Unstable replays
An unstable replay is a replay which can produce different outcomes depending on how it is played back, as if the replay tape could split into diverging timelines at some specific frame. Unstable replays are a rare occurrence, but not vanishingly so, and typically are RH racing laps with aggressive powergear slides.
Whether a divergence will appear in a replay, and what it will look like if it does, depends on a large number of factors, including but not limited to: the chosen camera, in-game graphics settings, the number of cycles in DOSBox and the system (be it an emulator or real hardware, as the effect has also been observed in native DOS systems) used to run the game. Since, in contrast, reproducing a replay with only the bare game state loop (be it by fast-forwarding the tape or by using the repldump tool) gives consistent outcomes across systems, instability is believed to somehow arise from the graphics layer. Since live gameplay can't help but involve graphics, the exposure of drivers to instability can be limited only so far.
While unstable replays are by and large just one more amusing curio of Stunts, the conflicting outcomes can lead to confusion when validating laps in a race, specially given how the system dependence of the effect can make it hard to reproduce. That being so, this article aims at clarifying what to expect upon facing an unstable replay and how to effectively navigate in it, which should help avoiding, or getting out of, messy situations in competitions.
Definitions
Note: Replays mentioned in what follows can be downloaded from Southern Cross.
To avoid mixing up things, let's begin by defining some key terms that we'll use in the discussion below. To begin with, for the replay controls, we'll mostly use the names in the game manual: skip to the start, stop, play, fast play and, in the bottom row, rewind and fast forward. (Note that fast play is often referred to as "double speed".)
Next, some key notions about instability:
- Fast-forward timeline
- The events in a replay as seen by fast-forwarding it from the beginning, loading it from the Options menu, or reproducing it with repldump. The fast-forward timeline is the same across systems and settings, and so is a sensible choice of a default validation method for potentially unstable replays. (Note the relevant replay control is fast forward, and not fast play/"double speed".)
- Divergent timeline
- A sequence of events in a replay different from the one in the fast-forward timeline. A divergent timeline might be seen by playing the replay (that is, watching with the play button), or through some other use of the replay controls. Whether it will actually be seen can depend on the computer running the game, the DOSBox cycles setting, camera choice, graphics settings, and so forth.
- Divergence point
- A point in the replay at which two or more timelines diverge. Divergence points are typically vertex points in a powergear slide, in which the car stops for a moment before shooting into a different direction (note the converse is presumably not true: there are many powergear slides which, as far as we know, don't lead to divergences). By convention, divergence points will be quoted using the last frame before the timelines split (for instance, FRI15695.RPL has a divergence point at 1:46.00).
- Unstable replay
- A replay known to have a divergence point and multiple timelines when reproduced in some way on at least one system. (Given the way factors external to the game affect the observations, "known to be unstable" is arguably more accurate than "unstable", but let's try to keep the language straightforward.)
