RenderMan "rgba" display mode produces 3 channel image instead of 4?

Question

I have a .rib file with the following display and quantize statements:

Display "output.tif" "tiff" "rgba"
Quantize "rgba" 65535 0 65535 0

and I expect to get an image with 4 channels (16-bit per channel).

I however get an image with only 3 channels.

How do I get the alpha channel in the output?

I am using prman.

Answer 1

Just remove the line Quantize and it should work as you expected.