docs/ISSUES.md

@@ -46,112 +46,9 @@ Copy this block when adding a new issue:
 
 # Active issues
 
-## #49 — Scenery (X, Y) placement drifts from retail at some landblocks
+## #48 — A few specific scenery trees hover above terrain (per-GfxObj Z misplacement)
 
 **Status:** OPEN
-**Severity:** MEDIUM (visible misplacement; species-specific or per-cell, not a global offset)
-**Filed:** 2026-05-06
-**Component:** scenery placement / `SceneryGenerator`
-
-**Description:** While verifying the `#48` Z fix at Holtburg
-landblock `0xA9B30001`, the user spotted a scenery tree placed at
-the **wrong (X, Y)** in acdream relative to retail at the same
-character coords. Specifically: a large tree that retail places far
-across the road on the right (east) side appears in acdream on the
-left (west) side, near a chess board / picnic-bench area. Side-by-
-side screenshot pair captured 2026-05-06.
-
-This is **not** a Z bug — every tree in the same screenshot has its
-trunk meeting the visible terrain (the `#48` `SampleTerrainZ` fix is
-working). It's also **not** the LandBlockInfo Stab path — the chess
-board / bench themselves are correctly placed, so the landblock
-origin and `lbOffset` math are right.
-
-**Hypotheses (need cdb retail trace to disambiguate):**
-
-1. The displacement-noise math in `SceneryGenerator` differs from
-   retail's `chunk_005A0000` LCG by a constant or a sign flip. Audit
-   `eeee4c5` claimed "all MATCH" against the decomp, but a runtime
-   trace would prove or disprove.
-2. Coordinate-system handedness: cell-local `(lx, ly)` in our path
-   may map to retail's `(ly, lx)` somewhere, rotating tree XY 90°
-   around the cell's NW corner.
-3. The `obj.Align != 0` path in retail (`FUN_005a6f60`, aligns the
-   object to the landcell polygon's normal) may use a different
-   reference point than ours, drifting placement on sloped cells.
-4. Slope filter could reject a cell retail accepts (or vice versa),
-   pushing trees into adjacent cells.
-5. Region-table / `SceneInfo` lookup might select a different
-   scenery list for the cell type.
-
-**Investigation plan (gold-standard, per `project_retail_debugger.md`):**
-
-1. Run the existing `ACDREAM_DUMP_SCENERY_Z=1` diagnostic to capture
-   acdream's full per-spawn (gfx, world XY, scale, partT) for
-   landblock `0xA9B3FFFF`.
-2. Attach cdb to a live retail client at the same Holtburg spot
-   (`tools/pdb-extract/check_exe_pdb.py` confirms PDB pairs with
-   v11.4186). Set a breakpoint on `CLandBlock::get_land_scenes` (or
-   the inner `chunk_005A0000` placement function); capture every
-   `(gfxObjId, worldX, worldY, scale, heading)` retail emits for
-   the same landblock.
-3. Diff the two tables. The spawn that's offset will be obvious;
-   the offset pattern (one tree, all trees, one species, constant
-   delta, etc.) determines which hypothesis above is correct.
-
-**Files:**
-
