为什么在 canvas 中使用 Chrome 为 Android 调整大小时图像变得很暗？

Question

我正在使用 this fiddle 重现我的问题。使用版本 57 中的 Chrome 作为 Android，调整大小后的图像变得非常暗。如果您足够大地调整图像大小（从 phone 到 phone），黑暗会消失并且图像会正确调整大小。

任何人都知道是什么导致了这种奇怪的行为？我在 Chromium 中有 filed a bug，但我不希望它很快得到解决。我需要尽快修复它，因为我在生产中使用这段代码并且它导致了重大问题。

所以要我放一些代码，因为我已经将 link 用于 jsfiddle，所以这是导致此问题的代码的一部分，我使用 HTML5 [调整图像大小] =24=]:

/**
     * Hermite resize - fast image resize/resample using Hermite filter. 1 cpu version!
     * 
     * @param {HtmlElement} canvas
     * @param {int} width
     * @param {int} height
     * @param {boolean} resize_canvas if true, canvas will be resized. Optional.
     */

this.resample_single = function (canvas, width, height, resize_canvas) {
    var width_source = canvas.width;
    var height_source = canvas.height;
    width = Math.round(width);
    height = Math.round(height);

    var ratio_w = width_source / width;
    var ratio_h = height_source / height;
    var ratio_w_half = Math.ceil(ratio_w / 2);
    var ratio_h_half = Math.ceil(ratio_h / 2);

    var ctx = canvas.getContext("2d");
    var img = ctx.getImageData(0, 0, width_source, height_source);
    var img2 = ctx.createImageData(width, height);
    var data = img.data;
    var data2 = img2.data;

    for (var j = 0; j < height; j++) {
        for (var i = 0; i < width; i++) {
            var x2 = (i + j * width) * 4;
            var weight = 0;
            var weights = 0;
            var weights_alpha = 0;
            var gx_r = 0;
            var gx_g = 0;
            var gx_b = 0;
            var gx_a = 0;
            var center_y = (j + 0.5) * ratio_h;
            var yy_start = Math.floor(j * ratio_h);
            var yy_stop = Math.ceil((j + 1) * ratio_h);
            for (var yy = yy_start; yy < yy_stop; yy++) {
                var dy = Math.abs(center_y - (yy + 0.5)) / ratio_h_half;
                var center_x = (i + 0.5) * ratio_w;
                var w0 = dy * dy; //pre-calc part of w
                var xx_start = Math.floor(i * ratio_w);
                var xx_stop = Math.ceil((i + 1) * ratio_w);
                for (var xx = xx_start; xx < xx_stop; xx++) {
                    var dx = Math.abs(center_x - (xx + 0.5)) / ratio_w_half;
                    var w = Math.sqrt(w0 + dx * dx);
                    if (w >= 1) {
                        //pixel too far
                        continue;
                    }
                    //hermite filter
                    weight = 2 * w * w * w - 3 * w * w + 1;
                    var pos_x = 4 * (xx + yy * width_source);
                    //alpha
                    gx_a += weight * data[pos_x + 3];
                    weights_alpha += weight;
                    //colors
                    if (data[pos_x + 3] < 255)
                        weight = weight * data[pos_x + 3] / 250;
                    gx_r += weight * data[pos_x];
                    gx_g += weight * data[pos_x + 1];
                    gx_b += weight * data[pos_x + 2];
                    weights += weight;
                }
            }
            data2[x2] = gx_r / weights;
            data2[x2 + 1] = gx_g / weights;
            data2[x2 + 2] = gx_b / weights;
            data2[x2 + 3] = gx_a / weights_alpha;
        }
    }
    //clear and resize canvas
    if (resize_canvas === true) {
        canvas.width = width;
        canvas.height = height;
    } else {
        ctx.clearRect(0, 0, width_source, height_source);
    }

    //draw
    ctx.putImageData(img2, 0, 0);
};

Answer 1

看来，这里已经提出了同样的问题：

我的错误报告已合并到这个铬问题中：https://bugs.chromium.org/p/chromium/issues/detail?id=713632#c24

为什么在 canvas 中使用 Chrome 为 Android 调整大小时图像变得很暗？

Why image gets very dark when resized in canvas using Chrome for Android?

javascript

android

google-chrome

html5-canvas

image-resizing