Darker shadows on Matte objects

[paraganek]

We are having problems to composite the following layer structure:



Link to full size image http://img127.imageshack.us/img127/6...shadowsnm7.jpg

There are two problems:

a. there is a halo on the borders between shadows from normal pass and matte pass

b. the shadows on matte objects in the matt pass are slightly darker then the shadows in the normal pass

There is no GI turned on in the scene, just one direct light and 3 fill lights.

I know one solution would be to render NO shadows in the normal pass and render ALL shadows (NO GI on other mattes unchecked) in the matte pass, but I am just wondering if there is any other way to get rid of those compositing errors.

Daniel

[vlado]

The bright edges are an issue that often gets people and I've had to explain it time and time again... it is not related to V-Ray, but is a general compositing problem and the solution boils down to this: do not multiply anti-aliased elements/layers which have high-contrast transitions along the same edges. In this case, for example, the bright edge on the greenish box in the normal pass, and the shadow boundary along the same edge in the matte alpha pass.

The reason is that because of antialiasing, the light is diffused along the edge boundary, leaking into areas which wouldn't be illuminated in a regular rendering.

One way to reduce the effect is to render and composite in higher resolution, and then scale down the result.

Best regards,
Vlado

[paraganek]

The reason is that because of antialiasing, the light is diffused along the edge boundary, leaking into areas which wouldn't be illuminated in a regular rendering.

Thank you Vlado for a quick response.
It is clear to me now why I am getting the halo on the edges but that still does not explain the darker shadows in the matte pass.
Any ideas ?

Daniel

[RErender]

Maybe the different shadows are a result of differing GI amounts between normal and mattes? Do yo have the same material on the mattes?

[vlado]

It is clear to me now why I am getting the halo on the edges but that still does not explain the darker shadows in the matte pass.
Any ideas ?

How are you compositing the renderings more precisely?

Best regards,
Vlado

[paraganek]

It is a simple two layer compostition in Combustion, each image rendered and saved as PNG with an alpha channel.
It looks the same in Photoshop too .

[vlado]

And the extra lighting in the shadows is coming from ambient light?

Best regards,
Vlado

[paraganek]

And the extra lighting in the shadows is coming from ambient light?

I do not think so. This is the lighting setup:



Just one direct light with Vray shadows and 3 fill lights. No GI.
I can post the scene if it is going to be any way helpfull to figure this out.

Daniel

[vlado]

Yes, the scene will be very helpful. You can either post it here or email it to vlado@chaosgroup.com

Best regards,
Vlado

[paraganek]

Problem solved

It was my fault, those boxes had no material assigned using just plain max wireframe colors.
As soon as I assigned either standard or vray materials, the shadows got consistent.

Daniel

[vlado]

Ok then, good to know

Best regards,
Vlado