brush strokes disappearing during sequence

[lovebiscuit]

I've always noticed that when I track in, one or two brushstrokes disappear once they are outside the camera frame, but still visible in the general drawing area.

I was never sure why this happened - but assumed that while in the process of previewing a moving scene the software or computer didn't need to worry about painting something once it had fallen outside the area of concern.

Maybe I was wrong to think it was that, because now it's happening inside the frame.

I am tracking in slowly, past a very densely packed scene and towards the end of the tracking movements parts of the main layer are disappearing shortly before they leave the frame.

Has anyone else experience this?