How to separate image from alpha using Quartz Composer?

Question

Cross posting here and in the developers forums... (sorry for any duplication - also posting to the QC mail list)
I'm attempting to construct a Quartz Composer composition that will take an image containing transparency, and — within the structure of that composition — disect the image so that I can perform separate operations on the alpha mask and the full image (i.e. as if the mask did not exist).  This would be akin to opening a PNG or TIFF in GraphicConverter, then accessing the images I want via Show Alpha Channel and Remove Alpha Channel.
I'm able to easily access the alpha channel by applying a Color Martix patch to the image, but I can't seem to figure out how to get to the full "unmasked" image.
Can anyone nudge me in the right direction as to how I could remove the alpha mask from an image?
Thanks!!
John

Answer 1

Hi John,
You could use the Core Image Filter to extract the Alpha channel as a mask, and then other filters to use that mask.
In the 'Settings' pane of 'Patch Inspector' for the 'Core Image Filter' place the following Core Image Kernel routine code:
A Core Image kernel routine
Alpha to Mask White Opaque, Black Transparent
kernel vec4 alphaToBlackWhite(sampler image)
vec4 pixelValue = sample(image, samplerCoord(image)); // Get a pixel
          unpremultiply(pixelValue);
          pixelValue.r = pixelValue.a; // Red channel
          pixelValue.g = pixelValue.a; // Green channel
          pixelValue.b = pixelValue.a; // Blue channel
           pixelValue.a = 1.0; // Alpha Channel make Opaque
          return premultiply(pixelValue);
Alternatively you could rewrite the above Kernel, more details on writing Core Image Kernels is in the developer documentation 'Image Tutorial: Writing Kernels'
Hope this helps,
Neil.