Rendering reflections only

[tricky]

Is it possible to render ONLY the reflections? I don't want the reflections on a black background - just the 100% reflection.

[vlado]

I suppose you want to get the "alpha" channel for the reflections. You cannot do this at present. This is because reflections are always added to a non-reflective scene, in which case, you don't need any alpha.

Best regards,
Vlado

[tricky]

Could this be an easy 'tweak' for you to make? It would save me hours messing around with rough looking hand-made reflection alphas.

[vlado]

Could this be an easy 'tweak' for you to make? It would save me hours messing around with rough looking hand-made reflection alphas.

Unfortunately it's not that simple. Reflections don't have an alpha, they have a filter color, so you don't need just a single alpha value - you need two separate RGB channels - one for the reflections themselves, and another - for their filter color. You will need to multiply those two channels, to get the actual reflection channel, which is then added to the image. This can be implemented relatively easily, but I'm not sure if it will be any simpler to use.

Best regards,
Vlado

[cpnichols]

This would be a very important wishlist, especially if you are doing live action + CG.

[Morbid Angel]

i guess the simplest way is to comp the reflection using addtive or multiply...

[cpnichols]

That will not really work... you need an alpha to occlude the objects that are in front of the plate that is being reflected. You could do it, but doing some sort of reflection occlusion and pipeling that into the alpha.

[Morbid Angel]

you can have forground objects as matte. or not visible to camera, i think scanline can do reflection alpha tho...myabe im wrong

[cpnichols]

do the reflection... then do a pass where the object you want reflected is black, the enviroment is white and reflect that. Then pipe that into the alpha of the reflection pass.

[Morbid Angel]

actually i think we misunderstand what tricky wants, he wants the reflection not to have black, but have alpha where black, that has not to do with the surrounding objects, but i think all he wants is to put the reflection on top of his base pass and have it work rightaway and not do tricks with it.

[tricky]

You are both sort of right. I want the reflection entirely seperate and alpha'd out so I can fine tune reflections in post. Basically, at the moment we apply a 100% opaque 100% reflective material to all glass and run a 'reflections only' pass. In PShop, we then boost all the levels to some extreme level and delete all the black. This method leaves very dirty edges to the reflections and isn't a nice way to work.

cpnichols - you seem to be exactly on my wavelength and your suggestion of the 100% white/black pass may work - I'll give it a try in my next job. The only thing is its another step in the pipeline.

I truly thought it would be a relatively straightforward step to create a true reflection only pass.

[tricky]

Pld thread, but any update on getting a 'matte' for just the reflections at rendertime?