composite colors: CALayer and blend mode on iPhone

Question

I'm trying to use core image on the iphone. I'm able to composite my colors using quartz to draw an uiview, but I want to separate each component into CALayer (UIview consume more resources).

So I have a white mask I want to use to filter a background bitmap, and I want to try different blending mode. Unfortunately, the layers are only "adding" their colors.

Here is my code:

@implementation WhiteLayerHelper

    - (void)drawLayer:(CALayer *)theLayer
            inContext:(CGContextRef)myContext
    {
        // draw a white overlay, with special blending and alpha values, so that the saturation can be animated
        CGContextSetBlendMode(myContext,kCGBlendModeSaturation);
        CGContextSetRGBFillColor(myContext,1.0,1.0,1.0,0.9);
        CGContextFillRect(myContext,[UIScreen mainScreen].bounds);

    }

@end

And here is the main view drawrect code, where I use my CALayer:

- (void)drawRect:(CGRect)rect {
    //get the drawing context
    CGContextRef myContext = UIGraphicsGetCurrentContext();
    // draw the background
    [self fillContext:myContext withBounds:m_overlayRect withImage:m_currentImage];
    [whiteLayer renderInContext:myContext];

}

Is there something wrong?

Answer 1

I managed to get the affect of compositing multiple CALayers by drawing them directly into a UIView's graphics context.

-(void)drawRect:(CGRect)rect {
 CGContextRef c = UIGraphicsGetCurrentContext();
 CGContextSetBlendMode(c, kCGBlendModeDifference);
 [myLayer drawInContext:c];
}

BTW, I did not add the layers as sublayers of the view's layer (that is I never called [myView.layer addSublayer:myLayer])