Maya Use Background Trouble..
 

Hi guys :),
I have a showreel to make but I am having alot of trouble rendering of this car for it.
I am trying to use the "use background" shader for the shadows but its only giving me solid black in my render. The camera is set up with an image plane for the backplate. I have also tried using a blinn with primary visibillty turned off, but no luck there either :(. If anyone could help me out here that would be great :). (it is HDR lighting)
here are some pics of my problem:
http://i204.photobucket.com/albums/b...seBGshadow.jpg
http://i204.photobucket.com/albums/b..._Car_useBG.jpg
http://i204.photobucket.com/albums/b...BGscreenie.jpg
oh, I have also tried disabling the image plane and just using the use background for the ground but still no shadow in alpha channel..

not sure how you are rendering this out but to take advantage of the use background shader you use it in tandem with the IBL node for your HDRI and render with Mental ray with final gather on .
also turn off the reflectivity and the reflection limit by setting them at 0 and the specular to completely black in the Use Background shader.You may have to turn off the default light also in the render settings of MR.

Hope this makes it a bit clearer for you .

steps for you
1) open the render settings window and set the renderer to Mental ray.

2) go to the Indirect lighting tab and create an Image based lighting node.

3) In your scene scale the IBL sphere to size surrounding your car and encompassing your scene.

4) with the sphere selected go to the attribute editor and in the field marked Image name browse to your HDR image.

5) create a plane for your ground and assign the Use Background shader to the plane.

6) select the plane and in the attribute editor go to the useBackground 1 tab and set the Specular color all the way left to make it black and reflectivity and reflection limit to 0

7) go back to the render settings and in the Indirect lighting tab turn on final gather.

8) go to the common tab and under render options turn off Enable default light .

9) set your render size and camera ect and render then save file using a format that will allow an alpha channel ..ie targa

You can render without primary visibility turned on for your environment if you want to use a low res HDR and comp it later

Thank you so much :), seems to be workin now, I think mabye the reason it wasn't working before was because i assigned my hdr to a polygon sphere instead of creating IBL.. not sure though?

yes that would be it..you need the IBL node to work the magic .