71b1622293
Keep local physics authoritative at the retail 30 Hz MinQuantum, but expose a render-only position that lerps between completed physics ticks for the player mesh and chase-camera target. Network outbound continues to use the discrete physics position. Also make the visually confirmed #47 humanoid close-detail DIDDegrade path default-on, with ACDREAM_RETAIL_CLOSE_DEGRADES=0 left as a diagnostic opt-out. Verification: dotnet build AcDream.slnx -c Debug; focused #38 interpolation tests passed; visual confirmed smooth 2026-05-06. Full dotnet test AcDream.slnx -c Debug --no-build still has the known 8 AcDream.Core.Tests baseline failures. Co-authored-by: Codex <codex@openai.com>
98 lines
3.4 KiB
Markdown
98 lines
3.4 KiB
Markdown
# Issue #38 render interpolation pseudocode
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## Problem
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Phase L.5 correctly restored retail's `CPhysicsObj::update_object`
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physics gate: the body only integrates when accumulated frame time reaches
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`PhysicsBody.MinQuantum` (1/30 second). That keeps collision behavior aligned
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with retail, but the renderer currently reads `_body.Position` directly.
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At 60+ FPS that means several rendered frames can show the same physics
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position, then jump to the next 30 Hz position. Chase camera makes the stepping
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obvious because both the player mesh and camera target follow that discrete
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physics sample.
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## Evidence
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- Named retail: `CPhysicsObj::update_object` at `0x00515d10`
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(`docs/research/named-retail/acclient_2013_pseudo_c.txt:283950`) skips or
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consumes time outside the valid quantum window and calls
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`CPhysicsObj::UpdateObjectInternal` only for accepted physics quanta.
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- ACE mirrors the same constants in `PhysicsGlobals`: `MinQuantum = 1/30`,
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`MaxQuantum = 0.1`, `HugeQuantum = 2.0`.
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- ACE's `InterpolationManager` / `PositionManager` prove client-style
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smoothing is a known AC-family concept, but their queue chases network target
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positions. Issue #38 is narrower: local render-only interpolation between the
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last two authoritative local physics ticks.
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- Glenn Fiedler's "Fix Your Timestep!" describes the canonical fixed-timestep
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accumulator plus render interpolation pattern:
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https://gafferongames.com/post/fix_your_timestep/
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## Intentional divergence
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Retail did not need a separate high-FPS render interpolation layer because the
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2013 client effectively presented near the same cadence as its physics gate.
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acdream renders at modern display rates, so we preserve retail's 30 Hz physics
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truth but draw a blended visual position between the last two physics truths.
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## Pseudocode
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State stored by `PlayerMovementController`:
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```text
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physicsAccum: leftover render time not yet consumed by physics
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prevPhysicsPos: body position at the start of the most recently completed tick
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currPhysicsPos: body position at the end of the most recently completed tick
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```
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On spawn, teleport, or any authoritative SetPosition:
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```text
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body.Position = newPosition
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prevPhysicsPos = newPosition
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currPhysicsPos = newPosition
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physicsAccum = 0
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```
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Each Update(dt):
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```text
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physicsAccum += dt
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if physicsAccum > HugeQuantum:
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physicsAccum = 0
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prevPhysicsPos = body.Position
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currPhysicsPos = body.Position
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return/render body.Position
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if physicsAccum >= MinQuantum:
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oldTickEnd = currPhysicsPos
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preIntegratePos = body.Position
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tickDt = min(physicsAccum, MaxQuantum)
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calc_acceleration()
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UpdatePhysicsInternal(tickDt)
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postIntegratePos = body.Position
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resolve collision from preIntegratePos to postIntegratePos
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body.Position = resolved physics position
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update contact/cell/velocity exactly as before
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prevPhysicsPos = oldTickEnd
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currPhysicsPos = body.Position
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physicsAccum -= tickDt
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alpha = clamp(physicsAccum / MinQuantum, 0, 1)
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renderPosition = lerp(prevPhysicsPos, currPhysicsPos, alpha)
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```
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Important constraints:
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- Never write `renderPosition` back to `_body.Position`.
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- `MovementResult.Position` remains the authoritative physics/collision/network
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position.
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- Add `MovementResult.RenderPosition` for mesh and camera drawing only.
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- Outbound `MoveToState` / `AutonomousPosition` keep using
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`MovementResult.Position`.
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- If no physics tick has completed yet, `prevPhysicsPos == currPhysicsPos`, so
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interpolation is a no-op.
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