-- [`src/AcDream.Core/World/SceneryGenerator.cs`](src/AcDream.Core/World/SceneryGenerator.cs) — placement math (LCG noise, displacement, rotation, scale, slope filter)
-- `acclient!CLandBlock::get_land_scenes` (`docs/research/named-retail/acclient_2013_pseudo_c.txt`) — retail entry point
-- `chunk_005A0000.c` — referenced retail source per `SceneryGenerator.cs` comments
-- [`docs/research/named-retail/symbols.json`](docs/research/named-retail/symbols.json) — for cdb breakpoints
-
-**Acceptance:** Side-by-side outdoor screenshot pair (acdream vs
-retail, same character coords, same time of day) shows scenery
-positions matching at multiple landblocks. The cdb trace + diagnostic
-diff documents quantitative agreement (zero offset within float
-precision) on at least one landblock end-to-end.
-
-**Out of scope here (kept under `#48`):** Z floating. That's fixed.
-
----
-
-## #48 — [DONE 2026-05-06 · a469395] A few specific scenery trees hover above terrain (per-GfxObj Z misplacement)
-
-**Resolution:** Hypothesis 2 (physics-sampler vs bilinear-fallback Z
-mismatch). The bilinear fallback in `GameWindow.SampleTerrainZ` had
-its two diagonal arms swapped — used the SEtoNW triangle test on
-SWtoNE cells and vice versa. Every scenery hydration in our
-diagnostic ran through the bilinear path (`source=bilinear` in all
-`[scenery-z]` log lines) because physics hadn't yet built a
-`TerrainSurface` for the streaming-in landblock — so on sloped
-cells, scenery sat at a different Z than the visible terrain mesh
-by up to ~1.5 m. The bug was latent since `ff325ab` (2026-04-17)
-which upgraded the fallback from naive 4-corner bilinear to
-triangle-aware barycentric, but with the diagonal-pair tests
-swapped. `TerrainSurface.SampleZ` (used by the physics path / player
-Z) was always correct, so player feet stayed flush — the two paths
-just disagreed and only scenery noticed.
-
-Fix: extracted the canonical triangle-pick math into
-`TerrainSurface.InterpolateZInTriangle` (private static); added
-`TerrainSurface.SampleZFromHeightmap` (public static) that reads
-heights directly from the landblock byte array using the same
-canonical math; redirected `GameWindow.SampleTerrainZ` to delegate
-to it. New conformance test
-`SampleZFromHeightmap_AgreesWithInstance_AcrossWholeLandblock` pins
-both sampler paths together at 1500 sample points across both
-diagonals, so future drift gets caught. User visually confirmed
-2026-05-06.
-
-The diagnostic dump (`ACDREAM_DUMP_SCENERY_Z=1`,
-`GameWindow.cs:4661`) is kept committed — it's gated by env var,
-zero cost when off, and is the right starting point for `#49`
-(scenery X/Y placement) too.
-
-Pseudocode: [`docs/research/2026-05-06-issue-48-fix-pseudocode.md`](docs/research/2026-05-06-issue-48-fix-pseudocode.md).
-
-**Status:** DONE
 **Severity:** LOW (cosmetic; ~3 trees per landblock, easy to ignore but obvious once spotted)
 **Filed:** 2026-05-06
 **Component:** rendering / scenery placement / terrain Z sampling

