Reflection Alpha

[jonnybefree]

Hi all,
i have a veeery urgent problem here..
can i render a reflection pass as alpha???
i mean, just the reflections.
cause i have to add the reflections on a white bg.
when i render it before black, and add in on bg in ae, he removes all black, and the object is white too, so nothing remains....

do i talking shit?

[vlado]

You will need to render this in a separate pass - make the object perfectly white (e.g. with a VRayLightMtl material) and render without any lighting or GI.

Best regards,
Vlado

[jonnybefree]

thanks for the answer
mr. lele just got me same advice . and i've got it to work like that.

thanks again mates!

[sylk]

Is this really correct? When I try this method I get a black outline on my reflection.

1. I have a black enviroment a red sphere and a mirror floor.

2. I render my reflection with color

3. I then render the same render but with self-illumination 100% white

4. Then use my second render as alpha in comp.

I get a black outline because the reflection is AntiAliased against the black enviroment. Kind of premultiplied, and I cant get it un-premultiplied or straight.

Is there a way of doing this without the black outline? And without "screen" or "add" in comp. I want it "normal" in comp.

[joconnell]

Reflection is always calculated as adding on top so it removes the black line - in your main render it's being antialiased against the colour it renders on - Why are you using it normal?

[pjkw]

I'd really like to see this as a renderelement or something, like reflectionfilter but black when it reflects the enviroment.

[sylk]

Im using it normal because I want it to cover some other reflections. If I use screen I can see through it and thats no good.

[joconnell]

Im using it normal because I want it to cover some other reflections. If I use screen I can see through it and thats no good.

Yep you see that's where the rendering some in - in the areas where you see reflections, you see no diffuse - the more reflective an object is, the less you see of it's own colour - for example a mirror has no diffuse colour of its own since it's reflecting 100% of its environment. If you render a surface that's white for example with 50% reflectivity it means you'll get 50% of the white value (it'll appear as a 50% grey) with 50% strength reflections on top - the reflections effectively cut down the strength of the white surface lying underneath them. It's something that the vray material does automatically - in real life you can't have something that's pure white and fully reflective.

[rivoli]

Is this really correct? When I try this method I get a black outline on my reflection.

1. I have a black enviroment a red sphere and a mirror floor.

2. I render my reflection with color

3. I then render the same render but with self-illumination 100% white

4. Then use my second render as alpha in comp.

well, it should work, unless you're using ps and it's messing up somewhere along the way.

here's something quickly put together:

background plate



beauty pass (plus typo)



matte



comped together



it should look about the same if you render them three teapots in one pass. I guess.

[gagui]

What is the workaround if you want your reflections over a white background?

[rivoli]

shouldn't be that different.
I'd say you can try rendering exactly as above plus an alpha for the teapot alone, so that you can later tweak the reflection as needed (so that it blends the way you like over the white backdrop).
or you can render your object + reflection pass over a white plane, then render the 100% self illumination pass, then comp. object's alpha may or may not be needed, don't know. at the end of the day I guess it'd give more control when compositing, so..

[judderman]

Hi All,

I'm resurrecting this old thread rather than starting a new one because the problem I'm having is the same. Well, I say problem but I did get a render out, but my workflow might not have been the best, in fact it was a bit hacky.

I did what Vlado said and rendered out my object pure white with no GI to get a reflection pass that I could use as a track matte in AE. That worked grand, but it meant saving my scene as a new file so I could apply a vray light material to it - that's grand if your scene isn't changing, but since I am not finished working on it, after I've applied white to everything I have to go back to my main file, change the view and redo all the above again.

Is there a way I can temporarily apply a white vray light material to a selection of objects, like an override?

The next thing I'm having some problems with are the shadows. My model is being composited on a photograph of a wet road, so I have the model, it's shadows, and it's reflections on the road. I have normally just clicked on the road surface, and done alpha contribution -1.0 and set affect shadows and alpha. This looks grand sometimes, but the shadows end up coloured, and they're baked in to the diffuse render. I suppose it'd make more sense to have the shadows separate from the main diffuse render, but I've tried this, and it never looks right.

I don't need a million passes, just diffuse, reflections and shadows. Hopefully someone can point me in the right direction

Cheers,

Judderman.