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pixelranger
10-14-2003, 03:31 AM
Hi all.

I'm not sure this fits in here or if I should have posted in the "tips and tricks"-section, but I fire away anyway. Moderators: feel free to move this post if you find it better suited in the t&t-section.

Question:
Are there anyone here with experience from head replacement?
That is, taking a live fottage and insert cg elements tracked to a moving object? The camera is static.

Luckily, a persons head is going to be fully replaced, so I only have to track a point on the persons neck, and not the motion and rotiation of the head itself.

I have LightWave, 3d Studio Max, Digital Fusion and After Effects so any combination of those will work.
Do any of you have some tips or could you refer to a tutorial/case study on this matter?

Thank you!

munky
10-15-2003, 03:56 AM
Hi pixelranger

I wouldn't try to track just one point if I were you. If you are removing the head then I'd have tracking points on your persons head. Any scale change in your figure will necessitate at least two point tracking. Afterfx six has a much improved tracker (so I hear; I'm a combustion boy myself) A good place to ask this question would be in the afx section of CG talk forum and also you could get a dvd of a film that has similar fx and see if it has a making of bonus dvd ( just to give you an overall idea). If you get really stuck let me know and I'll ask some of my SFX pals how to really go about it.

regards

paul

Mylenium
10-15-2003, 04:28 AM
For a real 3d-tracking, you would need tools other than AE or DFX and about 12 tracking points minimum (head rotation for all axes, shoulders). If you just want to overlay a photo/ image sequence, your best guess would be a corner pin type track. However this would not be very precise depending on how much of the actual head is visible at any given time.

Mylenium

ponder_it
10-15-2003, 11:35 PM
Working with big hollywood VFX houses all the time, my best approach to it would be to track the head in 3-D, and get major on set refrence. Meaning get real world measurments of where the head is placed in the real world. Is the head 1m away from the camera or 2m? Record as much data as possible. I've used the nose and ears to track the head within' Lightwave, no DFX needed until you have to erase the head from the plate. Another quick tip is to take about 36 photo's in a 360 degree rotation for refrence of the real head in case of distortion from the shot plate. Remember to get a clean plate and aproximate the camera move and the focus changes, for the clean plate. A locked off camera is always nice, but not necessary. Don't bother to try and track a 3-d real world object in a 2-d compositing program, your wasting your time. It's easy enough to adjust things within' lightwave. We did it 200 times on the feature Cats and Dogs as well as many others like it. Create the model that is to be put in and rotate it based on the plate. If you are having him look right into camera, as silly as this is gonig to sound... Draw a big cross on the forehead of your subjects head, record how long the cross is and use that to track him a bit better. This will not work if there is any morphing involoved. Trying to paint out the cross would cause you more grief than it's worth.



Cheerz