docs/research/2026-05-06-issue-48-fix-pseudocode.md

@@ -1,86 +0,0 @@
-# Issue #48 fix — bilinear fallback triangle-pair test was swapped
-
-## Diagnosis (from the diagnostic dump)
-
-ACDREAM_DUMP_SCENERY_Z=1 in landblock 0xA9B3 around the user's spawn
-showed:
-
-- Every scenery line had `source=bilinear` (physics engine had not
-  registered the landblock at hydration time — typical streaming race).
-- The trees flagged as "floating" had multi-part setups whose lowest
-  vertex extended into the ground per our calc (`partWorldZMin =
-  groundZ - 3.825`) — i.e., per the diagnostic the tree should sit
-  flush. The retail client at the same world coords does sit flush.
-- Player Z at the same world coords (~91 m) came from the physics
-  sampler (`TerrainSurface.SampleZ`), which is correct.
-
-That meant the bilinear fallback in `GameWindow.SampleTerrainZ` was
-producing a different ground Z than the visible terrain mesh and the
-player physics path on sloped cells, so trees were placed at a
-ground-Z that didn't match the terrain the player was walking on.
-
-## Root cause
-
-Two terrain Z samplers exist:
-
-1. `AcDream.Core.Physics.TerrainSurface.SampleZ` (instance) — used by the
-   physics engine for player + entity ground-snap. Triangle-aware,
-   matches the visible terrain mesh.
-2. `AcDream.App.Rendering.GameWindow.SampleTerrainZ` (private bilinear
-   fallback) — used by scenery hydration when physics has not yet built
-   a `TerrainSurface` for a streaming-in landblock.
-
-Both choose a per-cell **diagonal** with the AC2D `FSplitNESW` formula
-(constants `0x0CCAC033`, `0x421BE3BD`, `0x6C1AC587`, `0x519B8F25`).
-Both then choose one of the cell's two **triangles** based on the
-fractional position within the cell. **The fallback's two arms were
-swapped** relative to the chosen diagonal:
-
-| Diagonal | Correct dividing test | Correct triangles | Bilinear-fallback test (BUGGY) |
-|---|---|---|---|
-| `SWtoNE` (BL→TR, line y=x) | `tx > ty` | {BL,BR,TR} below / {BL,TR,TL} above | `s + t <= 1` (wrong — that's the SEtoNW test) |
-| `SEtoNW` (BR→TL, line x+y=1) | `tx + ty <= 1` | {BL,BR,TL} below / {BR,TR,TL} above | `s >= t` (wrong — that's the SWtoNE test) |
-
-On sloped cells the wrong triangle's plane gives a Z that disagrees
-with the rendered terrain by up to ~1.5 m. Flat cells happen to mask
-the bug because all four corners share one Z.
-
-## Fix
-
-1. Extract the correct triangle-picker math from `TerrainSurface.SampleZ`
-   (instance) into a new public static method
-   `TerrainSurface.SampleZFromHeightmap(byte[] heights, float[] heightTable,
-   uint landblockX, uint landblockY, float localX, float localY)`.
-   Same algorithm, but reads the four corner heights directly from the
-   landblock's raw heightmap byte array instead of the pre-resolved
-   instance cache. One source of truth for the triangle math.
-2. Replace `GameWindow.SampleTerrainZ` body with a call to that static.
-3. Conformance test in `tests/AcDream.Core.Tests/Physics/TerrainSurfaceTests.cs`
-   exercising a sloped heightmap on both diagonals, asserting that the
-   new static and the existing instance method return the same Z at the
-   same `(localX, localY)` (especially at points near the cell diagonal
-   where the previous bug manifested).
-
-## Why this is the right fix
-
-- Retail (`docs/research/named-retail/acclient_2013_pseudo_c.txt`) places
-  scenery via `CLandBlock::get_land_scenes` (0x00530460) →
-  `Plane::set_height(plane, &pos)` (0x0052f050), which projects the
-  position onto the terrain triangle's plane. The split direction comes
-  from the same `FSplitNESW` formula. Our `TerrainSurface.SampleZ` is a
-  faithful port of that algorithm and is already used by physics; the
-  bilinear fallback should be identical.
-- The bug is purely in the fallback path. Player Z is unaffected.
-- No retail behavior changes; only acdream consistency.
-- "Don't break" constraints from the handoff are satisfied:
-  - Player Z snap untouched (different code path).
-  - Species that already render flush still do (their Z was correct
-    on cells where bilinear and physics agreed; now it's correct
-    everywhere).
-
-## Pre-existing bugs out of scope
-
-The user reports separate (X, Y) misplacement at other locations.
-That's a different bug — likely in `SceneryGenerator`'s placement
-math or one of the terrain-mesh / region tables — and outside the
-scope of this fix. File as a follow-up issue.

src/AcDream.App/Rendering/GameWindow.cs

@@ -183,15 +183,6 @@ public sealed class GameWindow : IDisposable
     private static readonly bool s_retailCloseDegrades =
         !string.Equals(Environment.GetEnvironmentVariable("ACDREAM_RETAIL_CLOSE_DEGRADES"), "0", StringComparison.Ordinal);
 
-    // Issue #48 diagnostic — dump per-scenery-spawn placement evidence
-    // (rendered gfx id, sample source physics-vs-bilinear, ground/baseLoc/finalZ,
-    // mesh vertex Z range, DIDDegrade slot 0). One log line per spawn lets
-    // the user identify a floating tree by its world coordinates and tell
-    // whether the cause is BaseLoc.Z addition (H1), bilinear-fallback drift
-    // (H2), or DIDDegrade selection (H3). Diagnostic-first per CLAUDE.md.
-    private static readonly bool s_dumpSceneryZ =
-        string.Equals(Environment.GetEnvironmentVariable("ACDREAM_DUMP_SCENERY_Z"), "1", StringComparison.Ordinal);
-
     /// <summary>
     /// Issue #47 humanoid-setup detector. Matches Aluvian Male
     /// (<c>0x02000001</c>) and the 34-part heritage sibling setups
@@ -2532,28 +2523,62 @@ public sealed class GameWindow : IDisposable
     }
 
