Bounced Illumination from Live Action Plate
Sometimes our CG objects doesn’t seem to quite sit in although our IBL is working just fine. We might need to incorporate bounced light that may come from CG surfaces such as the floor or wall.
To do that, we first need to activate the mental ray production shaders. It’s just locating that mel script and changing the value to 1. Restart Maya and we should see the mip shaders.
Now we just need to connect whatever image sequence or texture outColor into the map attribute of mip_cameramap node, and then connect it’s output to the background attribute of mip_matteshadow. Finally we assign the matteshadow material to our road or wall.
These shaders seems to be a better replacement to the use Background shader.
In order to render these out as passes, we might need to isolate our CG object from the floor or wall, which means catching only the shadows just like the use background shader. The slight tweak to the shader network would be as follow:
Plug the cameramap output to the environment attribute of mip_rayswitch_environment. Create a mib_color_alpha (under data conversion), set factor to 0 and plug its output to the background attribute of rayswitch node. Finally plug the rayswitch output to the background attribute of matteshadow.
Now we have a nice alpha channel to work with!