在Firefox中调整base64图像的大小

Question

I'm still new in coding and HTML5. I have a HTML5 stage with multiple canvas. I need to get the base64 image from that stage and resize it, then get the base64 image of the resized canvas. This is the code I use:

stage.toDataURL({
    callback: function(dataUrl) {
        var tmp_img = new Image();
        tmp_img.src = dataUrl;

        //resize image to 45x75
        var canvas = document.getElementById('hidden_canvas');
        canvas.width = 45;
        canvas.height = 75;
        var ctx = canvas.getContext("2d");
        ctx.drawImage(tmp_img, 0, 0, canvas.width, canvas.height);
        dataUrl = canvas.toDataURL();
)};

The dataUrl is correct in Chrome but when I run the code in Firefox, it seems that the Firefox didn't generate the correct base64 code.

I would really appreciate any help

Answer 1

You've stumbled upon a common problem.

tmp_img.src loads the dataUrl into your new image--but that takes time. So javascript continues with your code below that even before the image is loaded. The result is that you are sometimes trying to ctx.drawImage before tmp_img has been fully loaded and is ready to use.

The fix is to refactor your code to use tmp_img.onload . The .onload triggers a callback function when tmp_img is finally ready to use. The refactoring might look like this:

var tmp_img = new Image();

tmp_img.onload=function(){

    //resize image to 45x75
    var canvas = document.getElementById('hidden_canvas');
    canvas.width = 45;
    canvas.height = 75;
    var ctx = canvas.getContext("2d");
    ctx.drawImage(tmp_img, 0, 0, canvas.width, canvas.height);
    dataUrl = canvas.toDataURL();

}

tmp_img.src = dataUrl;

// more javascript

JavaScript will execute the above code in this order:

1. Create a new Image object
2. See tmp_img.onload and make a not that it should do that code when the image is fully loaded
3. Set tmp_img.src and start loading the image.
4. Continue with "more javascript"
5. When the image is fully loaded, execute everything in the .onload callback function.

It looks like you're using KineticJS--are you?

If so, then instead of stage.toDataURL you could use stage.toImage . That way you already have the image loaded when the stage.toImage callback is executed (you don't have to worry about waiting for the image to load). Then just .drawImage the image that KineticJS provides in the callback.