     /// <summary>
-    /// Triangle-aware terrain Z sample directly from a landblock's raw
+    /// Bilinear sample of the landblock heightmap at (x, y) in landblock-local
-    /// heightmap. Used as the bilinear fallback in scenery hydration when
+    /// world units. Matches the x-major indexing convention of LandblockMesh.
-    /// physics hasn't built a <c>TerrainSurface</c> for the landblock yet
-    /// (streaming race). Delegates to
-    /// <see cref="AcDream.Core.Physics.TerrainSurface.SampleZFromHeightmap"/>
-    /// so this fallback and the player-physics path stay in lock-step on
-    /// sloped cells.
-    ///
-    /// <para>
-    /// Issue #48: the previous in-place implementation here had its two
-    /// diagonal arms swapped (SWtoNE cells used the SEtoNW triangle test
-    /// and vice versa), so scenery on hilly terrain sat at a different Z
-    /// than the visible terrain mesh — a multi-meter offset in some
-    /// cells, the user-reported "floating trees" symptom.
-    /// </para>
     /// </summary>
-    private static float SampleTerrainZ(DatReaderWriter.DBObjs.LandBlock block, float[] heightTable, float localX, float localY)
+    private float SampleTerrainZ(DatReaderWriter.DBObjs.LandBlock block, float[] heightTable, float worldX, float worldY)
     {
+        // Exact port of WorldBuilder TerrainUtils.GetHeight (line 59-108).
+        // Barycentric interpolation over the cell's triangle pair, respecting
+        // the cell's split direction (SWtoNE vs SEtoNW).
+        const float CellSize = 24f;
+
+        uint cellX = (uint)(worldX / CellSize);
+        uint cellY = (uint)(worldY / CellSize);
+        if (cellX >= 8) cellX = 7;
+        if (cellY >= 8) cellY = 7;
+
         uint landblockX = (block.Id >> 24) & 0xFFu;
         uint landblockY = (block.Id >> 16) & 0xFFu;
-        return AcDream.Core.Physics.TerrainSurface.SampleZFromHeightmap(
+        var splitDirection = AcDream.Core.Terrain.TerrainBlending.CalculateSplitDirection(
-            block.Height, heightTable, landblockX, landblockY, localX, localY);
+            landblockX, cellX, landblockY, cellY);
+
+        // 4 cell corners (heightmap x-major: Height[x*9 + y])
+        float h0 = heightTable[block.Height[cellX * 9 + cellY]];           // BL
+        float h1 = heightTable[block.Height[(cellX + 1) * 9 + cellY]];     // BR
+        float h2 = heightTable[block.Height[(cellX + 1) * 9 + (cellY + 1)]]; // TR
+        float h3 = heightTable[block.Height[cellX * 9 + (cellY + 1)]];     // TL
+
+        float lx = worldX - cellX * CellSize;
+        float ly = worldY - cellY * CellSize;
+        float s = lx / CellSize;
+        float t = ly / CellSize;
+
+        if (splitDirection == AcDream.Core.Terrain.CellSplitDirection.SWtoNE)
+        {
+            if (s + t <= 1f)
+            {
+                return h0 * (1f - s - t) + h1 * s + h3 * t;
+            }
+            else
+            {
+                float u = s + t - 1f;
+                float v = 1f - s;
+                float w = 1f - u - v;
+                return h1 * w + h2 * u + h3 * v;
+            }
+        }
+        else // SEtoNW
+        {
+            if (s >= t)
+            {
+                return h0 * (1f - s) + h1 * (s - t) + h2 * t;
+            }
+            else
+            {
+                return h0 * (1f - t) + h2 * s + h3 * (t - s);
+            }
+        }
     }
 
