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Compositing with live footage: holdout objects
show user profile  PaulJBis
Hi all:

I'm working on a shot where a series of 3D-generated objects will be "flying" around the head of a person that we've shot (which means that those 3D objects will be getting behind that person's head during part of the shot). From what I understand, the traditional workflow in these cases is:

1) Create a holdout object in 3ds max, which would represent in this case the person's head.
2) Add a matte material to it.
3) Make the 3D objects fly around the holdout object (so that they will be hidden by it during part of the shot).
3) Render out and composite.

Now, my questions are:

-Is this correct? Am I missing anything?
-What to do when the holdout object needs to have "soft edges"? In this case, let's say that the person in question has long, uncombed hair. Will I have to actually model the hair in 3ds max just for the holdout object?
-And what if I want to add the shadows projected by the 3D objects on the person's face? In order to make them realistic, I'd have to model the holdout object as an actual person's head, right? (It would not do to just create an sphere in the place of the head and such). Are there any tools to help you in these kind of situations? Software, 3D scanners... whatever.

read 779 times
2/13/2012 10:02:52 PM (last edit: 2/13/2012 10:02:52 PM)
show user profile  9krausec
As far as the shadows go on the face, you probably will have to model a rough head and apply a material to it that only renders the shadows. I just did something like this and will reply when i am home later tonight.

What renderer???

- Portfolio-

read 768 times
2/13/2012 10:22:53 PM (last edit: 2/13/2012 10:32:05 PM)
show user profile  Garp
I'd shoot the head on a green screen. That way you don't have to deal with edges.
Or if the backround it different enough, you might be able to key it out.
The idea is to have background/head/flying object piled in that order, and then use parts of the head footage to cover the transition when an object goes behind the head.

read 755 times
2/13/2012 11:23:00 PM (last edit: 2/13/2012 11:23:00 PM)
show user profile  PaulJBis
I still haven't decided on the renderer. Any particular recommendations?

Garp: so you are suggesting that I hide and show the 3D object "by hand", depending on when it goes behind the head? Isn't that... erm, terrible cumbersome? Also, what concerns me about the "soft edges" isn't to much the keying, but the fact that, if the holdout object has "hard" edges, the resulting animation will have the flying 3D object showing up when it shouldn't.

read 747 times
2/13/2012 11:55:08 PM (last edit: 2/13/2012 11:55:08 PM)
show user profile  Garp
You're not hiding by hand the objects, like cutting frame by frame around the head's edges or anything.
When an object is about to go behind the head and before it overlaps it, you cut a piece of the head footage and add it on a new layer on top of the object layer for the duration of the transition: from object fully visible to fully hidden. Same process when the object reappears on the other side.
Of course, the quality of the feathering around the head would depend on the quality of the head footage and how good the background is keyed out.

read 741 times
2/14/2012 12:57:47 AM (last edit: 2/14/2012 1:02:48 AM)
show user profile  Dub.
no-one really uses 3D holdouts on real footage as you can never get the edges quite right.

Usually you would use the roto tools in your compositing program to make mattes for the duration of the crossover.

read 731 times
2/14/2012 6:17:57 AM (last edit: 2/14/2012 6:17:57 AM)
show user profile  PaulJBis
Ah, I see.

The problem I'm having in this case is that I have several 3D objects flying around the head at the same time (think of the typical "cartoon birds" when a character is hit in the head); there will be some objects moving on the head's back, while others are moving on the front. So Garp's solution isn't really doable. What can I do then?

read 722 times
2/14/2012 11:55:14 AM (last edit: 2/14/2012 11:55:14 AM)
show user profile  Bolteon
shoot on green screen, roto and then use the roto'd plate in 3d as an object. animate the things flying around and create mattes from the render for each of the two main parts of you're render.

comp back onto you're environmental plate and tada.

-Marko Mandaric

read 720 times
2/14/2012 12:02:47 PM (last edit: 2/14/2012 12:03:16 PM)
show user profile  Dr. Jim
roto. seriously. You wanna know how it's done for real? Or weird tried-and-failed ways?

roto.......or shoot GS depending.

btw - Bolts hybrid suggestion works well too,....I have done it that way.
read 709 times
2/14/2012 1:17:33 PM (last edit: 2/14/2012 1:17:33 PM)
show user profile  PaulJBis
Hey, I'm not doubting the need for roto (or greenscreen). *I get that*. What I'm saying is: if I have a 3D object going behind the head, and *at the same time*, another 4 objects travelling in front of the head, how do I organize things?

Bolteon's solution (using the roto'ed head as a holdout object in 3DS Max) sounds good to me, and that's what I'll be doing, but I'm always willing to hear if there's a better solution... bearing in mind the above (there will be 3D objects in front of and behind the head *at the same time*).
read 703 times
2/14/2012 1:42:43 PM (last edit: 2/14/2012 1:42:43 PM)
show user profile  reeves1984
Seperate those flying bastards by rendering them in different passes.
Or make a pass with ID's so you can later matte them seperately.
Or use a Zdepth pass to define them.
OR if they don't overlap much and it's easy, garbage matte them (simple quick roto) just for separating the 'layers' of the elements.

If they animate from being in front and behind, you just have to animate stuff.

(unfortunately you're using max so passes aren't your friend)

Simon Reeves - VFX Artist & Blog
twitter <-- I work here

read 701 times
2/14/2012 1:47:46 PM (last edit: 2/14/2012 1:48:57 PM)
show user profile  Bolteon
the way i described is the best way to do it.

if there was a better way; i would have posted that.

-Marko Mandaric

read 679 times
2/14/2012 10:44:37 PM (last edit: 2/14/2012 10:44:37 PM)
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