Render shadows only for 1 object?

[notamondayfan]

Hello,

I'm trying to figure out how to render only the shadows for an object within the scene.

In this instance, I have a bathroom. I want to show several bath models, so I have rendered the room without the freestanding baths. I have then rendered the baths. Now I need to render the shadows for the baths, as a separate pass, so I can overlay the shadow in post.

I can use the matteshadow element, but this doesn't achieve what I'm after, because the walls, windows, etc all need to block the light, without casting shadows. If I turn off shadow casting for the walls, then the light spills through, casting false shadows.

Is there a way to do what I'm trying to achieve?

Thanks,
Dean

[notamondayfan]

Figured it out so don't panic everyone!

I ended up doing it in post. Rendered the room without the bath, then with the bath.

In Photoshop, I then took the raw lighting layer from the empty room, and the raw lighting from the room with the bath. Then used the difference blending mode, and masked out the bath itself, and BOOM, shadow only data

Thank you and good night!

[Oleg_Budeanu]

Phew, panic mode postponed!

[notamondayfan]

Phew, panic mode postponed!

Panic might be a bit extreme more like frustration! All happy now though!