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I was trying to composite a 3D cgi dragon animation (as an image stream) with some hand held location footage (not very wobbly but not tripod solid). The problem was that the area of ground where I needed the dragon placed was pretty indistinct so it was difficult to accurately move the quad for the image stream to follow the movement of the ground. There was a dark rock in the background clip which would have been easy to follow but it was some way away from the dragon and there was no way to correlate the position of the two.
In the same movie I had to composite a broken window tiff file over a clip of a window (to make it look broken!) and that was easier because the window frame formed a clear reference area to follow with the handles of the quad for the tiff image.
So I was thinking what if you could add an anchor point to a superimposition layer to follow an easily recognisable part of the background clip? So for example with the first clip I described above I could draw a small square around the rock and this is on or anchored to the layer with the dragon image stream. Then all I have to do is move the small box to make sure it stays around the rock and the dragon moves in the same way (because the two are anchored together) and so follows the movement within the clip.
Most background clips will usually have some distinct point of reference to follow and this facility would enable to use it to move your superimposed image even when it is away from the area where that quad is placed. It's basically the technique you use with programs such as After Effects but you have to move the tracking square by hand rather than the program doing it for you automatically.
Is that possible? To my non-programmer brain it seems simpler than expecting the program to analyse the motion in the clip (the user's brain does that) - all you need the program to do is link two elements together. Obviosuly the anchor point would have to be visible on the canvas but not appear in the final render.
I hope that makes sense - sorry it's a bit wordy but I wasn't sure how best to describe it. If it isn't clear please ask and I'll try again. I might be able to mock up some pictures to help.