77 lines
3.7 KiB
Markdown
77 lines
3.7 KiB
Markdown
# Character Presentation Interpolation
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The authoritative game simulation still advances in exact 100 ms ticks. Character
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interpolation is a rendering-only feature and does not modify `CharacterState`,
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network commands, tick IDs, state hashes, or deterministic lockstep behavior.
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## How it works
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Each runtime `Character` keeps two presentation samples:
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- `previous`: the visual values received at the preceding completed simulation tick;
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- `current`: the visual values received at the latest completed simulation tick.
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Rendering calculates an alpha from the real elapsed time since the latest completed
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tick, clamped to the 100 ms simulation interval. It renders the delayed
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`previous -> current` transition. This introduces at most one simulation tick of
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presentation latency, but it avoids predicting a state that has not arrived from
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the authoritative simulation.
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The following values are interpolated:
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- Position uses linear interpolation.
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- Facing angle uses the shortest angular path, including wraparound at ±π.
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- Animation frame remains fractional and is interpolated when both samples use
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the same animation state. Looping clips treat the last frame to first frame
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transition as a cyclic interval instead of interpolating backward through the
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entire clip.
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The interpolated values are used by mesh, weapon, shadow, night-lighting, name,
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and health-bar rendering. The local camera and shadow-map center follow the same
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interpolated player position; otherwise the camera would jump to the newest
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authoritative position while the player model was still in its presentation
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transition. Gameplay queries, commands, callbacks, collision, and presentation
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events continue to use the authoritative snapshot.
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## Other presentation animations
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Teleport-zone star particles are advanced by `Location::updatePresentation()`
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once per render frame using the real frame delta. They are no longer advanced
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only from the 100 ms simulation-tick callback, so their visual motion remains
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continuous without affecting teleport activation or other authoritative state.
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Dialogue typewriter text and cutscene presentation timers use the same
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per-render-frame path. This keeps character reveals, subtitle timing, fades, and
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cutscene image poses smooth instead of stepping in 100 ms increments.
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Interactive objects retain authoritative move, rotate, scale, and fade tasks on
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the simulation tick. Their previous and current visual transforms are sampled
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at tick boundaries and interpolated only while drawing, so doors and similar
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objects remain logically deterministic while their presentation is smooth.
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The same render-only sampling is used for `NpcCar` taxi visuals. Its position,
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body rotation, wheel rotation, and steering angle are interpolated for drawing;
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the authoritative taxi movement and waypoint logic remain fixed-step.
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## Animation skinning
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GPU and CPU bone interpolation receive the fractional render frame instead of an
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integer-truncated frame. Bone positions are linearly interpolated and bone
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rotations use spherical interpolation as before. Frames at the end of a clip are
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clamped to the final keyframe.
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## Discontinuities
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The presentation buffer is reset instead of blended when a character is first
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observed, changes animation state or enabled state, or has not received a tick
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for a sufficiently long gap. This prevents stale positions, animation clips, or
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disabled characters from producing visible interpolation artifacts.
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## Local and networked games
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Local games continue accumulating wall-clock remainder time and executing exact
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100 ms simulation updates. Networked games continue consuming only contiguous
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server tick messages. Both paths publish completed snapshots to the same runtime
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presentation objects; interpolation only changes how those snapshots are drawn
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between updates.
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