Suppose I have a movie showing an object (such as a bird) moving around. I would like to use the video editor to add a line behind the object showing the trajectory. Does anyone have suggestions on how to accomplish this? I've tried using the UV/Image editor to add a dynamically changing mask but am unable to figure out how to apply a color to this dynamic mask. Perhaps this is the wrong approach? Any help is appreciated, thank you!
Combining colored elements is easiest in the compositor.
I would also suggest using 3D to create the colored line as Blender has good visual tools in 3D. Just use a Shadeless or emitting material to make it monochromatic.
Then mask out the part of the bird, which shouldn't be covered by the line.
Render it with transparency and use a mix node in the compositor. Check the invert button in the mask options (in the image editor) to invert the mask.