javascript - Change html canvas black background to white background when creating jpg image from png image

Question

I have a canvas which is loaded with a png image. I get its jpg base64 string by .toDataURL() method like this:

 $('#base64str').val(canvas.toDataURL("image/jpeg"));

But the transparent parts of the png image are shown black in the new jpg image.

Any solutions to change this color to white? Thanks in advance.

Answer 1

This blackening occurs because the 'image/jpeg' conversion involves setting the alpha of all canvas pixels to fully opaque (alpha=255). The problem is that transparent canvas pixels are colored fully-black-but-transparent. So when you turn these black pixels opaque, the result is a blackened jpeg.

The workaround is to manually change all non-opaque canvas pixels to your desired white color instead of black.

That way when they are made opaque they will appear as white instead of black pixels.

Here's how:

// change non-opaque pixels to white
var imgData=ctx.getImageData(0,0,canvas.width,canvas.height);
var data=imgData.data;
for(var i=0;i<data.length;i+=4){
    if(data[i+3]<255){
        data[i]=255;
        data[i+1]=255;
        data[i+2]=255;
        data[i+3]=255;
    }
}
ctx.putImageData(imgData,0,0);