Question

如何在html5中的canvas下实现ps中的叠加效果

Answer 1

指定两个canvas的postion:absolution,然后设置者两个canvas的left与top属性，需要注意的是建议把小的canvas放到上面，方法是设置两个canvas的z-index为不同的值，值大的在上面。
绘图的过程可以参考一下代码：
var mycanvas=document.getElementByIdx_x_x_x("mycanvas");
var context = mycanvas.getContext("2d");
var imageObj = new Image();
imageObj.onload = function(){

// draw cropped image
var sourceX = 350;
var sourceY = 50;
var sourceWidth = 350;
var sourceHeight = 350;
var destWidth = sourceWidth;
var destHeight = sourceHeight;
var destX = mycanvas.width / 2 - destWidth / 2;
var destY = mycanvas.height / 2 - destHeight / 2;
context.drawImage(imageObj, sourceX, sourceY, sourceWidth, sourceHeight, destX, destY, destWidth, destHeight);