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View Full Version : Right tool for the job - Advanced compositing in LW



HM1X
01-23-2004, 09:17 PM
Creating models (or images) in LW (or Photoshop), it's seems easy enough to, say, have a person standing on a floor (modeled image) with surrounding walls and features (created images or models).

But my question is, what do you do when you want to composite an animated character or object into the scene which can move around or interact with the real character (shot against blue/green screen)?

As an example, the way the 'Halt Droid' moves around, in front of and in back of the 'emperor' in the Duality short (www.crewoftwo.com).

If this were to be done strictly in LW, my first instinct was to simply create a flat poly and import it into Layout, then project the blue/green screen footage of my character onto the flat poly. Then I could have my model, say a sphere for simplicity, move around the poly. It could be in front or move behind. Shadows could be cast, etc. Later the blue/green could be removed in After effects.

Only problem is, as the sphere moves behind the poly, it will become invisible to the camera, regardless at which point it actually intersects with and goes behind the polygon. Obviously you would only want the sphere to disappear as it goes beyond the outline of the actual character and it would be pretty hard to create a morphing poly that changes shape with the character at all times.

Am I close?

Cageman
01-23-2004, 11:32 PM
Yep!! That is the way to go if you only got LW to do the composite. In your example, I wouldnīt dare use LW for compositing... I think that AE, Combustion or DFX would be easier to use.

Having said that, the limitation with LW is that you canīt control or animate points in a mesh. So, if LW8 were here, you should be able to just animate the points of the flat poly (mask) to follow the outlines of the character. There is a plugin that can do this, but it opens a small window, and you really donīt have the same controll of the points as you have in modeler (and layout in LW8).

In either case (AE, Combustion, DFX or whatever) you will face a lot of work. Not hard work, but alot of keyframing... and, yep... itīs a booring process to mask stuff, but in the end youīll see the result, and that makes you forget the amout of time spent! :)

I have some shots coming up... composited stuff, Iīll post later and tell how things were done... :)

JCG
01-23-2004, 11:39 PM
About your problem with going behind the poly and being hidden from the camera...
You can do most in LW, but you'd still need something to create the chroma key, I think, or else it would look too rough.

1- I'd create an alpha sequence with a simple chroma key.
2- I'd use that to create a clip map for the poly that holds the footage.
3- I'd use that for animating the sphere, camera, lights, etc.
4- When that's done, dissolve both objects and render the background.
5- Then kill the background and undissolve the sphere. Render it.
6- Now import the three layers to the compositing program composite them together and do a nice chroma key of the footage this time.
7- Not really a seventh step. Just an alternative. You could skip #4. Then, in #5, you could just remove the clip map and use a nice chromakeyed alpha as a transparency map. (Might not be as neat depending on the filmed subject but, except for the creation of the alpha sequence, it'd be all LW.)

The footage poly doesn't have to cast shadows since it should include the filmed shadows as part of the footage. They key should place those shadows in the background if everything was planned correctly, but careful when the sphere crosses them.

HM1X
01-24-2004, 10:03 AM
Thanks for the info everyone.


Originally posted by JCG
The footage poly doesn't have to cast shadows since it should include the filmed shadows as part of the footage. They key should place those shadows in the background if everything was planned correctly, but careful when the sphere crosses them.

It wouldn's be so much a matter of the footage on the poly casting shadows, but rather the object (the sphere in this case) casting shadows on the poly as it moves in front, between the poly and the light source.

Someone else had suggested something as well. What do you think about rendering the 3d element in LW as a seperate pass against black with an alpha channel (a TGA sequence) and then comp it in a compositing application such as After Effects. If the situation then arose that it needed to pass behind a live-action character, one could just use some roto-splines in the composite to matte the foreground character as and when required.

Thinking about it, it doesn't seem there woud be any way to get away with it unless you roto-spline or mask the live-action character at some point, when the 3D object need to move behind. I just hate masking moving characters in animations... a real P.I.T.A :)

Jockomo
01-24-2004, 01:17 PM
I think the way I would approach this is to 1st create the animation, without worrying about when it is in front and when it is behind.

Then shoot your footage over the blue/green background ( you can use your animation to make sure your subject is placed correctly.

Now the comp is done in editing. Let's say the sphere circles around your live subject, moving in front first, then behind and around again in front.

In your background layer you put your background video, lets call that layer 1. Leave layer 2 empty for now, and put your chromakey subject on layer 3, set up your key.

Now your place your sphere animation on layer 4. Play the video... the sphere moves in front of your character...once the sphere is at the location on the side, about to go around back, make a cut there in your animation video. Now drop the rest of the video on layer 2. Continue the sphere around the back of the character. When the sphere gets to the other side of your character, cut, and place the remaining video on layer 4 again.

Now of course this is just a very basic situation, with ideal requirements, casting shadows on your subject, or making the sphere go through your subject could add more complexity, but hopefully this will give you an idea of one option.

wiremuse
01-26-2004, 09:50 AM
To those who may be interested in this thread but arn't completely familiar with the topic. There is a great 3 part series (and supsosively a part 4 coming) on specifically compositing in Lightwave by Eki Halkka.

The URL is here:
http://www.lw-fin.org/tutorials/eki/