Integrating CG into bg plate: issue with receiving shadows from bg plate

[Fredrik Averpil]

Hi,

I have a still photo which I intend to integrate CG objects into. My issue is that when I make my CG object sit on the floor, there is not contact shadow 1.) applied to the CG object and also 2.) no shadow cast on the floor visible in the CG object's reflections.





This is my setup:

I've reconstructed the room in Maya...



...and applied a material wrapper on all meshes. As base in the wrapper, I'm using a light material. The light material is projecting the photo onto the meshes.



When I render this, I get all my CG objects against black and the shadow is present in the alpha channel (which is great for compositing later on):



However, I notice (in the render against black above) that I don't get contact shadowing (of the floor) in the CG object's reflections. A CG object sitting on the floor should also get shadowed at the bottom of itself by the fact that it is close to the floor.

If I place a plane at the base of the CG object and turn primariy visibility off, I see that this contact shadow I am looking for is appearing. However, I am trying to find a solution in the shader network rather than having to work with double meshes...

Any ideas on what I should do to make that work?
My issue is basically that I can't cast shadows onto a light material, I guess...?

The light material is all at default values. This is what my wrapper materials looks like:



Any help much appreciated!
I'm on one of the most recent nightlies.

[Svetlozar Draganov]

Hello Fredrik

In general Wrapper material is designed to work with background image, so instead of using Light Material with projected texture plugged into Wrapper just assign bg-plate into Background Texture slot and use regular VrayMtl in Wrapper Material.

Here is one simple example with the above setup:




WrapperMtl with BaseMtl=VrayMtl is used for the plane objects: Matte Surface is activated with Shadows and Affect Alpha activated. This setup will turn plane into shadow-catcher object and Background-texture behind it will be visible too.

Global Illumination is activated and this will ensure contact shadows from sphere to the plane and vice-versa.
The sphere objects has VrayMtl with reflections activated and the shadows on the floor are visible in sphere's reflection.

Since in your scene you won't have just one Matte object you may consider turning off NO GI on other Mattes - I don't think that it will be needed in the scene you show us but have in mind that this option by default is ON and when there are more than one Matte Object they won't influence each other.

SceneFile ==> cg.zip

[david m.]

I think he want to use a light material projection for an exact lighting of the real scene like this:

http://www.youtube.com/watch?v=K3nPBrESJeE

?


Best regards

[Fredrik Averpil]

Thank you Svetlozar, I'm sorry I haven't gotten back in a timely fashion -- your solution seems to have solved what I was looking for and I'm getting great results in my particular scenes. When final rendering I have to disable the background (in the environment overrides section) as I will get the bg plate rendered in motion blurred parts of the image as well as in the shadows, it seems.

David, I haven't had time to check out the video clip, but I'll do that.

Svetlozar, just out of curiosity, I have been meaning to try and mimic mental ray's mip_matteshadow setup but in V-Ray. Are you familiar with this and how this should be set up properly?
If possible, please look at the last three pages of this PDF to see what I'm talking about: http://www.nvidia-arc.com/fileadmin/...production.pdf
What's great about this setup is that it makes integration of cg objects incredibly easy and it renders super fast too. But ... it's mental ray.

[Svetlozar Draganov]

Since I am not very familiar with Mental-Ray I do not know what all these nodes do and thereafter I do not now how to mimic such setup in Vray.
Would it be possible to prepare a simple scene for us with all these nodes connected and give us a short description of how they work - then we'll think about how to replicate the same setup in Vray if it is possible.

Thank you very much in advance.

[Fredrik Averpil]

Definitively, I'll will get back with an example scene.