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crisp
03-26-2007, 03:25 PM
Greetings,

I'm trying to composite a 3D object into some live action footage. I can get the object tracked and placed over the footage with no problems, but how do I get the shadows from my render to carry over to the live action footage? Is there a way I can render the shadows by themselves as a separate pass and multiply them in? I have a couple of guesses, but frankly it's a new area for me. I've done a lot of overlay stuff, but nothing that was supposed to actually be part of the scene. I've been looking through the LW documentation but can't find anything on it so far.

If I can't render out a separate pass, then how would I go about getting the cast shadows from the 3D scene to my 2D footage? Any ideas would be greatly appreciated! :help:

Thanks.

mlinde
03-27-2007, 08:01 AM
Leave on the cast shadows option, turn off camera visibility on your primary objects, and then bring in the shadow image sequence to your compositing app.

crisp
03-27-2007, 08:36 AM
Thanks for you response mlinde.

Just to be sure I understand, I obviously need to have an object in the scene to catch the shadows. I assume it would need to be white. It that correct?

marble_sheep
03-27-2007, 05:00 PM
Thanks for you response mlinde.

Just to be sure I understand, I obviously need to have an object in the scene to catch the shadows. I assume it would need to be white. It that correct?

Something you could try is changing the Alpha Channel drop-down to "Shadow Density" under the Advanced surface tab (in your ground plane's surface settings). This will create an alpha based on where your shadows are cast. (I don't remember off the top of my head, but I think this method only works for ray-traced shadows.)

Then, in your compositing app, extract the alpha channel and use it as a matte to color correct your background layer. Try to avoid the temptation to just multiply the shadow pass over top the BG... it's easier, but won't give as realistic of a result as if you actually color-corrected (darkened) the area affected by the shadow matte.

mlinde
03-28-2007, 07:42 AM
Thanks for you response mlinde.

Just to be sure I understand, I obviously need to have an object in the scene to catch the shadows. I assume it would need to be white. It that correct?
Nope. See example below, but you just want to check the "Unseen by Camera" box for the object you need a shadow from.

crisp
03-28-2007, 06:04 PM
marble_sheep: Thanks for your recommendations. Changing the alpha made all the difference. It was pretty smooth sailing after that. It hadn't occured to me about color correcting vs multiplying, but not that you mention it it totally makes sense. I'm definitely doing it that way from now on. I don't know if you have other ideas about getting the shadows from other lighting situations. I will experiment and see what I figure out, but if you have any ideas, I'd love to hear them.

mlinde: Thank you for your patience. I'm pretty green. Can you tell:D

mlinde
03-29-2007, 08:16 AM
marble_sheep: Thanks for your recommendations. Changing the alpha made all the difference. It was pretty smooth sailing after that. It hadn't occured to me about color correcting vs multiplying, but not that you mention it it totally makes sense. I'm definitely doing it that way from now on. I don't know if you have other ideas about getting the shadows from other lighting situations. I will experiment and see what I figure out, but if you have any ideas, I'd love to hear them.

mlinde: Thank you for your patience. I'm pretty green. Can you tell:D
No worries. Everyone was green at one time...