--Issue Summary--

This is one of the features that are on the request roadmap and it's been requested a few more times.

In traditional animation Flipping & Rolling are two similar techniques that allow the animator to quickly preview their current animation work by flipping ( jump between the first, last and inbetween drawing of an action) or rolling (preview drawings in ordered sequence flipping with all your available fingers).

Explanation of Flipping Techniques

1. The "Inbetween" Flipping: This technique is used mostly for the process known as in-betweening and the minimum amount of drawings you can use is two, but 99% of the time you will use three drawings. These are usually, the first drawing in the action (previous key), the last drawing (next key) and the currently edited inbetween.

REF: https://www.youtube.com/watch?v=0hw5eQF99ng

2. The rolling or "roll flipping" technique is used to check how a specific part of the action is working out, and the minimum amount of drawings are at most 5, one for each finger to manipulate plus one sitting at the bottom of the stack.

REF: https://www.youtube.com/watch?v=0C8arzQWUoI

3. There's also the "flip" or "stack flipping" technique which is when you take a large amount of drawings, organize them like a giant flipbook and let it "flip" down to check a large portion of your animation at a relative non-real time speed (it's hard to get constant fps using your fingers lol). This could already be considered as partially covered by the playback behavior though.

REF: https://www.youtube.com/watch?v=6Lmwz7M7SsY

In order to mimic this behavior in a digital environment timeline this functionality should aim to modify the playback behavior as it should be based upon the current drawing / frame container that is being edited, that is, where the playback scrub is located at the moment of activating the function.

I have to note this functionality is semantically different from the Range function as well how its applied during a job. Whereas the current Range function allows you to preview a portion of the timeline from start to end including every frame in the range (in a way this would fulfill the stack flipping technique), the inbetween flipping and roll flipping functions have to behave differently to allow the animator to see the only the drawing(s) (normally keys or main drawings) behind and /or in front but always coming back to the currently edited drawing.

In the sense of previewing only the keyframe drawings, we have a similar behavior exhibited by the "onion skin toggle match keyframes" command (the little diamond button on the timeline)

For reference you can read about TVPaint's "Flip" function panel: https://www.tvpaint.com/doc/tvp11/index.php?id=lesson-tradigital-animation-advanced-flip-presentation

I think we can extend the current range function so it can Ideally contain the additional Flipping & Rolling behaviors. Pencil2D's own inbetween flipping, roll flipping and stack rolling should comply with the following criteria (Updated 02/05/2019):

• Implement Linear Playback (current default): starts at the first frame, ends at the last frame of the range (this would comply with the requirements for stack flipping and the roll flip to see the drawings in sequence)
• Implement Flip-Playback: Would allow the scrub to jump from the current drawing to the next, go back to current drawing, jumps to previous drawing, go to current drawing. This can be done:
  • Automatically: Pressing the button executes the inbetween behavior once
• Implement a toggled behavior that allows you to playback from and to the currently active drawing / frame container.
• Implement a toggled behavior that allows you to playback from the beginning of the roll range up to the current drawing. Note: this was implemented similar to the onion skin range where there is a set number of drawings you can playback before the current frame, but it's independent from the frame range feature.