How do I render an image that is just an alpha channel?

#1

I am using mentalray and I go into framebuffer and select Alpha (Byte) 1X8 Bit (and I’ve tried the others too, no difference) but I keep getting images with color as if I hadn’t given a different setting.

I see in the script editor when I render (not batch render) “output data type “a” not directly supported by image format tif, conversion performed by mental ray” and I think that might have something to do with it.

Yeah, I know I can just use the alpha channel from an RGBA but I want to have the option (to save on render time and hard drive space) to output footage that is JUST an alpha channel.

How do I do this? I’m not finding anything online that specifically addresses this.

#2

If I am understanding you right. Your just trying to make an alpha pass to be com posited. What I usually do at the studio is I render multiple passes for Motion Blur Depth of Field and if anyone needs a specific element to be cut out from that background I will just make an Alpha pass specific to that object.

What I do is select the object that I want to make an alpha layer for. Create a new render layer and just shade that object with a white surface shader. Which I usually render it in Maya Software to make a quick render of.

Now in Vray passes are insanely fast to make you just set up the passes and then your good to go!

#3

Moved to the rendering subforum for you. Please, do use the subforums when appropriate (rigging/rendering/dynamics/programming).

#4

@Jaco: Fair enough, didn’t know if it was appropriate to put it in the rendering subforum just because it didn’t seem to me to be a super technical question to ask.

@Crezz: Thank you for the advice, that’ll be the way I go about it, no can do for vray for me.

#5

You can also check out mental ray render passes ( “Coverage” or “Opacity” in your case )

#6

You could consider using contribution maps which are a convenient way of making matte images of groups of geometry.
The Maya manual doesn’t include much detail about the setup so
here is a link found with google search describing the process.
https://www.youtube.com/watch?v=9s99jQfd9nY