Is it possible?

I mean a simple app that takes in a video(of a person acting) as an input and creates markers on it's body parts and somehow record any motion on them. Then save as a BVH file.

In fact, i think i can do this as alittle java project. I'm going to try.

The only thing that i don't know how to do is How do i build a BVH file. WHat do BVH files contain, if i know that then i can make the java program write a bvh file.

Notice i said simple because, i just want it to capture motion out of a 2D video.. So i dn't care if it doesn't work perfectly in a poser( since it is a 3d world). I just want to at least give it X and Z motion. Ok i'm going to try this..

Well... good luck with that.

There's a discussion of the BVH file spec here, and also at Wotsit. It's a text format that pretty much just has position data for bones. You'll want to make sure and set your bone names in the BVH file to match the bone names in the character you're going to attach to.

I think you'd be a lot better off putting markers on your actor before the video is made; it's going to be awfully hard to get consistent results by trying to set markers from just looking at the video. If your accuracy is off, the motion isn't going to look right. If you've got no budget at all, and your actor is a really good sport, you might put the actor in a swimsuit, and use a water based marker to paint crosses directly on the actor's skin, at the joints. That'll be a lot easier to match up, I think.

Most systems in existence that use this technique use more than one camera, too, to get depth information. If your actor is perfectly facing the camera, you can get a shoulder dip down or up correctly, but you can't really get any subtle front-to-back motion.

You may possibly get good results if you try to set your points every few frames, say every fifth to tenth, then do a curve-fit to the data points, then use the new curve to generate the actual motion. The math there is a little hairy, though.

Let us know how it works... maybe you could post a sample animation of just the bones, showing how you did it. I'm sure there are lots of people who would be interested.

BVH file specification

Quote - BVH file specification

A lot of that information is specific to character studio.