如何在每次画布变换后实现 Lanczos 重采样，而无需制作新画布？

Question

更新：一旦我让这个演示工作......天哪，它的速度很慢，比如 12-16 秒仅用于 2 级渲染（当图像约为 1000x2000 像素时）。这根本不值得操心。

我在这里的最佳答案中发现了这个非常棒且充满希望的代码：Resizing an image in an HTML5 canvas

//returns a function that calculates lanczos weight
function lanczosCreate(lobes){
  return function(x){
    if (x > lobes) 
      return 0;
    x *= Math.PI;
    if (Math.abs(x) < 1e-16) 
      return 1
    var xx = x / lobes;
    return Math.sin(x) * Math.sin(xx) / x / xx;
  }
}

//elem: canvas element, img: image element, sx: scaled width, lobes: kernel radius
function thumbnailer(elem, img, sx, lobes){ 
    this.canvas = elem;
    elem.width = img.width;
    elem.height = img.height;
    elem.style.display = "none";
    this.ctx = elem.getContext("2d");
    this.ctx.drawImage(img, 0, 0);
    this.img = img;
    this.src = this.ctx.getImageData(0, 0, img.width, img.height);
    this.dest = {
        width: sx,
        height: Math.round(img.height * sx / img.width),
    };
    this.dest.data = new Array(this.dest.width * this.dest.height * 3);
    this.lanczos = lanczosCreate(lobes);
    this.ratio = img.width / sx;
    this.rcp_ratio = 2 / this.ratio;
    this.range2 = Math.ceil(this.ratio * lobes / 2);
    this.cacheLanc = {};
    this.center = {};
    this.icenter = {};
    setTimeout(this.process1, 0, this, 0);
}

thumbnailer.prototype.process1 = function(self, u){
    self.center.x = (u + 0.5) * self.ratio;
    self.icenter.x = Math.floor(self.center.x);
    for (var v = 0; v < self.dest.height; v++) {
        self.center.y = (v + 0.5) * self.ratio;
        self.icenter.y = Math.floor(self.center.y);
        var a, r, g, b;
        a = r = g = b = 0;
        for (var i = self.icenter.x - self.range2; i <= self.icenter.x + self.range2; i++) {
            if (i < 0 || i >= self.src.width) 
                continue;
            var f_x = Math.floor(1000 * Math.abs(i - self.center.x));
            if (!self.cacheLanc[f_x]) 
                self.cacheLanc[f_x] = {};
            for (var j = self.icenter.y - self.range2; j <= self.icenter.y + self.range2; j++) {
                if (j < 0 || j >= self.src.height) 
                    continue;
                var f_y = Math.floor(1000 * Math.abs(j - self.center.y));
                if (self.cacheLanc[f_x][f_y] == undefined) 
                    self.cacheLanc[f_x][f_y] = self.lanczos(Math.sqrt(Math.pow(f_x * self.rcp_ratio, 2) + Math.pow(f_y * self.rcp_ratio, 2)) / 1000);
                weight = self.cacheLanc[f_x][f_y];
                if (weight > 0) {
                    var idx = (j * self.src.width + i) * 4;
                    a += weight;
                    r += weight * self.src.data[idx];
                    g += weight * self.src.data[idx + 1];
                    b += weight * self.src.data[idx + 2];
                }
            }
        }
        var idx = (v * self.dest.width + u) * 3;
        self.dest.data[idx] = r / a;
        self.dest.data[idx + 1] = g / a;
        self.dest.data[idx + 2] = b / a;
    }

    if (++u < self.dest.width) 
        setTimeout(self.process1, 0, self, u);
    else 
        setTimeout(self.process2, 0, self);
};
thumbnailer.prototype.process2 = function(self){
    self.canvas.width = self.dest.width;
    self.canvas.height = self.dest.height;
    self.ctx.drawImage(self.img, 0, 0);
    self.src = self.ctx.getImageData(0, 0, self.dest.width, self.dest.height);
    var idx, idx2;
    for (var i = 0; i < self.dest.width; i++) {
        for (var j = 0; j < self.dest.height; j++) {
            idx = (j * self.dest.width + i) * 3;
            idx2 = (j * self.dest.width + i) * 4;
            self.src.data[idx2] = self.dest.data[idx];
            self.src.data[idx2 + 1] = self.dest.data[idx + 1];
            self.src.data[idx2 + 2] = self.dest.data[idx + 2];
        }
    }
    self.ctx.putImageData(self.src, 0, 0);
    self.canvas.style.display = "block";
}

...

img.onload = function() {
  var canvas = document.createElement("canvas");
  new thumbnailer(canvas, img, 188, 3); //this produces lanczos3
  //but feel free to raise it up to 8. Your client will appreciate
  //that the program makes full use of his machine.
  document.body.appendChild(canvas);
}

然而，这个实现加载一个图像并渲染它，故事结束。

我一直在尝试重新实现此代码，以便在每次缩放现有画布（例如，放大和缩小图像或文档）时进行过滤，而无需加载新图像或创建新图像画布。

我怎样才能让它以这种方式工作？或者这甚至可能吗？

Answer 1

您想要做的是类似于singleton 来重用您的画布对象。这将让您节省每次创建新画布对象的成本，并且您将重复使用相同的对象

function getCanvas(){ 
  var canvas; 
  if (typeof canvas === "undefined"){ canvas = document.createElement("canvas");}
  return canvas;
}
img.onload = function() {
  var canvas = getCanvas("canvas");
  .... THE REST OF YOUR CODE .......
}

。

但这并不是减慢代码速度的原因， 图像缩放算法是非常繁重的算法，需要大量使用 cpu，“通常在非常低的水平上使用 gpu 加速”，并使用像 @ 这样的高级技术987654322@r 等等。这是一个interesting tutorial in java.net，关于图像缩放的工作原理，它是在 java 中，但您可以插入到任何语言。

Javascript 还没有为这种技术做好准备，所以我建议您使用 canvas api 中提供的transformations，因为在您阅读的教程中，您阅读的有效方法是使用 canvas2Dcontext。 P>

var ctx = canvas.getContext("2d"); 
ctx.scale(2,2);