3D Track Motion Lens Equiv?

Orcatek wrote on 12/28/2004, 7:22 AM
Here is basically what I am trying: I have a solid rectangular object in the real world which is being rotated relative to the camrea.

The object starts with the plane of interest being parallel to the viewer. I then crop the a secondary video to match refrence points on the object leaving a border.

Now I rotate the object so I need to rotate the secondary video track to keep the video "On" the object. The problem is that the 3D track appears to have a different virtual lens than I use in shooting, so the object and the video do not maintain the perspective relation as the rotate. The front edge can be made to match the video, but then the rear edge of the 3D track is shorter than the real video rear edge.

If someone knows the lens equivalent, I can shoot it without tons of trial and error. Or better yet knows how to change it in Vegas, I can't find a setting.

Hope you can understand what I am talking about here.


BillyBoy wrote on 12/28/2004, 7:32 AM
Hard to be sure without "seeing" it, but it sounds like you're talking about the "Z" value. In 2D space there is only X and Y or relative width and height. In 3D space you add a reference that respresents how close or far a object is to the camera. So you have three intersecting points. Moving in any direction X, Y or Z is referenced as a plus or minus value from the point to begin at. Hope that makes sense.

If you have version 5 of Vegas they have some new 3D tools. Also the 3D plug-in from Satish.

What I think you want to do is move in "Z" space, not just crop. This will simulate moving further or closer to the object. It gets more complex if you also wish to rotate a object in 3D space.

A simple simulated 3D example can be found at my Flash tutorial site.

Try the special effects button, then the eagle thumbnail.

Orcatek wrote on 12/28/2004, 9:14 AM
Thanks - I understand that it is the Z value - I work in 3D all the time - the issue virutal lens. BTW I do have Vegas 5

When you draw an object in 3d space, the lines flow away and do not remain parallel and would eventually intersect at some distance.

With 3D track motion the lines form an angle of 30 degrees(just an example not sure the real value) when the object is rotated 45 degrees. This is fine except when you are trying to match an object in the video.

Depending on your lens, when you rotate an object 45 degrees on video, the lines of angle can be 20 degrees or 35 or 12 etc. By knowing the "lens" for 3D motion I can use the same in the video so it will match. With trial and error I could eventually find it, but that is time consuming.

Hope this clears it up. If not I will capture a sample to show what I mean.

Sullivan wrote on 12/28/2004, 2:25 PM
Yeah, I do understand what you mean and it's a very good question. I don't know the answer, though!

Maybe we can determine it, however. Of course, you're not going to get a focal length in mm. You're going to get the FL in pixels and you can scale that to match your camera, given the image sensor size.

We know that when Z=0 the image size is exactly the same size as the "film" size so at Z=0, the distance from the nodal point to the subject plane is the "focal length" in pixels of the virtual camera.

Now try an experiment; I don't have Vegas here (:
Move the image away from the camera until it is 1/2 the original width.
Now the distance from the subject plane to the nodal point is twice the "focal length"

Thus, whatever "Z" is at that point is the FL, in pixels, of the imaginary camera.