对象不支持此属性或方法 IE8

Question

我正在使用这个 HTML5 grayscialer 代码 - http://webdesignerwall.com/demo/html5-grayscale/ 但是，在 javascript 中，IE8 返回错误“对象不支持此属性或方法”。有人知道 JavaScript 有什么问题吗？我很确定这里的这条线会导致错误“var ctx = canvas.getContext('2d');”谢谢。

// $(".item img").css({"display":"none");

// On window load. This waits until images have loaded which is essential
$(window).load(function(){

    // Fade in images so there isn't a color "pop" document load and then on window load
    $(".item img").animate({opacity:1},500);

    // clone image
    $('.item img').each(function(){
        var el = $(this);
        el.css({"position":"absolute"}).wrap("<div class='img_wrapper' style='display: inline-block'>").clone().addClass('img_grayscale').css({"position":"absolute","z-index":"998","opacity":"0"}).insertBefore(el).queue(function(){
            var el = $(this);
            el.parent().css({"width":this.width,"height":this.height});
            el.dequeue();
        });
        this.src = grayscale(this.src);
    });

    // Fade image 
    $('.item img').mouseover(function(){
        $(this).parent().find('img:first').stop().animate({opacity:1}, 1000);
    })
    $('.img_grayscale').mouseout(function(){
        $(this).stop().animate({opacity:0}, 1000);
    });     
});

// Grayscale w canvas method
function grayscale(src){
    var canvas = document.createElement('canvas');
    var ctx = canvas.getContext('2d');
    var imgObj = new Image();
    imgObj.src = src;
    canvas.width = imgObj.width;
    canvas.height = imgObj.height; 
    ctx.drawImage(imgObj, 0, 0); 
    var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
    for(var y = 0; y < imgPixels.height; y++){
        for(var x = 0; x < imgPixels.width; x++){
            var i = (y * 4) * imgPixels.width + x * 4;
            var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3;
            imgPixels.data[i] = avg; 
            imgPixels.data[i + 1] = avg; 
            imgPixels.data[i + 2] = avg;
        }
    }
    ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height);
    return canvas.toDataURL();
}

Answer 1

Internet Explorer 8 及更早版本不支持canvas element。

Answer 2

如果您想在 IE8 上使用画布，则需要查看 Google 的 excanvas。在这里查看：http://code.google.com/p/explorercanvas/wiki/Instructions