畫布：屏蔽圖像並保留其alpha通道？

Question

這是我正在嘗試做的事情：

1. 獲取圖像A和圖像B.圖像B是黑白掩模圖像。
2. 將圖像A的alpha通道替換為圖像B的紅色通道。
3. 在畫布上繪制圖像C.
4. 在圖像C上繪制圖像A.

在步驟4之前，一切似乎都沒問題。圖像C根本看不到，圖像A應該是透明的，那里是白色。

cx.putImageData(imageA, 0, 0);
var resultData = cx.getImageData(0, 0, view.width, view.height);

for (var h=0; h<resultData.data.length; h+=4) {
    resultData.data[h+3] = imageB.data[h];
}

cx.putImageData(imageC, 0, 0);
cx.putImageData(resultData, 0, 0);

Answer 1

西蒙是對的： putImageData方法沒有注意合成; 它只是復制像素值。 為了獲得合成，我們需要使用繪圖操作。

我們需要使用像素數據混淆通道（將紅色轉換為alpha），將更改后的像素數據放入圖像中， 然后使用復合操作來獲得所需的屏蔽。

//copy from one channel to another
var assignChannel = function(imageData, channelTo, channelFrom) {
  if(channelTo < 0 || channelTo > 3 || channelFrom < 0 || channelFrom > 3) {
    throw new Error("bad channel number");
  }
  if(channelTo == channelFrom)
    return;
  var px = imageData.data;
  for(var i = 0; i < px.length; i += 4) {
    px[i + channelTo] = px[i + channelFrom];
  }
};
/**============================================================================ 
  * this function uses 3 or 4 canvases for clarity / pedagogical reasons:
  * redCanvas has our mask image;
  * maskCanvas will be used to store the alpha channel conversion of redCanvas' image;
  * imageCanvas contains the image to be masked;
  * ctx is the context of the canvas to which the masked image will be drawn.
============================================================================**/
var drawOnTopOfRed = function(redCanvas, maskCanvas, imageCanvas, ctx) {
  var redImageData = redCanvas.getContext("2d").getImageData(0, 0, w, h);

  //assign the alpha channel
  assignChannel(redImageData, 3, 0);

  //write the mask image
  maskCanvas.getContext("2d").putImageData(redImageData, 0, 0);

  ctx.save();

  //draw the mask
  ctx.globalCompositeOperation = "copy";
  ctx.drawImage(maskCanvas, 0, 0);

  //draw the image to be masked, but only where both it
  //and the mask are opaque; see http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#compositing for details.
  ctx.globalCompositeOperation = "source-in";
  ctx.drawImage(imageCanvas, 0, 0);
  ctx.restore();
};

jsfiddle的例子

帶有示例的塗鴉：

Answer 2

因為在步驟4中您使用的是完美替換像素的putImageData 。 您想在圖像C的頂部繪制圖像A，因此您無法執行此操作。 相反，你會想要使用drawImage()

所以：

cx.putImageData(imageC, 0, 0); // step 3
// create a new canvas and new context,
// call that new context ctx2 and canvas can2:
var can2 = document.createElement('canvas');
// set can2's width and height, get the context etc...
ctx2.putImageData(resultData, 0, 0);
cx.drawImage(can2, 0, 0); // step 4 using drawImage instead of putting image data