Render option for last-n animations

[mscuthbert]

Description of proposed feature

In rendering manim videos, I often want to render just the last few animations. I'll use the flag -n 35 and then when I've added a few more animations, I'll update to -n 40, etc.

It would be helpful to have a flag like -n -5 or --last_n_frames 5 which would only render the last few frames of the video; saving time when debugging with early returns, etc.

How can the new feature be used?

as described above -- to save time when developing, especially in CairoRenderer where interactive debug isn't available.

Additional comments

I'm still a manim novice -- finishing my first longish video (medieval music animations!) so I've only just gotten a grasp of how the Scene and renderers interact with each other, but my sense is that the difficulty of this feature is that when render() is called and construct() is run, the system has no idea how many play() calls will be run, so counting backwards isn't easy.

Two implementation paths come to mind, both involving running the scene twice.

1. use the existing save_last_snapshot mechanism to count the number of plays and use that information to set -n start-end appropriately and run again.
2. create a new Renderer class NullRenderer whose .play() function does nothing except update_skipping_status. Creating a BaseRenderer that Cairo and OpenGL (and NullRenderer) both inherit from would probably save some code-dup anyhow.

The problem with both of these approaches is if the construct() call changes properties on self while running and does not reset them they will be incorrect the second run:

class MyScene(Scene):
    def __init__(self, ...):
          ...
          self.start_height = 10

    def construct(self):
          ...
          self.play(obj.animate.shift(self.start_height))
          self.start_height += 5
          self.play(obj.animate.shift(self.start_height))

This could be a documented limitation of this method or doing something like creating a deepcopy of the Scene object before running construct() could work. It still would have issues though if there were side effects like reading/writing state from disk, but that is quite rare.

If the community supported this, I'd be willing to implement it myself.

[tameTNT]

The implementation of this feature (specifically --last_n_frames) will likely have some overlap with #2589

[ad-chaos]

I often want to render just the last few animations

Have you taken a look at Sections ?