javascript - HTML5 Canvas : Reproduce a picture with getImageData and putImageData

Question

我几天来一直在尝试仅使用 Canvas API 以及 getImageData 和 putImageData 来重现图像，遵循我在网上找到的一些教程。使用正方形或从 Canvas 上绘制的任何其他形式很简单，但使用外部图片似乎更困难。

你可以看看我是如何尝试的，如果你有解决方案，我很乐意阅读。我尝试读取放置在 Canvas 左侧的图像，然后将其复制到同一 Canvas 的右侧。

var c       = document.getElementById("canvas"),
    ctx     = c.getContext('2d');
    img     = new Image(),
    img.src = "http://nagisalloum.com/wp-content/uploads/2012/01/white-duck-water2.jpeg";

img.onload = function(){
    ctx.drawImage(img, 0, 0, 400, 300);
}


//read the width and height of the canvas
var width   = c.width,
    height  = c.height,
    w2      = width / 2,
    imgData = ctx.getImageData(0, 0, 400, 300);

function drawing(){
    var index= (height * w2) * 4;   
    for(index= 0; index<=imgData.data.length; index+=4){
        red     = imgData.data[index];
        green   = imgData.data[index+1];
        blue    = imgData.data[index+2];
        alpha   = imgData.data[index+3]; 
    }
        ctx.putImageData(imgData, w2, 300);
}

drawing();

这是一个 fiddle :http://jsfiddle.net/6g82o9Lk/

感谢您的帮助!

Answer 1

首先:您的 var 定义开头存在语法错误:

var c       = document.getElementById("canvas"),
    ctx     = c.getContext('2d'),
    img     = new Image();
img.src = "http://nagisalloum.com/wp-content/uploads/2012/01/white-duck-water2.jpeg";

其次，图像数据不会在onload之前，所以这样会更好:

img.onload = function(){
    ctx.drawImage(img, 0, 0, 400, 300);
    drawing();
}

function drawing(){
    //read the width and height of the canvas
    var width   = c.width,
    height  = c.height,
    w2      = width / 2,
    imgData = ctx.getImageData(0, 0, 400, 300);

    var index= (height * w2) * 4;   
    for(index= 0; index<=imgData.data.length; index+=4){
        red     = imgData.data[index];
        green   = imgData.data[index+1];
        blue    = imgData.data[index+2];
        alpha   = imgData.data[index+3]; 
    }
        ctx.putImageData(imgData, w2, 300);
}

但最重要的是:在 Chrome 中，您会收到安全错误。

Uncaught SecurityError: Failed to execute 'getImageData' on 'CanvasRenderingContext2D': The canvas has been tainted by cross-origin data.

我猜这可能相关:How to fix getImageData() error The canvas has been tainted by cross-origin data?