[Request] Implement Loop Selected Frames

[ghost]

By @sfepa

This function can save a lot of time when you need repeating animated details in complicated scenes, for example snow, rain, flag, smoke etc.
Instead of copying frames manually we could select them and define as loop.

[blurymind]

this can be implemented as frame "instances" or "clones" - similarily to how toonboom and flash handles it. This way editing a clone, affects all the other clones. But you can still create a brand new frame by copying from a clone.

[davidlamhauge]

Will this be taken care of in #1106 , or is there more to it?

[Jose-Moreno]

@davidlamhauge Unfortunately there's more to it, as I wrote on Discord a few weeks ago in the #feature-requests channel, an later on the Google drive nested timeline proposal there are always two main expected use cases for loops: timeline dependent cycles and timeline-independent cycles.

The issue you referenced would be able to cover the first case and address the OP suggestion I think. But the second comment would go more in line with the second use case, and would require the "nested" images container I spoke of.
This container can be made to be manipulated independently from the main timeline, and looped "indefinitely" while using global setting to control the exposure of the nested "timeline" over the main timeline. In order to cycle the keyframe exposures it would use "clones" or instances that always keep refering to the original drawing resources (the raw PNG's) for a defined range of frames that the user sets.

That said, once 1106 is merged I think we could close this issue and open a specific issue for the other case, since that one depends on a different set of requirements to be met before being fulfilled.