Maya - Render Shadows without Groundplane

Alphavader

Hi there,


a simple question, how is it possible
to render an object with shadows but without the
Groundplane.

Id like to do postprocessing in Photoshop and iam looking
for a way to get rid of the groundplane.

(I only want to have the object with shadows (transparent)

Here a little image thats explains better



Thanks

kodde

What does the Alpha channel look like in this render? It should at least have all the geometry as white, hopefully for your sake also the shadows. If not you can do a second pass with the use background shader(if I'm not mistaking) on your geometry which should give you just the shadows in your alpha.

Essentially, the Alpha channels can be used as masks to alter/get rid or areas you don't want later in post.

Alphavader

Hmm no it displayed all or nothing,
isnt it possible to set recieve shadow but dont render ?!
Maybe the Alpha thing doesnt work so right, because i
set different renderlayers...

kodde

Alphavader> I'm not used to working with render layers, but that might be it. I'm fairly certain this should work though.

Another thing worth mentioning is that if you do use this technique and your using premulitplied alpha (this is default) it might be worth setting your render background color to a color close to what the actual background is that you will be using later. This eases away matting problems you otherwise might get. For instance rendering to a white background, removing the white background with the alpha, adding a black background. This could cause white matting issues around the edges of the masked area.

tharle

you could try rendering out a key pass (i think thats what they're called). basically render the scene once normally with mental ray and nice colour bleed etc. then just apply a bunch of primary colour surface shaders to the objects and rerender again using scanline (or something that won't cause colour bleed) you can then use the second pass as a set of selection masks for the first one. it's a pretty common vfx thing in film i think.

chronic

the mip_matteshadow node is probably what you are looking for.

its not exposed to the interface by default, but you can create one from the Mel command line like this

createNode mip_matteshadow

rube

use the 'Use Background' shader. Turn off the spec and reflectivity settings. Shadow Mask will affect the opacity of the shadows. That way your ground plane will be transparent except where the shadows are cast.

Alphavader

Hi, thanks for the replay..
@kodde, youre right, but it doesnt work well with my settings, i need the white background for self illu
@Rube, "Use Backgroundshader" is an option, but doesnt work well with mental ray !
@ Chronic, the mip_matteshadow is kinda the right thing, but i dont know well how to use the shader..
I googled and there is about rayswitch and matteshadow_camera, can you give me a quick quide, how to use the shader ?


thanks !

ps: found this :
http://www.djx.com.au/blog/2008/04/08/mip_matteshadow-and-how-i-have-used-it/

i can say more, if it works - if someone has a better solution - let me know

Alphavader

I cant connect the shader to the object (groundplane)
any suggestions ?!


but in the digital tutors tutorial (wich is awesome)
http://www.digitaltutors.com/11/training.php?vid=13469&autoplay=1
he can

Alphavader

hmm sorry but still didnt get it right,
i set up the shader and it rendered well, but its not
seperated from the bg.

(i just connect the groundplane to the matteshader)
1. turned black
2. set up the lights (mental ray need a white bg for better self illu
3. rendered it out - worked well but still no seperation...

plz help

Alphavader

Okay, the right definition of renderlayers + mip_matteshadow + transparentshader (under matteshadow bg) did the trick,
everythink has an alpha channel now..
Its not complete that what i wanted, but i can work with that.

EDIT:
Solved it: In the layeroptions (AO/Shadow/Beauty and so on) there is a checkerbox : 3 Stands for the RGB - chance it to 4 to get the Alpha channel !

thanks guys for your help