Javascript drawImage，getImageData和putImageData无法正常工作

Question

我想用Javascript在一组图像中用透明的颜色替换紫色。我正在使用drawImage，getImageData和putImageData：我错了（在这里看到JsFIddle：http://jsfiddle.net/2pGfX/）？

HTML：

<img onclick="scegli(this.id)" id="c0">
<img onclick="scegli(this.id)" id="c1">
<img onclick="scegli(this.id)" id="c2">
<img onclick="scegli(this.id)" id="c3">

<canvas id="myCanvas"></canvas>

使用Javascript：

var items = ["image1.gif", "image2.gif", "image3.gif"];

var arraioParti = [
["a",0, 100,  100, 181, 43],
["b",0, 100,  100, 213, 43],
["c",0, 100,  100, 173, 0],
["d",0, 100,  100, 190, 84]
];

function scegli(parte) {   
scelto = Math.floor(Math.random() * items.length);       
document.getElementById("c" + parte).src = items[scelto];
}

function inizia() {

for (fh=0; fh<arraioParti.length; fh++) { 

        scegli(fh);

        document.getElementById("c" + fh).style.width = arraioParti[fh][2] + "px";
        document.getElementById("c" + fh).style.height = arraioParti[fh][3] + "px";

        traspa("c" + fh);

}
}

function traspa(quale) {

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");

var img = new Image();   

img.src = document.getElementById(quale).src;
img.width = document.getElementById(quale).width;
img.height = document.getElementById(quale).height;

       img.onload = function() {
           c.width = img.width;
           c.height = img.height;
           ctx.drawImage(img, 0, 0, c.width, c.height);
       }        

var imgData = ctx.getImageData(0, 0, c.width, c.height);

for (var i=0; i<imgData.data.length; i+=4) {

    if (imgData.data[i] == 255 && imgData.data[i+1] == 0 && imgData.data[i+2] == 255) {

        imgData.data[i] = 255;
        imgData.data[i+1] = 255;
        imgData.data[i+2] = 255;
        imgData.data[i+3] = 0;

    }

}

ctx.putImageData(imgData, 0, 0);
document.getElementById(quale).src = c.toDataURL("image/png");

}

window.onload = inizia();