How to draw a shape on top of a UIImage while respecting the image's alpha mask

Question

I need a UIImageView that can draw itself in color or b/w according to a flag:

  BOOL isGrey;

I'm trying to do it by drawing a black rectangle on top of the original image with the Quartz blendmode set to Color. This works except it doesn't respect the image's alpha mask.

See illustration: alt text http://img214.imageshack.us/img214/1407/converttogreyscaleillo.png

Searching Google and SO, I found and tried several solutions but none respect the mask either.

Here is the code that produces the 'What I get' image above:

 - (void)drawRect:(CGRect)rect {

    if (isGrey) {
        CGContextRef context = UIGraphicsGetCurrentContext();

        // flip orientation
        CGContextTranslateCTM(context, 0, self.bounds.size.height);
        CGContextScaleCTM(context, 1.0, -1.0);

        // draw the image
        CGContextDrawImage(context, self.bounds, self.image.CGImage);

        // set the blend mode and draw rectangle on top of image
        CGContextSetBlendMode(context, kCGBlendModeSaturation);
        CGContextSetRGBFillColor(context, 0.0, 0.0, 0.0, 1.0);
        CGContextFillRect(context, rect);       
    } else {
        [self.image drawInRect:rect];
    }
}

Is there some Quartz drawing mode that I'm forgetting to set? I've looked thru the Quartz Programming Guide but is so hard to extract the one bit of info you need from the overlapping and hyperlinked subjects.

Obviously I'm looking for a general solution that will apply to images with any masked shape, not just the circle shown.

Answer 1

To draw a shape while respecting an image's alpha mask, just add one line before you draw:

 CGContextClipToMask(context, self.bounds, image.CGImage);

 // example usage
  - (void)drawRect:(CGRect)rect {

    if (isGrey) {
            CGContextRef context = UIGraphicsGetCurrentContext();

            // flip orientation
            CGContextTranslateCTM(context, 0.0, self.bounds.size.height);
            CGContextScaleCTM(context, 1.0, -1.0);

            // draw the image
            CGContextDrawImage(context, self.bounds, self.image.CGImage);

            // set the blend mode and draw rectangle on top of image
            CGContextSetBlendMode(context, kCGBlendModeColor);
            CGContextClipToMask(context, self.bounds, image.CGImage); // respect alpha mask
            CGContextSetRGBFillColor(context, 0.0, 0.0, 0.0, 1.0);
            CGContextFillRect(context, rect);               
    } else {
            [self.image drawInRect:rect];
    }

}