Javascript HTML5画布像素操作

我只是试着养成像素操作的习惯，它不会改变任何事情

我使用以下循环反转每个像素：

var newImage = context.createImageData(canvas.width, canvas.height);
var arr = context.getImageData(0, 0, canvas.width, canvas.height);
var pixels = arr.data;

for(var i = 0; i < pixels.length; i+=4){
    var r = 255 - pixels[i];
    var g = 255 - pixels[i + 1];
    var b = 255 - pixels[i + 2];
    var a = pixels[i + 3];
    newImage.data[i] = r;
    newImage.data[i + 1] = g;
    newImage.data[i + 2] = b;
    newImage.data[i + 3] = a;
}

为什么这不起作用？我做错了什么

---

问题来自图像（尚未加载）。 你的代码可以工作（看这个没有图像的小提琴-）

你必须这样做：

var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
canvas.width = 500;
canvas.height = 322;

var image_obj = new Image(); 
image_obj.onload = function(){
    var newImage = context.createImageData(canvas.width, canvas.height);
    var arr = context.getImageData(0, 0, canvas.width, canvas.height);
    var pixels = arr.data;

    for(var i = 0; i < pixels.length; i+=4){
        var r = 255 - pixels[i];
        var g = 255 - pixels[i + 1];
        var b = 255 - pixels[i + 2];
        var a = pixels[i + 3];
        newImage.data[i] = r;
        newImage.data[i + 1] = g;
        newImage.data[i + 2] = b;
        newImage.data[i + 3] = a;
    }

    context.clearRect(0, 0, canvas.width, canvas.height);
    context.putImageData(newImage, 0, 0);
}

image_obj.src = 'http://www.johnchapple.com/jcwp/wp-content/uploads/2012/09/photography_quotes.jpg';

var canvas=document.getElementById（'canvas'）；
var context=canvas.getContext（'2d'）；
画布宽度=500；
canvas.height=322；
var image_obj=新图像（）；
image_obj.onload=函数（）{
var newImage=context.createImageData（canvas.width、canvas.height）；
var arr=context.getImageData（0，0，canvas.width，canvas.height）；
var像素=arr.data；
对于（变量i=0；i

我认为您必须将这些说明放入图像的

onLoad

回调中；）此外，除非您是从运行或设置了适当的标志，否则您将被良好的ole“无法从画布获取图像数据，因为画布已被跨源数据污染”消息阻止。您是对的：因为图像尚未加载，所以没有显示任何内容。

var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
canvas.width = 500;
canvas.height = 322;

var image_obj = new Image(); 
image_obj.onload = function(){
    var newImage = context.createImageData(canvas.width, canvas.height);
    var arr = context.getImageData(0, 0, canvas.width, canvas.height);
    var pixels = arr.data;

    for(var i = 0; i < pixels.length; i+=4){
        var r = 255 - pixels[i];
        var g = 255 - pixels[i + 1];
        var b = 255 - pixels[i + 2];
        var a = pixels[i + 3];
        newImage.data[i] = r;
        newImage.data[i + 1] = g;
        newImage.data[i + 2] = b;
        newImage.data[i + 3] = a;
    }

    context.clearRect(0, 0, canvas.width, canvas.height);
    context.putImageData(newImage, 0, 0);
}

image_obj.src = 'http://www.johnchapple.com/jcwp/wp-content/uploads/2012/09/photography_quotes.jpg';