A couple of questions

[trixian]

I'm curious as to why:

• The alpha channel is not used in the multimatte render element for a 4th channel mask?
  Right now it inherits the "scene" alpha, witch is included in many other elements in addition to the separate Alpha output. Seems a waste.
• The render element VRayMtlSelect would be a hell of a lot more useful if its alpha was the active material and not the global scene alpha as presently.
  Is getting the materials "mask" complicated or prone to edge artifacts the reason?

Its possible I have completely missed the point of the VRayMtlSelect element, as I can't seem to see how it would be used in its present state, so please enlighten me.

[vlado]

The alpha channel is not used in the multimatte render element for a 4th channel mask?

V-Ray does not support RGBA render elements right now; this is because the alpha is not really a single number, but a color (for the case where you have colored transparency or refraction). I know that having an alpha that can be used as "mask" for some of the elements (material select, reflections, refractions) etc can be useful, although still limited to grayscale transparency only. This is on the "to do" list and will be implemented.

Right now it inherits the "scene" alpha, witch is included in many other elements in addition to the separate Alpha output. Seems a waste.

This "alpha" is written only in the output files; it is not stored in memory so it does not waste any resources.

The render element VRayMtlSelect would be a hell of a lot more useful if its alpha was the active material and not the global scene alpha as presently.

See the first comment above.

Best regards,
Vlado

[trixian]

I'm not sure I really understand what you are getting at when you say the alpha is not really a single number, but a colour. I was under the impression that, for 8 bit images et least, the alpha channel is just another place to store pixels in a 0-255 range, equal to the red or other channels. This is the norm for storing utility textures in games to minimize memory usage and state changes for shaders.

Why would coloured transparency need a coloured alpha channel when the colour is already in the RBG channels pre-multiplied against one's BG (Black)?
AFAIK, Nuke also seems channel-agnostic for most formats where one can shuffle them in and out practically at will.

Regarding my comment on it being a waste; what I meant was a waste of cpu cycles and time to write, waste of disk space (the files are larger with an alpha than without regardless if the alpha is usefull, whitch it is not as it can be considered spam since we already have this information in other files we output), and the fact that those of us stuck using photoshop lose pixels when opening certain formats (png\exr etc. thanks Adobe dev. team. Keep ignoring your users requests).

More specifically, VRayMtlSelect only needs this grayscale information to mask out the specified material. Isn't that the whole point of the element, to render out only one material?
How is this element supposed to be used in a practical situation the way it is now, with large parts of the image black, with select areas showing the desired material? You can't add or multiply, and you wouldn't anyway as it contains the beauty pass of your material. Are we supposed to hand mask it out?