     private void OnLiveEntityDeleted(AcDream.Core.Net.Messages.DeleteObject.Parsed delete)
@@ -4614,76 +4639,10 @@ public sealed class GameWindow : IDisposable
             // fall back to the local bilinear sample.
             var worldPx = localX + lbOffset.X;
             var worldPy = localY + lbOffset.Y;
-            float? maybePhysicsZ = _physicsEngine.SampleTerrainZ(worldPx, worldPy);
+            float groundZ = _physicsEngine.SampleTerrainZ(worldPx, worldPy)
-            float groundZ = maybePhysicsZ
                 ?? SampleTerrainZ(lb.Heightmap, _heightTable, localX, localY);
             float finalZ = groundZ + spawn.LocalPosition.Z;
 
-            // Issue #48 diagnostic. One log line per (spawn, rendered-mesh)
-            // disambiguates H1 (BaseLoc.Z / mesh-zMin per-species), H2
-            // (physics-vs-bilinear sampler drift), and H3 (DIDDegrade slot 0).
-            // User identifies a floating tree visually, finds the matching
-            // line by world coords + gfx id, the data picks the hypothesis.
-            if (s_dumpSceneryZ)
-            {
-                string source = maybePhysicsZ.HasValue ? "physics" : "bilinear";
-                foreach (var mr in meshRefs)
-                {
-                    var dgfx = _dats.Get<DatReaderWriter.DBObjs.GfxObj>(mr.GfxObjId);
-                    if (dgfx is null) continue;
-
-                    float zMin = float.PositiveInfinity, zMax = float.NegativeInfinity;
-                    foreach (var v in dgfx.VertexArray.Vertices.Values)
-                    {
-                        if (v.Origin.Z < zMin) zMin = v.Origin.Z;
-                        if (v.Origin.Z > zMax) zMax = v.Origin.Z;
-                    }
-                    if (float.IsPositiveInfinity(zMin)) { zMin = 0f; zMax = 0f; }
-
-                    // Per-part transform offset inside the setup (post-spawn-scale).
-                    // For setup spawns this is Setup.PlacementFrames[Default].Frames[i] *
-                    // spawn.Scale. For single-GfxObj spawns it's identity * spawn.Scale.
-                    var partT = mr.PartTransform.Translation;
-
-                    bool hasDD = dgfx.Flags.HasFlag(DatReaderWriter.Enums.GfxObjFlags.HasDIDDegrade);
-                    string ddInfo = string.Empty;
-                    if (hasDD && dgfx.DIDDegrade != 0)
-                    {
-                        var ddi = _dats.Get<DatReaderWriter.DBObjs.GfxObjDegradeInfo>(dgfx.DIDDegrade);
-                        if (ddi is not null && ddi.Degrades.Count > 0)
-                        {
-                            uint slot0Id = (uint)ddi.Degrades[0].Id;
-                            float slot0Min = 0f;
-                            var slot0Gfx = _dats.Get<DatReaderWriter.DBObjs.GfxObj>(slot0Id);
-                            if (slot0Gfx is not null && slot0Gfx.VertexArray.Vertices.Count > 0)
-                            {
-                                slot0Min = float.PositiveInfinity;
-                                foreach (var v in slot0Gfx.VertexArray.Vertices.Values)
-                                    if (v.Origin.Z < slot0Min) slot0Min = v.Origin.Z;
-                                if (float.IsPositiveInfinity(slot0Min)) slot0Min = 0f;
-                            }
-                            ddInfo = $" deg[0]=0x{slot0Id:X8} deg[0]ZMin={slot0Min:F3}";
-                        }
-                    }
-
-                    // partWorldZMin = the lowest vertex of this part in world space.
-                    // = finalZ (setup origin in world Z) + partT.Z (part offset) + zMin (mesh-local lowest vertex)
-                    // If everything is right and the lowest part of the tree should
-                    // touch the ground, we expect partWorldZMin <= groundZ for at
-                    // least one part of a multi-part setup.
-                    float partWorldZMin = finalZ + partT.Z + zMin;
-
-                    Console.WriteLine(
-                        $"[scenery-z] lb=0x{lb.LandblockId:X8} root=0x{spawn.ObjectId:X8} gfx=0x{mr.GfxObjId:X8}" +
-                        $" source={source}" +
-                        $" world=({worldPx:F2},{worldPy:F2}) localXY=({localX:F2},{localY:F2})" +
-                        $" groundZ={groundZ:F3} BaseLoc.Z={spawn.LocalPosition.Z:F3} finalZ={finalZ:F3}" +
-                        $" partT=({partT.X:F2},{partT.Y:F2},{partT.Z:F3}) spawnScale={spawn.Scale:F3}" +
-                        $" zRange=[{zMin:F3}..{zMax:F3}] partWorldZMin={partWorldZMin:F3} delta={partWorldZMin - groundZ:F3}" +
-                        $" hasDIDDegrade={hasDD}{ddInfo}");
-                }
-            }
-
             var hydrated = new AcDream.Core.World.WorldEntity
             {
                 Id = sceneryIdBase + localIndex++,

