Question

目前，我正在使用画布动态生成带有javascript的灰度图像。灰度代码如下：

// Grayscale w canvas method
function grayscale(src)
{
    var canvasUrl = false;

    try
    {
        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);
        canvasUrl = canvas.toDataURL();
    }
    catch(err)
    {
        canvasUrl = false;
    }

    return canvasUrl;
}

在我的代码中，我的图片放在div中。 div的设置为“display：none”，直到javascript执行并完成生成所有图像的灰度表示，此时它将div设置为“display：block”。

大部分时间都可以使用，但不是全部。偶尔，div将不会显示，因为以下行发生错误：

var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);

error

基于SO的相关问题，一般解决方案似乎是使用$(window).load(func);来运行灰度生成代码。

然而，正在做的事情：

<script type="text/javascript">
    var animateTime = 250;

    // 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},animateTime);

        // 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":el.css("width"),"height":el.css("height")});
                el.dequeue();
            });
            this.src = grayscale(this.src);

            if(!this.src)
                alert('An error occurred.'); // handle the occasional DOM error...
        });

        // Fade image
        $('.item img').mouseover(function(){
            $(this).parent().find('img:first').stop().animate({opacity:1}, animateTime);
        })
        $('.img_grayscale').mouseout(function(){
            $(this).stop().animate({opacity:0}, animateTime);
        });
        $('.item').mouseover(function() {
            $(this).children('h3').css('display', 'block');
            $(this).children('h3').stop().animate({opacity:1}, animateTime);
        });
        $('.item').mouseout(function() {
            $(this).children('h3').css('display', 'none');
            $(this).children('h3').stop();
            $(this).children('h3').css('opacity', '0');
        });

        $("#loading").css('display', 'none'); // hide the loading GIF
        $(".outer_content_container").css('display', 'block');
        $(".outer_content_container").animate({opacity:1}, animateTime*4);
    });
</script>

这让我想到：我可以设置“display：none;”在包含图像的父div上导致浏览器根本不加载图像并继续调用window.onload？

Answer 1

由于您有竞争条件，因此在加载图像之前不会设置图像的宽度和高度。使用onload事件可以了解何时可以读取尺寸。

    var canvas = document.createElement('canvas');
    var ctx = canvas.getContext('2d');
    var imgObj = new Image();
    imgObj.onload = function() {
        //do the processing here
    };
    imgObj.src = src;

Answer 2

canvas.height = imgObj.height;gagein

什么是gagin？

imgObj.src = src;

在该行之前使用图像加载方法并将其余功能包装在其中。

imgObj.addEventListener("load", function() {
    canvas.width = imgObj.width;
    canvas.height = imgObj.height;gagein
    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);
    canvasUrl = canvas.toDataURL();
}, false);

带有“display：none”的父元素会导致子imgs无法加载吗？

2 个答案: