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View Full Version : help in compositing motchmove + live plate

halogenic139
07-15-2003, 07:36 PM
ok, i am a complete beginner and I want to composite a 3d model into a 2d plate that i shot. And I have a lot of questions ;)

Hopefully you guys can just sort of read quickly through and then answer any ones you know about.

First of all, lets say I've rendered out a 3d object and want to composite into a video. Do I render the object with an alpha channel and then use that as a mask? Do I also render it with an opacity channel? (Becuause I still want the ground to show up under the shadow of the object.) Also, how do I ensure that the 3d object will be in exactly the right place in my footage? Should I render it against a single frame of the movie plate in the 3d program, place it, then render out the whole thing, making sure the resolution is the same as the video so they will be the same, or is there another way to do it?

I think that's it...

Really, thanks for the help, dont know what I would do without you guys :cool:
jlelievre
07-15-2003, 08:14 PM
Hi halogenic139 !

First step is: does the 2d plate contain movement, or is it a locked off shot? If it locked off (i.e camera doesn't move) you can line it up in your 3d app to make sure the model is in the right place, scale etc. if the 3d plate contains movement such as rotation, then you will have to track the footage in some sort of tracking app. (Matchmover, Boujou).

When compositing a 3d object onto footage you will want to render it out as a file type that contains an alpha channel. This will allow your background to show through. For the shadows what you may want to do is render them out as a seperate layer. This will allow you to control the opacity, and the colour of them a lot easier.

Hope this helps! :)

Joel
http://www.intrinsia.net
halogenic139
07-15-2003, 10:19 PM
thanks Intrinsia, so if I render out the model as a sequence of images with an alpha channel, the compositor will automatically know it and use it as a mask?
jlelievre
07-15-2003, 10:28 PM
that is correct. :)
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