Shadow casting on invisible objects

[gmask]

Originally posted by Andrei2k
I am doing some realistic compositing for which I would like to use props to simulate shadows on surfaces which exist in the footage im compositing my 3D over. I tried using a use background shader and the reder passes i can't get to work like it should I don't think......what am I doing wrong? or what other method should I use if I want to render shadows on a surface but not the surface so I can just apply the shadow on top of my footage...:argh:

Thanks in advance

I think you'd use one or there other but not both. For the objects casting shadows if you do not wish tohem to render but still cast shadows you need to turn their primary visibilty off.

[Andrei2k]

The problem is with the objects recieving the shadows.....I need them not to render but the shadows on them to render.

[gmask]

Originally posted by Andrei2k
The problem is with the objects recieving the shadows.....I need them not to render but the shadows on them to render.

if you are using shadow pases then this should not be a problem. The shadow pass does exactly that.

[tauf]

Originally posted by Andrei2k
I am doing some realistic compositing for which I would like to use props to simulate shadows on surfaces which exist in the footage im compositing my 3D over. I tried using a use background shader and the reder passes i can't get to work like it should I don't think......what am I doing wrong? or what other method should I use if I want to render shadows on a surface but not the surface so I can just apply the shadow on top of my footage...:argh:

Thanks in advance

Some of this might be old knowledge, but it's easier to take the explanation from the top...

First you'll want to create an appropriate 3d geometry representation of the real world object that you want to receive the 3d shadow. Move that object so that it adequately sits on top of and matches perspective with the real object in the background plate. Assign the background shader to that object, and then add whatever other 3d object in the scene with a shadow casting light to cast a shadow on the first object. Use the attribute editor for the first object to turn off Primary Visibility in the Render Stats, and you should be good to go.

[Deepray]

¿ not using raytrace ?... without raytrace, transparent objects cast shadows.... :shrug:

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