src/AcDream.Core/Physics/TerrainSurface.cs

@@ -143,89 +143,23 @@ public sealed class TerrainSurface
         // and ACE's LandblockStruct.ConstructPolygons.
         bool splitSWtoNE = IsSplitSWtoNE(_landblockX, (uint)cx, _landblockY, (uint)cy);
 
-        return InterpolateZInTriangle(hBL, hBR, hTR, hTL, tx, ty, splitSWtoNE);
-    }
-
-    /// <summary>
-    /// Sample terrain Z directly from a landblock's raw heightmap. Same
-    /// algorithm as <see cref="SampleZ"/> (instance), but reads the four
-    /// corner heights through <c>heightTable[heights[x*9+y]]</c> on the fly
-    /// instead of from the pre-resolved instance cache. Use this when a
-    /// <see cref="TerrainSurface"/> hasn't been built yet for a landblock —
-    /// e.g. scenery hydration during streaming, before physics has registered
-    /// the landblock. Both paths produce the same Z, so scenery sits flush
-    /// with the visible terrain mesh and with the player physics path.
-    ///
-    /// <para>
-    /// Issue #48 root cause: the previous bilinear fallback in
-    /// <c>GameWindow.SampleTerrainZ</c> had its two diagonal arms swapped
-    /// (used the SEtoNW triangle test for SWtoNE cells and vice versa),
-    /// so on sloped cells scenery sat at a different Z than the visible
-    /// terrain by up to ~1.5 m. Routing the fallback through this static
-    /// helper guarantees both samplers stay in lock-step.
-    /// </para>
-    /// </summary>
-    public static float SampleZFromHeightmap(
-        byte[] heights, float[] heightTable,
-        uint landblockX, uint landblockY,
-        float localX, float localY)
-    {
-        ArgumentNullException.ThrowIfNull(heights);
-        ArgumentNullException.ThrowIfNull(heightTable);
-        if (heights.Length < 81)
-            throw new ArgumentException("heights must have 81 entries", nameof(heights));
-        if (heightTable.Length < 256)
-            throw new ArgumentException("heightTable must have 256 entries", nameof(heightTable));
-
-        float fx = Math.Clamp(localX / CellSize, 0f, CellsPerSide - 0.001f);
-        float fy = Math.Clamp(localY / CellSize, 0f, CellsPerSide - 0.001f);
-        int cx = (int)fx;
-        int cy = (int)fy;
-        cx = Math.Clamp(cx, 0, CellsPerSide - 1);
-        cy = Math.Clamp(cy, 0, CellsPerSide - 1);
-
-        float tx = fx - cx;
-        float ty = fy - cy;
-
-        // x-major heightmap indexing matches TerrainSurface's pre-resolution
-        // (heights[x * 9 + y]) and ACE LandblockStruct.
-        float hBL = heightTable[heights[cx     * HeightmapSide + cy    ]];
-        float hBR = heightTable[heights[(cx+1) * HeightmapSide + cy    ]];
-        float hTR = heightTable[heights[(cx+1) * HeightmapSide + (cy+1)]];
-        float hTL = heightTable[heights[cx     * HeightmapSide + (cy+1)]];
-
-        bool splitSWtoNE = IsSplitSWtoNE(landblockX, (uint)cx, landblockY, (uint)cy);
-        return InterpolateZInTriangle(hBL, hBR, hTR, hTL, tx, ty, splitSWtoNE);
-    }
-
-    /// <summary>
-    /// Pick the cell's triangle for the chosen diagonal and barycentric-
-    /// interpolate Z. Single source of truth shared by both
-    /// <see cref="SampleZ"/> (instance, pre-resolved cache) and
-    /// <see cref="SampleZFromHeightmap"/> (static, raw heightmap).
-    /// Triangle layout matches ACE <c>LandblockStruct.ConstructPolygons</c>:
-    /// SWtoNE cells split BL→TR (line y=x), SEtoNW cells split BR→TL
-    /// (line x+y=1).
-    /// </summary>
-    private static float InterpolateZInTriangle(
-        float hBL, float hBR, float hTR, float hTL,
-        float tx, float ty, bool splitSWtoNE)
-    {
         if (splitSWtoNE)
         {
             // Diagonal BL(0,0) → TR(1,1) — line y = x.
             // Triangles: {BL,BR,TR} below (tx > ty), {BL,TR,TL} above.
             if (tx > ty)
-                return hBL + (hBR - hBL) * tx + (hTR - hBR) * ty;   // BL+BR+TR
+                return hBL + (hBR - hBL) * tx + (hTR - hBR) * ty;   // BL+BR+TR triangle
-            return hBL + (hTR - hTL) * tx + (hTL - hBL) * ty;       // BL+TR+TL
+            else
+                return hBL + (hTR - hTL) * tx + (hTL - hBL) * ty;   // BL+TR+TL triangle
         }
         else
         {
             // Diagonal BR(1,0) → TL(0,1) — line x + y = 1.
             // Triangles: {BL,BR,TL} below (tx+ty <= 1), {BR,TR,TL} above.
             if (tx + ty <= 1f)
-                return hBL + (hBR - hBL) * tx + (hTL - hBL) * ty;   // BL+BR+TL
+                return hBL + (hBR - hBL) * tx + (hTL - hBL) * ty;   // BL+BR+TL triangle
-            return hTR + (hTL - hTR) * (1f - tx) + (hBR - hTR) * (1f - ty); // BR+TR+TL
+            else
+                return hTR + (hTL - hTR) * (1f - tx) + (hBR - hTR) * (1f - ty); // BR+TR+TL triangle
         }
     }
 

