Canvas reduces imagesize of Jpeg, but why?

Question

When I draw a JPEG-Image to Canvas using drawImage() and after that, using canvas.toDataURL() to make it saveable local (with right mouseclick), then the saved Jpeg-Image has a reduced filesize about 40%. It is only so, when using Jpeg. PNG, GIF (NON-COMPRESSION-FILES) rises up the size. I thought, if I convert a Image-File to Base64 the size grows up to about 130%. So I think the canvas-element has an own integrated compression-functionality? If so, can I deactivate it?

The Code looks like this:

var img = new Image();

img.onload = function () 
{
var width = img.width;
var height = img.height;
context.drawImage(img, 0, 0,width,height);
document.images[0].src = canvas.toDataURL('image/jpeg');//<-size = 30,2 KB (30.990 Bytes)
}
img.src = "http://www.roomeffect.de/pageslices/RSB.jpg"; //<-original file size = 58,5 KB (59.930 Bytes)

I don't know what the problem is?

Answer 1

You can specify the JPEG quality as the second parameter to the toDataURL function. The default quality in Firefox is 0.92 (92%).

https://developer.mozilla.org/en/DOM/HTMLCanvasElement

Answer 2

It's not a problem. JPG is a lossy format, there's no reason to expect a JPG which is expanded into a bitmap (so you can see it on the screen) to be the same size as a new JPG made from compressing that bitmap again. If you want the original file to be saved, save the original file.