html5 canvas clearRect无法在Android Froyo浏览器中清除图像

Question

I'm trying to use the following code to rotate the canvas on mobile browsers based on the devices's orientation

e=window.event;
        var w=window.innerWidth||window.screen.availWidth;
        var h=window.innerHeight||window.screen.availHeight;
        var ww = colouringImage.width;
        var hh = colouringImage.height;
        var c = document.createElement('canvas');

        var cntx = c.getContext('2d');
        if(this.canvas.height > h) { 
            c.setAttribute('width', hh);
            c.setAttribute('height',ww);
            cntx.rotate(-90 * Math.PI / 180);
            cntx.drawImage(colouringImage, -ww, 0);
            this.artboard.canvas.width = hh;
            this.artboard.canvas.height = ww;
            this.artboard.clearRect(0,0,hh,ww);
            this.artboard.drawImage(c,0,0);
            c = document.createElement('canvas');
            c.setAttribute('width', hh);
            c.setAttribute('height',ww);
            cntx = c.getContext('2d');
            cntx.rotate(-90 * Math.PI / 180);
            cntx.drawImage(this.canvas,-ww,0);
            this.canvas.width = hh;
            this.canvas.height = ww;
            this.context.clearRect(0,0,hh,ww);
            this.context.drawImage(c,0,0);
         }else if (this.canvas.width > w)
         {
            c.setAttribute('width', ww);
            c.setAttribute('height', hh);
            cntx.drawImage(colouringImage, 0, 0);
            this.artboard.canvas.width= ww;
            this.artboard.canvas.height = hh;
            this.artboard.clearRect(0,0,ww,hh);
            this.artboard.drawImage(c,0,0);
            c = document.createElement('canvas');
            c.setAttribute('width',ww);
            c.setAttribute('height',hh);
            cntx = c.getContext('2d');
            cntx.rotate(90 * Math.PI / 180);
            cntx.drawImage(this.canvas,0,-ww);
            cntx.translate(hh/2,ww/2)
            cntx.rotate(0 * Math.PI / 180);
            this.canvas.width = ww;
            this.canvas.height=hh;
            this.context.clearRect(0,0,ww,hh);
            this.context.drawImage(c,0,0);
         }

What I'm doing here is making a copy of whats alreayd drawn and rotating it on a temp canvas then drawing it back onto the canvas after clearing it. However, what is happening is that it doesn't seem to clear the image. the image is a png and needs to be transparent.

Is there some other way to clear images drawn onto the canvas?

Answer 1

Was able to accurately rotate the canvas without distorting the coordinates for drawing by creating a memory-based canvas and applying the necessary rotations then drawing the image back onto the main canvas using the memory-based canvas as the source.

Note: I'm currently using 2 canvases as designed for my project. Thus I had to create ememory based canvases as necessary to apply whatever translations needed