Photoshop -> using Z-buffer in compositing

[Ziggizag]

I have the following problem:

 

Very ooftenI would like to be capable of color correction of a single particular object in a 3D rendering. Though it is possible to make appropriate collor correction in 3D package - this is nasty as that require additional rendering of the complete scene what is time consuming.

 

Ideally, I would like to make do such minor color correction is compositing software - in case of stills - in Photoshop.

 

But... how to do this?

 

I tried to make two render passes:

 

A) rendering the scene having one particular object set invisible to camera (but visible to GI solution)

 

B) rendering the scene having all objects but this one set invisible to camera (but visible to GI solution)

 

That could work but it fails in case given object intersects with other in 3D space.

 

Normaly, 3D camera only renders these object's parts what are visible to camera, so these parts what are obscured by other object are not renederd. But if I render given object "separately" - then I have all parts being rendered - even these what are normally invisible as obscured by other objects. and in case of intersecting elements - alpha channel provides unsufficient information to perform appropriate selection to remove unneeded parts.

 

Normally, this information if provided with depth channel: the Z-buffer. But unfortunately, I can't see any use of it in Photoshop.

 

Ideally, some specialized "layer blending mode" should be implemented in Photoshop. It could work in the following manner:

 

Presuming there are two Photoshop layers:

 

- bottom layer containing (A) render image masked with its Z-buffer channel

- top layer containing (B) render image masked with its Z-buffer channel

 

Using specialized blending mode, only these pixels would be revealed what are actually "closer" based on appropriate Z-buffer data.

 

Unfortunately, there is no such blending mode in Photoshop, so I am stucked. Do you have any idea how to overcome this issue? How "the professionals" are doing it?

[Peter M. Gruhn]

Sounds like what you want is a render by material or object ID. Something like that is in there somewhere.

[JamesL]

You want to go down the route of Render elements like Peter says - if you're using Vray then I find that VrayWireColour is the way to go.

 

Alternatively, if you are using Vray and are desperate to get an alpha channel for one particular object then select all the other objects in the scene and go into the Vray Object Properties (right-click or in the Render Settings tab) - set all the objects to Alpha = -1.0.