Stair stepping is to be expected : the motion during three consecutive frames isn't exactly constant, so the interpolation created from frame 1 and 3 won't be exactly aligned with frame 2.
The method should be the following :
- fieldseparate
- aligning fields ( moving the second field half a pixel up or down, if needed )
- compute motion
- motion compensating odd frames on even frames, and merging a spatial interpolation with the result of the motion compensation.
It can be done using only avisynth scripting, but I can't implement it today.
Wilbert : I do not need the edges, a circle may be enough. But it will have to fit rather well ( ratio area of the face / area of the disk close to 1 ), in order to follow closely the face ( it will even be able to discard blocks inside the circle that don't belong to the face
). I provide the source code with the filter, there is a class GenericMotionFilter from with all my filters inherit. You could code it yourself, if you can make my code to compile properly. Fetching the motion vectors is quite easy ( look at MVBlur, for example ), and then the algorithm would be to make a mean of the motion vectors inside the circle, and then decide that all blocks whose vector is close to the mean are inside the face.
You know where these blocks went since you have their motion vectors. So now, you have a set of blocks which should overlay the face on the next frame. You repeat the algorithm and it should work.