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

Answer 1

Although it doesn't directly answer your question, you may be able to get the effect you are looking for by using the kCGBlendModeLuminosity blend mode:

- (void)drawRect:(CGRect)rect {
    CGSize imageSize = [image size];
    CGContextRef c = UIGraphicsGetCurrentContext();
    if (isGrey)
        CGContextSetBlendMode(c, kCGBlendModeLuminosity);
    CGContextScaleCTM(c, 1.0, -1.0);
    CGContextDrawImage(c, CGRectMake(0.0f, 0.0f, imageSize.width, -imageSize.height), [image CGImage]);
}

_{Also, the rect passed to drawRect: is the invalid area, not the entire area. You should be using the bounds instead}