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I noticed that one can extend timeline backward indefinitely. What's the purpose? Would there be any difference if I start my animation/simulation from frame -10 instead of frame 0?

Lai Yu-Hsuan
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    Purely there for user preference, as far as I know. People come from different programs with different backgrounds and different practices. Also, some people are just plain strange.... Either way, the space is there for everyone to use however they please ;) - One use I'm aware of is to make the 0 mark the midpoint for a cyclical looping animation, with an equal number of frames extending on either side. – Christopher Bennett Jun 27 '21 at 16:38
  • Here's an example problem where negative frames were used: Render from -100 frame as you can see, rendering negative frames is problematic, so you should avoid using them. – Markus von Broady Jun 27 '21 at 16:58
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    you may need to begin some simulations before 0, also, sometimes you need to move some keyframes before 0 – moonboots Jun 27 '21 at 17:20
  • Also, you would want to keyframe before frame 1 if using motion blur from frame 1. Because motion blur won't be generated on a frame with no motion on it. Something must already be moving before it enters the frame you want the blur to be rendered on. – Mentalist Jun 28 '21 at 00:34

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The most common use of negative frames is for a pre-roll simulation, not to be confused with a pre-roll ad. Sometimes you want a simulation, such as a smoke simulation, to develop for a bit before you start rendering it.

You could do this by starting the simulation at frame 1, and then instead of rendering frame 1-N, you would render frame M-N+N, where "M" represents the length you want the animation to run before the rendering starts, but starting the animation at -M can be more convenient.

Other uses have to do with making it easier to use the F-curve editor for repeating loops, such as when you want to start a walk somewhere but the first frame of a walk cycle.

There is no harm in starting in negative frames. However you should be aware that you need to enable allow negative frames for this to work, and over time there tend to be bugs in how Blender handles negative frames.

Marty Fouts
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