageCanvas = document.createElement('canvas');
var ctxGreyscaleImage=greyscaleImageCanvas.getContext("2d");
将灰度图像加载到greyscaleImageCanvas上，然后以下两个操作实现了步骤1，其中thresh_str是每个RGB的阈值在0-FF之间的十六进制字符串
//(1a) Threshold the image on the offscreen working canvas,
// reducing values above threshold to have threshold value
ctx.drawImage(greyscaleImageCanvas, 0, 0);
ctx.globalCompositeOperation='darken';
ctx.fillStyle=thresh_str;
ctx.fillRect(0,0, drawingCanvas.width, drawingCanvas.height);
//(1b) Set everything *below* threshold to 0 (black) since that part is unchanged
// from the original image. Pixels above threshold are all non-zero.
ctx.globalCompositeOperation='difference';
ctx.drawImage(greyscaleImageCanvas, 0, 0);
没有为HTML globalCompositeOperations定义直接的“除法”操作，但是有一个“颜色减淡”，它将底色层除以倒置的顶层。因此，通过首先对步骤1的输出进行反向复制，然后使用减色操作（确实定义了0/0 = 0）在除法之前对它进行“非反向”处理，可以实现所需的结果。结果是非零（阈值以上）像素变为1，零（亚阈值）像素保持为零。
//(2a) Copy the result of (1b) to the onscreen canvas
ctxOnScreen.globalCompositeOperation='copy';
ctxOnScreen.drawImage(drawingCanvas, 0, 0);
//(2b) Invert the result of step (1b) so that it can be 'un-inverted' by color dodge
ctx.globalCompositeOperation='difference';
ctx.fillStyle='white';
ctx.fillRect(0,0,onScreenCanvas.width,onScreenCanvas.height);
//(2c) 'color-dodge' the results of (1b) with it's own inverse (2b)
ctxOnScreen.globalCompositeOperation='color-dodge';
ctxOnScreen.drawImage(drawingCanvas, 0, 0);
[这种方法似乎比for循环快3-5倍，至少在Mac和android（Huawei P10）JSPerf上的Chrome 79上如此
function img2grey(canvasContext) {
canvasContext.globalCompositeOperation='color';
canvasContext.fillStyle='white';
canvasContext.fillRect(0,0,onScreenCanvas.width,onScreenCanvas.height);
}
//--- on-screen canvas
var onScreenCanvas=document.getElementById("canvasTest");
var ctxOnScreen=onScreenCanvas.getContext("2d");
//--- off-screen working canvas
var drawingCanvas = document.createElement('canvas');
var ctx=drawingCanvas.getContext("2d");
//--- off-screen canvas to store the greyscale image
var greyscaleImageCanvas = document.createElement('canvas');
var ctxGreyscaleImage=greyscaleImageCanvas.getContext("2d");
var image = new Image();
function thresholdImage(thresh_val) {
if(thresh_val.length == 1){
thresh_val = '0' + thresh_val;
}
thresh_str = '#'+thresh_val+thresh_val+thresh_val;
ctxOnScreen.clearRect(0, 0, onScreenCanvas.width, onScreenCanvas.height);
ctx.clearRect(0, 0, drawingCanvas.width, drawingCanvas.height);
//----- (1) Threshold the image on the offscreen working canvas,
// reducing values above threshold to have threshold value
ctx.drawImage(greyscaleImageCanvas, 0, 0);
ctx.globalCompositeOperation='darken';
ctx.fillStyle=thresh_str;
ctx.fillRect(0,0,onScreenCanvas.width,onScreenCanvas.height);
//----- (2) Set everything *below* threshold to 0 (black) since that part is unchanged
// from the original image
ctx.globalCompositeOperation='difference';
ctx.drawImage(greyscaleImageCanvas, 0, 0);
//----- (3) Copy the result to the onscreen canvas
ctxOnScreen.globalCompositeOperation='copy';
ctxOnScreen.drawImage(drawingCanvas, 0, 0);
//----- (4) Invert the result of step (2) so that it can be 'un-inverted' by color dodge
ctx.globalCompositeOperation='difference';
ctx.fillStyle='white';
ctx.fillRect(0,0,onScreenCanvas.width,onScreenCanvas.height);
//----- (5) 'color-dodge' the results of (2) with it's own inverse (4)
//----- This makes use of 0/0 defined as 0 in this globalCompositeOperation,
//----- so that non-zero (suprathreshold) pixels become 1, zero (sub-threshold) pixels stay zero
//~ ctxOnScreen.globalCompositeOperation='color-dodge';
ctxOnScreen.globalCompositeOperation='color-dodge';
ctxOnScreen.drawImage(drawingCanvas, 0, 0);
}
image.onload = function() {
onScreenCanvas.width = image.width;
onScreenCanvas.height = image.height;
drawingCanvas.width = image.width;
drawingCanvas.height = image.height;
greyscaleImageCanvas.width = image.width;
greyscaleImageCanvas.height = image.height;
//!!NB Doesn't work on chrome for local files, use firefox
// https://stackoverflow.com/questions/45444097/the-canvas-has-been-tainted-by-cross-origin-data-local-image
ctxGreyscaleImage.drawImage(image, 0, 0);
img2grey(ctxGreyscaleImage);
thresholdImage((Math.round(rng.value)).toString(16));
};
var rng = document.querySelector("input");
var listener = function() {
window.requestAnimationFrame(function() {
thresholdImage( (Math.round(rng.value)).toString(16) );
});
};
rng.addEventListener("mousedown", function() {
listener();
rng.addEventListener("mousemove", listener);
});
rng.addEventListener("mouseup", function() {
rng.removeEventListener("mousemove", listener);
});
image.src = "https://i.imgur.com/vN0NbVu.jpg";.slider-width100 {
width: 255px;
}<html>
<head>
</head>
<body>
<canvas id="canvasTest"></canvas>
<input class="slider-width100" type="range" min="0" max="254" value="122" />
</body>
</html>