I used a little ditty (and a layerX operation) to create the luminance channel.
I weighted the R, G, and B channels like so:
lum = 0.299r + 0.587g + 0.114b
It gives almost exactly the same result. (I may tweak in a bit)
Here it is, the node, documentation, etc.... no ShakeSDK needed!!
Enjoy!
-W



