Map to control alpha channel in Vray

[Tommy L]

Hello everyone,

I have a plane in a scene with a black and white .avi controlling the mix in a Blend material. I would like the .avi to also be an alpha channel. Any ideas?

Thanks, Tom

[Tommy L]

I guess I could just render a seperate pass with all black materials and have the b/w map on a vray light mat. It would be nice to do it in one pass though. This map is the only thing in the scene Id like to be able to pull as a alpha in post.

[Crazy Homeless Guy]

Not sure exactly what you are doing, but maybe you could use a Vray Override material with the alpha animation in the refract slot, and set the IOR to 1?

 

EDIT: Actually, I am a little confused on how you use a map to control the mix blend?

Edited June 22, 2010 by Crazy Homeless Guy

[Tommy L]

Hi Travis,

The image attached shows a WIP for a promo animation. The wall material is a Blend material with two Vray materials in the slots. The mask slot is an .avi (an animation in black and white that is the graffiti being built). I want to be able to pick up that mask in the alpha in aftereffects.

[-MerlyN-]

I dont think there's a way in doing it in just one pass. Since the wall is completely opaque there's no alpha in itself. Only thing that crosses my mind is rendering the material id's as renderpasses, but I dont know, if a blend material will generate different material id's. And coming to think of it, can you render material id's as render passes at all? never tried that, actually.

 

On a side note, you might want to adjust your camera angle or increase the resolution of your graffiti mask avi. There's some serious pixelation going on in the foreground...

[Tommy L]

Merlyn,

Thanks for that. The mask is a stand-in. The .avi is 6000x1000 uncompressed. Weighs in at 12gig!

[Crazy Homeless Guy]

Awww...ok. I follow now. You need the alpha of the graffiti for use in aftereffects, and you need it so that it matches the perspective. Potentially for reveal masking effect or something along those lines.

 

OK, ....I like to think of work arounds, and this is the best I have come up with so far. This is done with Max2010, maybe Max2011 will allow something different.

 

I selected everything but the box, and gave it an alpha contribution of -1 in the Vray properties. This means it will not contribute to the alpha channel. I also added a Vray Extra Texture to the Render Elements. I placed the mask texture in the texture slot. In your case, this would be your black and white AVI.

 

Now render the sequence, and combine in Aftereffects. If you set the alpha image to a layer transfer mode of multiply over the extra texture image, you should get the resulting image you are looking for.

 

There is probably a cleaner/easier/more direct way.

[Tommy L]

Excellent! Thanks Travis.

[Tommy L]

Travis,

This works a treat. I owe you a beer.

[Crazy Homeless Guy]

Glad it worked, and someday I will take you up on that beer.