CALayer alpha mask not working

Question

I'm trying to create a simple example of drawing an image to a layer, and then setting that layer's alpha mask. I added the code below to my sample app's viewDidLoad. without setting the mask, i can see the image of the leaf. after i set the mask, i see nothing where my sublayer was. what am i doing wrong?

Here are the two images i'm using (just samples i found on the net) http://sorenworlds.netfirms.com/Alpha/leaf.jpg

http://sorenworlds.netfirms.com/Alpha/leaf-alfa.jpg

self.view.layer.backgroundColor = [UIColor orangeColor].CGColor;
self.view.layer.cornerRadius = 20.0;
self.view.layer.frame = CGRectInset(self.view.layer.frame, 20, 20);


CALayer *sublayer = [CALayer layer];
sublayer.backgroundColor = [UIColor blueColor].CGColor;
sublayer.shadowOffset = CGSizeMake(0, 3);
sublayer.shadowRadius = 5.0;
sublayer.shadowColor = [UIColor blackColor].CGColor;
sublayer.shadowOpacity = 0.8;
sublayer.frame = CGRectMake(30, 30, 128, 192);
[self.view.layer addSublayer:sublayer];

CGImageRef img = [UIImage imageNamed:@"leaf.jpg"].CGImage;
sublayer.borderColor = [UIColor blackColor].CGColor;
sublayer.borderWidth = 2.0;
sublayer.contents = img;

CGImageRef imgAlpha = [UIImage imageNamed:@"leaf_alpha.jpg"].CGImage;   
CALayer *alphaLayer = [CALayer layer];
alphaLayer.contents = (id)imgAlpha;
sublayer.mask = alphaLayer;

Any help is greatly appreciated.

Answer 1

There are several problems with your code:

1. The CALayer class reference says that the mask layer's alpha is used to mask the parent layer. Your mask layer (alphaLayer) is created with a JPG, which doesn't contain alpha values. Quartz doesn't understand that you want to use the grayscale RGB pixel values in the mask image as alpha values, and there's no code that you can call to do this (as far as I know).

   Quartz probably interprets the lack of alpha in the mask image as 0 alpha for every pixel which is why nothing is displayed.

   I suggest getting a paint program, like Pixelmator, and making a PNG file (which has alpha values built in) out of the original leaf image. Doing this removes the need for adding a mask layer in code.

2. Your file name is "leaf-alfa.jpg", but it's "leaf_alpha.jpg" in your code.

3. Sublayers must be laid-out in their parent layer's coordinates. You did not do this, nor did you set a boundary for the mask layer. A layer's default boundary is the empty rectangle, so even if you used an image with an appropriate alpha mask, you would still see nothing (or maybe just the original leaf image with no masking). You would need to do something like this:

   alphaLayer.bounds = CGRectMake(0, 0, 128, 192);
   alphaLayer.position = CGPointMake(sublayer.bounds.size.width/2.0,
                                     sublayer.bounds.size.height/2.0);