On Tue, Sep 22, 2009 at 9:20 AM, David Duncan <david.dun...@apple.com>wrote:
> On Sep 21, 2009, at 9:32 PM, Roland King wrote:
>
> I'm trying to construct an image which is colored according to the alpha
>> value of a different image. I've been hunting around the Quartz Core docs
>> and I can't figure out a good way to do this. The motivation is to do
>> something similar to what apple does on the iphone tabbar items which are
>> drawn white or blue using the alpha of the image you supply and it's a
>> pretty good effect I want to reuse.
>>
>
>
> Just use your second image as a mask via CGContextClipToMask(). If you use
> a normal image, then the alpha values are used as alpha for the drawing you
> do which should do exactly what you want.
Also, this bit
So for instance if I my drawing color is D and the point on my underlying
> image is {r,g,b,a}, I want to draw a point of color S * a.
seems to have a typo, but there are compositing modes for whatever operation
you're talking about here.
DestinationIn is Result = DestinationColor * SourceAlpha. So you could do
CGContextBeginTransparencyLayer, draw mask with SourceOver, draw color to be
masked with DestinationIn, CGContextEndTransparencyLayer.
There are quite a few modes that use only the alpha channel from either
source or destination.
-Ken
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