Strange mask behavior when using Alt+J and Alt+U

Help & Support
1

Hello :slight_smile:

When Im in the Compositor window (to edit the different points forming a mask) and Ive recorded several keyframes frame by frame to advance the mask (I press F3 to bring up the mask), if I use the keyboard shortcuts Alt+J to advance one frame or Alt+U to rewind one frame, the mask doesnt really follow the frames! In fact, I have to advance two frames (2x Alt+J) and rewind one frame (Alt+U) to get the mask in the right place on the right frame. So if I only use Alt+J to position the mask points on the next frame, theres a strange behavior… I hope Ive explained myself clearly; could you shed some light on this behavior?

(However, when I move the images forward or backward by clicking on the keyframes in the track, everything works fine.)

Capture-decran-du-2025-11-09-00-04-52.png

2

@chapolin

Andrew-R suggests:

I suspect this might be related to how compositor/playhead count frames, I think there was explanation somewhere in manual …
may be this: ====When using frame advance functions the behavior may seem odd. If you frame advance forward and then frame advance backward, the displayed frame doesnot change. This is because the playback position is not the frame but the time between two frames. The rendered frame is the area that the playback position crosses. When you increment the time between two frames by one and decrementit by one, you cross the same frame both times and so the same frame is displayed. There is an option in Settings �.. Preferences, Appearance tab to Always show next frame that may help make this clearer for some users.

3

See also manual 2.1.5 Insertion Point and 17.2 Playing is different than Seeking/Position

4

Hello, Thank you so much for your quick help and sorry for the delay. I understand now and wanted to take the time to experiment. So I was able to adapt. I also used a grid outside of Cin to orient myself very precisely with the mask points. My video is still being edited, but Ill post it here soon as an example.