I'm doing something wrong. I want to reduced the glare of an item that comes into frame by adding a track mat -- at least I think that is what this is called -- that follows the subject as it enters and leaves the shot. I can make the mask just fine, and it follows perfectly, but the mask just sits on the sceen in that position before and after the subject has gone. I can figure out how to get the keyframe to start and end but not appear and disapear for a portion of the shot.
What am I missing?
You need to use the size control in the Program monitor bottom left corner, set to 10%. This gives you a lot of space around the frame of the image. Now ... when the subject goes off frame, follow them off with the mask.
So, then, the mask is supposed to start at the beginning of the clip and continue unti the end even if it's only necessary for a small part in the middle? I'm supposed to minimize mask until I need it, or have it start off screen, or change the opacity, is the idea?
I've been trying to work out how to make it start and stop mid clip, but that's no possible?
You can fade the mask out totally with its opacity controls, so it's not doing anything. That may be what you need.
Sorry I'm late to make this thread solved. After reading your first answer, I did just that. I'm just surprised that the mask can't actually start and finish at a certain time, but that it's carrys on through the clip. Good to know.
I often cut the clip to start and stop effects.