feat(physics): use sloped terrain plane in FindEnvCollisions
Our previous FindEnvCollisions built a FLAT contact plane (Normal = +Z)
at the sampled terrain Z, discarding the triangle's actual slope.
Retail uses the real terrain polygon's plane (ACE Landblock.cs:125-137
find_terrain_poly → walkable.Plane) which IS sloped.
Without a true slope normal, AdjustOffset's projection of horizontal
velocity onto the plane produces no slope-aligned Z component — fine
for step-subdivision on flat ground, visibly wrong whenever the contact
plane is carried across frames (via PhysicsBody.ContactPlane persistence
from commit 93cbabb): the projection is a no-op and movement is purely
kinematic. With the real slope normal, projected motion correctly
follows the slope.
Not a user-visible bug fix by itself (DIAG LocalZ shows delta≈0 for the
local player everywhere; the "looks too high in water" issue the user
reported is actually a missing water-rendering feature, not a physics
bug). Landing it anyway because it matches retail behavior and removes
the "flat-plane-is-fine" assumption that would bite on any future
contact-plane-dependent code.
Additions:
- TerrainSurface.SampleSurface(localX, localY) → (Z, Normal), deriving
the plane normal analytically from the triangle's height gradient.
Matches the same triangle SampleZ already interpolates through.
- PhysicsEngine.SampleTerrainPlane(worldX, worldY) → System.Numerics.Plane,
the wrapper that bridges terrain space to transition space.
- TransitionTypes.FindEnvCollisions uses SampleTerrainPlane instead of
synthesizing a flat plane from SampleTerrainZ.
All 717 tests green. Flat-plane case is unchanged (Normal.Z = 1 when
the triangle is level, identical to the old plane).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Erik
parent
93cbabbc87
commit
40f120617d
3 changed files with 139 additions and 8 deletions
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@ -110,6 +110,35 @@ public sealed class PhysicsEngine
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return null;
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}
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/// <summary>
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/// Sample the outdoor terrain plane (Z + sloped normal) at the given
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/// world-space XY position. The returned <see cref="System.Numerics.Plane"/>
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/// has the true terrain-triangle normal (NOT a flat <c>(0,0,1)</c>), and
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/// its <c>D</c> is set so the plane passes through the sampled point. Used
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/// by <see cref="Transition"/> to build a CORRECT contact plane — a flat
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/// plane breaks slope tracking because <c>AdjustOffset</c>'s projection
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/// onto a flat plane cannot impart the Z component that horizontal
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/// velocity needs to follow the slope.
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/// </summary>
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public System.Numerics.Plane? SampleTerrainPlane(float worldX, float worldY)
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{
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foreach (var kvp in _landblocks)
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{
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var lb = kvp.Value;
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float localX = worldX - lb.WorldOffsetX;
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float localY = worldY - lb.WorldOffsetY;
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if (localX >= 0f && localX < 192f && localY >= 0f && localY < 192f)
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{
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var (z, normal) = lb.Terrain.SampleSurface(localX, localY);
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// System.Numerics.Plane convention: dot(Normal, P) + D == 0
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// for points P on the plane. Pick P = (worldX, worldY, z).
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float d = -(normal.X * worldX + normal.Y * worldY + normal.Z * z);
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return new System.Numerics.Plane(normal, d);
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}
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}
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return null;
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}
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/// <summary>
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/// Resolve an entity's movement from <paramref name="currentPos"/> by
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/// applying <paramref name="delta"/> (XY only) and computing the correct Z
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@ -116,6 +116,99 @@ public sealed class TerrainSurface
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}
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}
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/// <summary>
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/// Sample both the terrain Z and the triangle-plane surface normal at
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/// (localX, localY). The normal is derived from the gradient of the same
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/// triangle <see cref="SampleZ"/> interpolates across, so returned
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/// <c>(Z, Normal)</c> is exactly the sloped plane the physics should
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/// contact against.
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///
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/// <para>
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/// This matters for <see cref="Transition"/>: when <c>AdjustOffset</c>
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/// projects the per-step movement offset onto the contact plane, a flat
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/// plane (<c>Normal = (0,0,1)</c>) cannot impart any Z component to a
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/// horizontal velocity — the character walks off a slope and the
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/// per-frame step-down budget (~4 cm) can't catch up with the slope
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/// descent rate, so the sphere floats above the terrain. A SLOPED plane
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/// gives AdjustOffset the info it needs to produce slope-aligned motion
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/// with the correct Z component baked in.
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/// </para>
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///
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/// <para>
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/// Retail does this via <c>LandCell.find_terrain_poly → walkable.Plane</c>
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/// (ACE <c>Landblock.cs:125-137</c>). We derive the equivalent plane
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/// analytically from the chosen triangle's three corner heights.
