I was just thinking a bit more about that onion skinning thing for drawings and having the next keyframe's drawing showing through...
It is definitely the way to go ! 
Here's what came out as tentative functionnal specification :
- One drawing is still associated to one and only one key image.
- You can enable/disable fading.
- When enabled, it means each drawing will be seen on a few frames before and after its reference key image.
- The opacity of the drawing peaks on its key image, and is lowered as the play head moves away.
- The default fading value for new drawings is modifiable via general settings.
- Each drawing can have a custom value, modifiable through its configuration window.
- You can disable fading altogether for all drawings if you want.
So far I guess its standard fading / onion skinning and exists in other packages. Now the specific stuff:
- You can set a drawing to have an infinite fading value, meaning it would be visible during all the video.
- It stays linked to its reference key image but it doesn't matter anymore.
- This solves the issue of not being able to have persisting drawings; all without introducing a new set of tools or a new drawing mode...
This also almost solves the custom guides feature:
- Manually draw a circle or more complex shape on the first frame and turn its fading to infinity.
- Save as .kva file.
- import it in another video and you can now move it around, change its color, etc. and use it like a reference frame.
- It won't do all you could expect from a custom guide though, but with some tweaks we could have something very useful, that you can import and resize.
and share with anyone. ( Heck I could even host a repository of custom guides on kinovea.org...)
Opens new horizons !
