View Full Version : 4 points tracking with one hidden ?

09 September 2004, 07:04 PM
Hi again !
I need to track a paralepiped that have one corner hidden by the hand of a character, actually it's the sceen of a mobile that I need to replace.
How do you get the 4th point's motion in order to apply a corner pin effect ?
In after effects there is the parallel-corner-pin effect to do that.
I was thinking of looking for an expression to retreive this missing point but I'm stuck...

09 September 2004, 09:52 PM
Just track it as close as possible to the point and then just offset the track with a move 2d.

09 September 2004, 10:12 AM
Ok actually this is not as easy as it seems.
What I need is to replace a mobile screen by a virtual one, so I need to track the corners to apply a corner pin effect to the virtual screen. But one corner is hidden by a hand, actually a big part of the screen is hidden.
I was wrong when I said "paralepiped", a rectangle seen in perspective is not really a paralepiped, oposite sides and angles are not equal. The mobile screen is a 3D object projected on a 2D plan.

I think the equation is too complicated for me. The tracker returns the x and y coordinates of the 3 corners but is it enough information to calculate the fourth corner ?


09 September 2004, 01:15 PM
Hi seb4d,

Can't you track the 4th point by hand?

If you have three points visible all the time shouldn't be that hard to figure out where the fourth one should be to manually track it.

Case you shot is shaking too much to try to go manually, you can stabilize it first, do the four corner and then put back the shaking in both elements.

Just an idea :)


09 September 2004, 02:02 PM
This could be a solution yes, but I was hoping there was a way to do that automatically because this kind of shot often happens it would save a precious time.

09 September 2004, 02:02 PM
yes, i tend to stabalise my footage and then reverse the transform, it easier to see what its doing, also if you are having problems with tracking points disapearing, take a look at this post, points out something i had missed while learning tracking. The tracking point offset button allows you to put your track point where it should be, and then offset where it tracks, this was useful in a project where i had to replace a poster on the wall, while people were walking infront of it constantly, it worked in that situation because it was a long shot with a hand held camera but the camer stayed parralel. If you are unlucky enough to have a camera shot with lots of changes in perspective, this technique becomes less useful, as the further away from the orriginal point there will be a greater difference in relation to the orriginal movement, (that probebly doesn't make a lot of sence)

I have tried manually corner pinning, and its not a pleasant or very pretty way of doing it. Thats when i run and use 3d tracking instead

09 September 2004, 07:39 PM
Yes ant- , I know the tracking point offset but unfortunately this does not the trick because of the complex motion of the screen to track.
I think I'll have to go with manual work.

Thanks for your help guyz !

09 September 2004, 08:11 PM
i don't know if you have access to 3d tracking with boujou or PFTrack or something, but the automatic tracking can work out motion from hundreds of points, I have used this techniqe sometimes, and then just rendered out a grid from a 3d program and tracked that with shake. Long way round, but more accurate.

09 September 2004, 03:31 PM
Yes this sounds like a good alternative, it deserves to be tested !
Thanks !

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