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/// </para>
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/// </summary>
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public (float Z, System.Numerics.Vector3 Normal) SampleSurface(float localX, float localY)
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{
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float fx = Math.Clamp(localX / CellSize, 0f, CellsPerSide - 0.001f);
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float fy = Math.Clamp(localY / CellSize, 0f, CellsPerSide - 0.001f);
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int cx = (int)fx;
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int cy = (int)fy;
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cx = Math.Clamp(cx, 0, CellsPerSide - 1);
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cy = Math.Clamp(cy, 0, CellsPerSide - 1);
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float tx = fx - cx;
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float ty = fy - cy;
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float hBL = _z[cx, cy ];
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float hBR = _z[cx + 1, cy ];
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float hTR = _z[cx + 1, cy + 1];
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float hTL = _z[cx, cy + 1];
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bool splitSWtoNE = IsSplitSWtoNE(_landblockX, (uint)cx, _landblockY, (uint)cy);
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// The SampleZ formula for each triangle is linear in (tx, ty):
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// Z = a + b * tx + c * ty
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// so dZ/dLocalX = b / CellSize, dZ/dLocalY = c / CellSize.
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// Surface normal = normalize((-dZ/dX, -dZ/dY, 1)) — a well-known
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// identity for a height-field plane.
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float z, dzdx, dzdy;
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if (splitSWtoNE)
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{
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// Diagonal BL(0,0) → TR(1,1). Triangles: {BL,BR,TR} / {BL,TR,TL}.
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if (tx > ty)
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{
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// {BL,BR,TR}: Z = hBL + (hBR-hBL)·tx + (hTR-hBR)·ty
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z = hBL + (hBR - hBL) * tx + (hTR - hBR) * ty;
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dzdx = (hBR - hBL) / CellSize;
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dzdy = (hTR - hBR) / CellSize;
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}
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else
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{
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// {BL,TR,TL}: Z = hBL + (hTR-hTL)·tx + (hTL-hBL)·ty
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z = hBL + (hTR - hTL) * tx + (hTL - hBL) * ty;
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dzdx = (hTR - hTL) / CellSize;
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dzdy = (hTL - hBL) / CellSize;
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}
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}
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else
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{
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// Diagonal BR(1,0) → TL(0,1). Triangles: {BL,BR,TL} / {BR,TR,TL}.
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if (tx + ty <= 1f)
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{
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// {BL,BR,TL}: Z = hBL + (hBR-hBL)·tx + (hTL-hBL)·ty
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z = hBL + (hBR - hBL) * tx + (hTL - hBL) * ty;
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dzdx = (hBR - hBL) / CellSize;
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dzdy = (hTL - hBL) / CellSize;
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}
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else
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{
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// {BR,TR,TL}: Z = hTR + (hTL-hTR)(1-tx) + (hBR-hTR)(1-ty)
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// Equivalent linear form: Z = [hBR+hTL-hTR] + (hTR-hTL)·tx + (hTR-hBR)·ty
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z = hTR + (hTL - hTR) * (1f - tx) + (hBR - hTR) * (1f - ty);
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dzdx = (hTR - hTL) / CellSize;
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dzdy = (hTR - hBR) / CellSize;
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}
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}
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var normal = System.Numerics.Vector3.Normalize(
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new System.Numerics.Vector3(-dzdx, -dzdy, 1f));
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return (z, normal);
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}
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/// <summary>
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/// Compute the outdoor cell ID for the given landblock-local position.
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/// </summary>
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@ -619,16 +619,25 @@ public sealed class Transition
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}
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// ── Outdoor terrain collision ────────────────────────────────────
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// Sample terrain Z at the foot sphere's world position.
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float? terrainZ = engine.SampleTerrainZ(footCenter.X, footCenter.Y);
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if (terrainZ is null)
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// Build the terrain contact plane at the foot sphere's world XY.
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//
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// Retail fidelity: plane Normal must be the terrain TRIANGLE normal
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// (sloped on slopes), NOT a flat (0,0,1). AdjustOffset projects the
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// per-step movement offset onto this plane — with a flat plane the
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// projection just zeros the Z component and horizontal velocity stays
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// horizontal, so the sphere cannot follow a descending slope within
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// one frame's step-down budget. A sloped plane projects horizontal
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// velocity INTO slope-aligned motion with the correct Z component.
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//
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// ACE reference: Landblock.GetZ (Landblock.cs:125-137) calls
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// find_terrain_poly and uses walkable.Plane — the actual triangle's
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// plane, not a reconstructed flat one. SampleTerrainPlane returns
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// the same thing analytically from the triangle's corner heights.
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var planeOpt = engine.SampleTerrainPlane(footCenter.X, footCenter.Y);
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if (planeOpt is null)
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return TransitionState.OK; // no terrain loaded here — allow pass-through
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// Build the terrain contact plane (flat ground: Normal = +Z, D = -terrainZ).
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var contactPlane = new System.Numerics.Plane(
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new Vector3(0f, 0f, 1f), -terrainZ.Value);
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return ValidateWalkable(footCenter, sphereRadius, contactPlane, isWater: false,
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return ValidateWalkable(footCenter, sphereRadius, planeOpt.Value, isWater: false,
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cellId: sp.CheckCellId);
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}
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