iOS HTML5 Canvas toDataURL

问题

I need some assistance. We seem to be having an issue with iOS with regards to getting the base64 of an image via HTML 5 / Canvas. Everything works fine if we use the default height / width of the canvas or hard code the height and width. However if we set the canvas height / width to that of the image src then the image won’t load into the canvas and therefore we won’t get the image as base64.

Code snippet which works:

function convertImageToBase64(imgUrl, callback) {
    var canvas = document.createElement("canvas");
    var context = canvas.getContext('2d');
    // load image from data url
    var imageObj= new Image();
    imageObj.onload = function () {
        var dataUrl;
        context.drawImage(imageObj, 0, 0, canvas.width, canvas.height);

        dataUrl = canvas.toDataURL("image/png");
        callback.call(this, dataUrl);
        canvas = null;
    };
    imageObj.src = imgUrl;
}

Code snippet which does not work on iOS but does work on Android:

function convertImageToBase64(imgUrl, callback) {
    var canvas = document.createElement("canvas");
    var context = canvas.getContext('2d');
    // load image from data url
    var imageObj= new Image();
    imageObj.onload = function () {
        var dataUrl;
        canvas.width = imageObj.width;
        canvas.height = imageObj.height;
        context.drawImage(imageObj, 0, 0, canvas.width, canvas.height);

        dataUrl = canvas.toDataURL("image/png");
        callback.call(this, dataUrl);
        canvas = null;
    };
    imageObj.src = imgUrl;
}

We need to be able to establish the canvas height / width based upon the image itself.

Any guidance or assistance is most appreciated.

回答1:

All this limits are actual for iOS:

The maximum size for decoded GIF, PNG, and TIFF images is 3 megapixels for devices with less than 256 MB RAM and 5 megapixels for devices with greater or equal than 256 MB RAM.
The maximum size for a canvas element is 3 megapixels for devices with less than 256 MB RAM and 5 megapixels for devices with greater or equal than 256 MB RAM.
JavaScript execution time is limited to 10 seconds for each top-level entry point.

This limits don't throw any errors, so then you will try to render or read 6MB image you will get a broken blob/dataURL string and so on. And you will think that File API is broken, canvas methods toDataURL/toBlob are broken, and you will be right. But bugs aren't in browser, this is a system limitation.

So this limitations create a broken behavior for javascript API.

来源：https://stackoverflow.com/questions/26152652/ios-html5-canvas-todataurl

标签

javascript

ios

html5

canvas