alpha masks

I am teaching the Blender part of a class where the compositing teacher who is using NUKE requires me to show the students how to output an Alpha mask from the model they have made in Blender with
“the Alpha being in the Alpha channel, R channel black, G black, B black
It is actually an image with 4 channels with R,G,B being black.
I don’t want to do it in Pshop because it would be too much time consuming on 600 hundred frames.”
He wants it rendered as a TIFF or EXR which I can do and I can set output to RGBA but individual channels? The final animation will be composited to run infront of a filmed background. I know Blender has its own compositor but he is a world renouned compositor (Happy Feet, Lord of the Rings, The Matrix Reloaded etc) who had never previously heard of Blender and was initially very dismissive of it so he used Nuke.

what if you turned off all the lamps and then rendered the character, it would be pure black, and the alpha would still be there. You would just have to isolate the character on its own layer.


You can use a very simple node setup. Use nodes for compositing, then link your render layer to your composite output by alpha only.

um, i think broken and I answered this question yesterday…? here’s the setup again.



From the initial description, it sounds like the alpha of the render layer should be connected to the alpha of the composite output. That will output the alpha to the alpha channel, leaving the red, green and blue as black. I might be misinterpreting the OP though.