Expert: Andre Hickman - 11/20/2006

Question
First of all 'Bahut-bahut Dhanyawad' (Thank you very much) for your great Explanation.

You mentioned 'rotoscoping the tracking--tracking by hand, one frame at a time', it means Tracking Manualy ? Do 'Analyze one frame forward' rather then 'Analyze forward' , if tracker goes wrong, we can keep tracker back at right place, am i right here ?

Please tell me about 'Rotoscoping' in which way you mentioned in your Answer.

And one more thing, are these basic Tracking features(in AFX) different from Cumbustion's Tracking features or same?

-------------------------------------------
The text above is a follow-up to ...

-----Question-----
When we track (in AFX) something then we have a Feature region and a Search Area, i want to know how it works ?
Once we tell the Feature region that this is your Search Area, then why sometimes Feature Region gets outside of Search Area ? should it not search the Feature inside the Search Area ?
-----Answer-----
Well, in a nutshell, this is how it works....

I will use the example of a brown human eye...
Say for instance, you bound your search area around the facial area from the forehead to the cheek bone, around one of the eyes, and the feature region around just the eye, which you are trying to track.

What happens is the software is looking for pixel patterns.  In essence it would be looking in the search area which is primarily a solid type flesh color for an area of pixels that have a white outside, with a dark brown to black area inside.  As long as that is prevalent in the search area, then it will track fine.

But what happens if the footage is zoomed out to the point where it can't distinguish the white area around the iris, or what if it doesn't zoom at all...what if the person blinks?  Or what if the lighting in the footage goes bad, or changes dramatically? Then that white area, with a black area inside, disappears! Now what are you tracking frame by frame? The tracker knows where the feature was just a frame ago, but it gets lost searching for it. It tries to continue in the natural direction that the feature was going, but can't find anything...

The reality is that tracking only works well with an area that is high contrast with it's surrounding, and with quality footage.  If you are trying to track something with low contrast, or something on some video footage with compression which causes "dancing pixel" changes in the footage, well...GOOD LUCK!  You'd probably be better off zooming in and rotoscoping the tracking--tracking by hand, one frame at a time.

If you've ever seen the making of a movie that used green screening, they have all these white "X"'s on the green screen stage, or white "X"'s with black outlines...these are their tracking points for compositing CG environments, and they work because they are in high contrast from the solid green paint...

I hope this information helps.

Cheers,
Andre

Answer
I am not exactly sure of that, because I have never used that feature.  What I am sure of is the way that you would have to track if you did not have a tracking feature at all...by hand.  Say for instance, you want to put a lens effect over a person's eyes.  You would put the flares on the eyes, then advance 1 frame...adjust the flares...advance another frame...adjust the flares...advance another frame...adjust the flares...etc...

You could also advance in multiples of frames like 5 at a time, and get the motion, then go back and go between the keyframes and create more keyframes where necessary.  That is Rotoscoping...

As far as the difference in any trackers in different softwares, they all are based on the same technology. The only difference may be the algorithms that they used to get to the end goal.  They work the same way in essence though.

Cheers,
Andre