Reflection Alpha problem

[Chris]

This is a known bug with raytraced reflections. The way I normally would get around it would be to select everything but the reflecting object & uncheck 'visible to camera' in the properties. Then assign a matte shadow material to the reflecting object & give it the right amount of reflection. Make the background black & render (without an alpha channel - you dont need one) Then just composite this pass over the top of the other passes in 'additive' mode...

[Oren Ben-Tov]

Thanks for the answer :)
but maybe I confused you I ment that I have a character image reflected like on a window, and I cant see his reflection in the compositing program :
something like this:

[Chris]

yep, that is what I'm talking about. it will work fine if you just render an RGB image, but if you have the background window partially transparent the reflection will not be considered in the alpha channel. so if you turn the alpha channel on in the compositing program, the reflection gets alpha'd out & dissappears... So are you compositing that image over another, so that the windows are transparent? If so I would do what I outlined above - create another pass just for the reflection & comp that over the top. You are also going to need to mask the reflection where his body obscures it. you could either do yet another pass with his body self illuminated white & everything else black - then use that as a stencil image for your reflection pass (thats what I'd do) or you could make a copy of the man, assign a black material & uncheck 'visible to reflections' in the properties. Then render out the reflection pass. the black man will then obscure the reflections so you could just comp the one reflection layer over the top...

[Oren Ben-Tov]

i found out how to see reflection.raytrace in combustion with out having to render a second pass of just the reflection.
if anybody interested let me know

[Boa]

Would be nice if you could post your solution here in the forum for all to read.:airguitar

Boa

[Oren Ben-Tov]

ok this is for combustion( you can probebly do the same in ae)

I make a copy of the layer that have the reflection, and make both of them - color + alpha.

The lower layer have the mode - "fully addativ mix", and opacity 1.

In the operators I gave it the operator - "alpha levels" and I play with the sliders untill i can see the reflection.

this is it more or less, play with it a little, it will now show the reflection that wasnt shown with the color + alpha mode.

[MasterZap]

Yeah I was gonna come to that: You'll have to tell the program you are compositing in that your alpha channel is pre-multiplied w. additive mixing. Unfortunately, my version of AfterEffects doesnt support that at all. *argh*

Its all from the old confusion of what the alpha channel "really is". *sigh* (there are multiple interpretations)

/Z

[CGTalk Moderation]

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.