javascript - 在 Canvas 上，drawImage 与 png 与使用笔触创建绘图之间的区别？

Question

为什么在 Canvas 中处理事物的方式不同？例如，如果我在 Canvas 上放一个 png vs 在 Canvas 上画一条线。当我将该 Canvas 的内容复制到另一个 Canvas 时，只会复制该行。

var thecanvas = document.getElementById('mycanvas');
var context = thecanvas.getContext('2d');

// put a png on the canvas
var img = new Image();
img.onload = function(){ context.drawImage(img,0,0); };
img.src = 'images/test.png';        

// draw a line on the canvas
context.moveTo(100, 150); context.lineTo(450, 50); context.stroke();

final_image = thecanvas.toDataURL("image/png");
copyimg = new Image();
copyimg.src = final_image;          
var newcanvas = document.getElementById('newCanvas');
var newcanvascontext = newcanvas.getContext('2d');

// why is only the line I drew copied over and not the png image???
newcanvascontext.drawImage(copyimg,0,0,397,397);

Answer 1

请注意图像加载事件。在加载图像之前复制 Canvas 。你必须这样做

var thecanvas = document.getElementById('mycanvas');
var context = thecanvas.getContext('2d');

// put a png on the canvas
var img = new Image();
img.onload = function(){ 

        context.drawImage(img,0,0); 

        var newcanvas = document.getElementById('newCanvas');
        var newcanvascontext = newcanvas.getContext('2d');
        newcanvascontext.drawImage(thecanvas,0 ,0);

};
img.src = 'http://www.mygreatiphone.com/wp-content/uploads/2011/11/google.png';        

// draw a line on the canvas
context.moveTo(100, 150); context.lineTo(450, 50); context.stroke();

查看演示:http://jsfiddle.net/diode/3NHXy/5/