View Full Version : Nodes: Manipulate Surface's Alpha Channel?

11-17-2008, 10:11 PM
Is there a way of rigging up nodes or something Lscript-ish to directly manipulate a surface's alpha channel rendering?

For example, I'm rendering a pass of HV sprites of dust being kicked up by a spaceship landing. The ship has layer-based gradient textured transparent exhaust plumes that need to "knock a hole" in the alpha of the HVs the same way as the ship is knocking holes in both the alpha and rgb via setting all its surfaces to constant value = 0 in surface editor -> advanced -> alpha channel and rgb channel rendering black via object properties -> Matte Object = black.

The problem is, constant value = 0 doesn't work for transparently textured surfaces (it's ignored by RT cameras rendering HVs even with ray trace transparency enabled) and obviously setting it to surface opacity creates a normal grayscale where white is opaque and black is transparent.

What I need is for this normal grayscale alpha created by the transparent surfaces to be SUBTRACTED from the grayscale HV alpha, leaving a "black hole" where the plumes are in front of the HVs. Clear as mud?

But HOW??

I cannot synthesize a reliable grayscale alpha channel equivalent from the rgb image by simply converting to grayscale and using levels to compensate because of the varying range of hues and luminosities inconsistent with an actual alpha when the rgb is desaturated.

Any help/ideas?