View Full Version : Removing Alpha While Retaining Reflect

05-21-2006, 06:41 PM
Hi guys,

I'm trying to render a scene for easy compositing, and I've ran into a problem I have yet to encounter till now, and I'm running out of time to finish this work off. So time spent trying to work this problem out, and failing, is digging into my rendering time, so any help would be much appreciated :)

Essentially I'm after a way to select certain objects, and get them to reflect the others, but not render these other meshes (primary visibility off) so I can save render time, but also have these reflected-only objects cut black-hole alphas if they happen to obscure the visible objects.

* I can apply a use-background, but then I lose the reflected objects shading.
* Primary Visibility alone does not remove alpha from the scene.
* Matte Opacity does the trick, but it still draws these other time-consuming objects at render time; this is not an option just for a simple alpha for a smaller object in the scene.
* I could render alpha passes, but I'm already dealing with a shit-load of layers, and I just don't want anymore :)

I would have hoped the render layers supported this kind of render partitioning.

Thank you

05-21-2006, 07:36 PM
The only way i know to do this is to use puppets mega TK shaders. It will give you a "true" reflection pass.


Take a look at the shake composite and you will see the reflection pass. Don't think there is a way to get that type of pass straight out of maya.

05-21-2006, 08:18 PM
Hi T_R, thanks for the reply, I've seen that shader before, but well.. I'm not after a reflection pass exactly, I guess I could make one, but it would further complicate things :)

Attached is an image (which I should have done on post #1) because it is a difficult thing to explain exactly.

Getting mentalRay to render a single sphere, while reflecting the other which is "cutting" the alpha away.


05-22-2006, 12:01 AM
Hi there Vincent - long time buddy :)

Is this what you had in mind? (no passes - just a normal alpha in Maya):

Movie here:
file here:


05-22-2006, 12:50 AM
I'm guessing you used the ctrl.rays shader Atwooki? I think that'd be my answer. You can use it to supply the regular shading network to the reflection rays while a black shader is supplied to the eye rays.

05-22-2006, 01:38 PM
Vincipoo my ill-accented friend were we too slow? Did you miss the deadline? I promise not to laugh if you did. Honest.

No really I may laugh briefly however I'll try not to enjoy it.

05-22-2006, 02:28 PM
Nah - just constrained a shading mapped sphere (primary visiblity ON with 'Black Hole') to the checkered one (primary visiblity OFF);

On reflection (no pun intended!), your ctrl_rays answer would have done the trick of course, Jared ;)


05-22-2006, 07:36 PM
Hey Chris, long time indeed. Just scrubbed through your galleries, looking good man! How is the project going exactly?

Oh and that work-around will do nicely, thanks a lot. Description, quicktime, and a scene file. You shouldn't have.. :)

Not sure of the affects of doubling my 350,000 polygon scene though, this might not be an optimum process, but I'll test it out after this render :) If all else fails, I can just spit out a geometry alpha pass... but I really shouldn't have too. Primary Visibility and right underneath a Primary Visibility + Alpha Knock-out attribute would be terrific, Autodesk.. (eh, still get the creeps saying that). :argh:

And Joz, the deadlines are personal ones, which can be moved, but ultimately you feel shitty not meeting them on a regular basis. But if I'm going to spend weeks making something, like hell I'm rendering with low-AA just to render quickly.. ;)

05-22-2006, 09:40 PM
Hope that's good for your huge file size Vince :D and:
Oh and that work-around will do nicely, thanks a lot. Description, quicktime, and a scene file. You shouldn't have..
You kidding me?!
I owe you one from time (sure know what I mean!) my friend

Yeah, the short's going really well; tho' many interruptions atm - been a 'daddy' now for a little over a year ;)



05-23-2006, 05:45 AM
Congratulations, dad! :)

Always on PM/e-mail if you want to fire through any significant progress on the short, would love to see more and catch up.

Thanks again too.

CGTalk Moderation
05-23-2006, 05:45 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.