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Getting only the reflections on a non-transparent floor

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Paulo Jan
Getting only the reflections on a non-transparent floor
on Jul 1, 2014 at 5:21:24 pm

Hello:

I have a problem similar to the one described here:

http://forums.creativecow.net/readpost/19/865051

Basically, I have an object reflecting on the floor. The rest of the floor is black, the only visible thing on it is the reflection, and I want to output only the reflection, without the rest of the floor, so that I can composite it later in After Effects on a non-black background.

The solution mentioned in the thread above was to render a separate reflection pass and composite it on top of the background, but in my case it doesn't work, because if I composite it with the "add" mode, the black (0,0,0) parts of the image get knocked out, as they should... but the reflection gets much lighter (since it gets added to the value of the underlying background).

Basically, I need to isolate the reflections from the floor object they are on, either on C4D or on After Effects. I need to output a pass that has only the reflections, with the rest of the image being transparent (not just black, but also transparent in the alpha channel). Is there a way to do this?


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Brian Jones
Re: Getting only the reflections on a non-transparent floor
on Jul 2, 2014 at 5:31:52 am

is the reflection at 100%


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Paulo Jan
Re: Getting only the reflections on a non-transparent floor
on Jul 2, 2014 at 8:51:44 am

I thought so, but now that I see it... no, it's at 70%. (I assume that you mean the reflection parameter in the floor material). Is that important?


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Brian Jones
Re: Getting only the reflections on a non-transparent floor
on Jul 2, 2014 at 3:44:24 pm

it should, the reflection is always going to appear lighter on something other than pure black as whatever colour your object is has to be adding to the reflection. Starting with the max amount of reflection image has to help though. If you are still not getting as much as you need you can always duplicate the reflection layer in AE.


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Adam Trachtenberg
Re: Getting only the reflections on a non-transparent floor
on Jul 2, 2014 at 11:18:39 pm

The effect can also depend a lot on the background you're compositing over. For example, if you're comping the reflections onto a solid color background it might look weird because we're used to seeing reflective objects reflect a world and not just objects placed on them. So instead of rendering just a black reflective plane with objects on it, you might also want to add a sky object with a gray color or some kind of blurred image applied to it.

For example, here's a render with no sky object:



And the same scene, but with a sky object on which I've placed a 50% gray material:



The object reflections aren't brighter in the first comp, but they appear to be because the objects are the only things being reflected.


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