canvas.toDataUrl()返回'data:,'

Question

我正在尝试调整图像大小并使用canvas.toDataUrl()获取base64字符串表示形式.

我的代码如下(见下文).我的问题是,每当我第一次启动它时,它都会返回'data:,'.

然后,当我重做重新调整大小(使用按钮调用)时,它工作正常,它返回一个非空的base64字符串.到底是怎么回事？

 function drawAndResizeFunction(images)
 var qDraw = $q.defer();

// 1
        drawCanvasWrapper().then(function(canvasData){
            qDraw.resolve(canvasData)
        });

        // 2
        function drawCanvasWrapper() {
            var pResults = images.map(function (imageObj) {
                //return drawCanvassIter(imageObj.tempURL); // tempUrl
                return resizeIter(imageObj.tempURL).then(function(result){
                    console.log("resized", result) // *** RETURNS data:, in first attempt
                     return result;
                })
            });
            return $q.all(pResults);
        };

        // 3inval
        // returns canvasdata
        function resizeIter(nativeURL) {

            console.log("resizeIter")

            var qResize = $q.defer();

            var canvas = document.getElementById("resizecanvas");
            var ctx = canvas.getContext("2d");
            var img = new Image();
            img.src = nativeURL;

            var newScales = resizeDimensions(img.width, img.height)

            var iw  =canvas.width   =img.width      =newScales.iw;
            var ih  =canvas.height  =img.height     =newScales.ih;

            img.onload = function () {

                // --> 4
                ctx.drawImage(img, 0, 0, iw, ih);
                $timeout(function(){
                    qResize.resolve(canvas.toDataURL("image/jpeg"));
                }, 200)


            };

            return qResize.promise;



            //
            //
            function resizeDimensions(iw, ih) {
                var scaleFactor = 1;
                var targetSize = 800;

                if (iw > targetSize || ih > targetSize) {
                    if(iw > ih) {
                        scaleFactor = targetSize/iw;
                    } else {
                        scaleFactor = targetSize/ih;
                    }
                }
                var iwAdj = Math.floor(iw*scaleFactor);
                var ihAdj = Math.floor(ih*scaleFactor);

                return {
                    ih: ihAdj, iw: iwAdj
                }
            };
        };

return qDraw.promise; 
    }; // done

Answer 1

原因

原因是画布的大小无效:

[...]一个例外是如果画布没有高度或没有宽度,在这种情况下结果可能只是"data:,".

无效大小包括<1.

当图像元素没有加载数据时,默认情况下宽度和高度属性为0,直到图像完全加载并解码为位图,此时触发onload处理程序:

var img = new Image();
document.write("w: " + img.width + " h: " + img.height);

解决方案

在从中读取任何大小之前确保图像已加载(简化示例,采用适合您的代码的方式):

var img = new Image();
var w, h;
img.onload = function() {
    w = this.width;     // here we can extract image size
    h = this.height;
    // set canvas size here as well before drawing the image in
    // continue you code from here using f.ex. a callback
    document.write("w: " + w + " h: " + h);
};
img.src = "http://i.imgur.com/eekEotAb.jpg";  // set src last