Compositing multiple shadows from passes

[robdeja]

Hi all,

We are currently working on a large animation project for which we need to render out many different passes due to the scene size. Basically we are seperating out our passes into the following elements

1. Landscape/Buildings/Street furniture
2. Trees/Plants
3. People
4. Leaves/Dust/Dirt etc. (particles as a pass)
5. Background Matte Paintings

Each of these will be rendered out to the following passes to EXR:
- Diffuse
- Specular
- Reflection
- Refraction
- Shadow
- Ambient occlusion
- Velocity
- z-depth
- object id
- coverage

While we have rendered out these passes many times before and successfully combined them back together in Fusion, we haven't had the need to render so many passes each with their own shadows which need to be composited all together - this is where the problem is.

I know there have been many posts regarding compositing of passes from a single render but I couldn't find any which cover combining of passes from separately rendered elements...

What is the methodology of getting each of these seperate passes to combine shadows (without them doubling up) using the vray shadow pass?
For example, the buildings will cast shadows on the landscape and then in a seperate pass, we require the trees to also cast shadows on the landscape and buildings. The people will receive shadows from the buildings and trees and also cast shadows back onto them!

I have successfully managed to get all elements & passes combined except for the shadows which seem to render differently in Vray. The vray shadow pass seems to be a diffuse pass for the areas in shadow and thus has diffuse colour. In other render engines, the shadow pass is a black/white mask which I could add to other shadow masks to build up a complete shadow for all elements.

If anyone could offer any advice on this it would be most appreciated.

Many thanks

[joconnell]

Yup - vray is taking into account ambient light and that the shadow is only blocking a certain colour of light (eg blocking yellow sunlight leaving a blue shadow where some of the skylight still leaks in). The main thing is that you have to make sure a similar light source isn't doubling up its shadows - say for example your people pass and your tree pass - You'd have to layer together the shadows for both trees and people in such a way as they don't darken where they overlap - probably by precomping them together first using blending modes and then using the result of that into the master render layers comp.

Try stuff like the math min and max modes or even simple screens or lightens to combine the shadow and reflection layers together first and then bung them into your master comp.

[robdeja]

Thanks for the reply.

Yeah I tried the method you suggested but it's not exactly ideal. You always get slight fringing around the edges of the individual shadow passes when they are combined.

There must be a better way?!

[vlado]

You should use the VRayMatteShadow render element then; it is a black/white pass without the diffuse color.

Best regards,
Vlado

[robdeja]

Ah yes!
But...the VrayMatteShadow pass seems to render incorrectly.
Notice the areas in shadow showing up some white artifacts.

[vlado]

You can deal with this either by increasing the poly count on the teapot, or by increasing the Shadow bias parameter for the light.

Best regards,
Vlado

[robdeja]

Ok, I'll try increasing the shadow bias.
Increasing the polygon count is not an option for my scenes which suffer from the same artifacts in the VrayMatteShadow pass.

Thanks