tests/AcDream.Core.Tests/Physics/TerrainSurfaceTests.cs

@@ -67,64 +67,6 @@ public class TerrainSurfaceTests
         Assert.Equal(42f, surface.SampleZ(300f, 300f));
     }
 
-    [Fact]
-    public void SampleZFromHeightmap_AgreesWithInstance_AcrossWholeLandblock()
-    {
-        // Issue #48 conformance: the static SampleZFromHeightmap (bilinear
-        // fallback used at scenery hydration before physics registers a
-        // landblock) must produce the same Z as the instance SampleZ
-        // (player physics path) at every (x, y). The previous fallback in
-        // GameWindow had its diagonal arms swapped — this test pins both
-        // paths to one source of truth.
-        //
-        // Heightmap with distinct per-(x,y) values so every triangle plane
-        // is genuinely different from the others; flat / planar heightmaps
-        // would mask a triangle-pick bug because all four corners would
-        // give the same interpolated Z.
-        var heights = new byte[81];
-        for (int x = 0; x < 9; x++)
-            for (int y = 0; y < 9; y++)
-                heights[x * 9 + y] = (byte)((x * 17 + y * 13) % 256);
-
-        var hTable = LinearHeightTable();
-
-        // Pick a landblock where IsSplitSWtoNE(...) returns BOTH true and
-        // false across the 64 cells — Holtburg coords (0xA9, 0xB3) work.
-        const uint lbX = 0xA9, lbY = 0xB3;
-        var instance = new TerrainSurface(heights, hTable, lbX, lbY);
-
-        // Sample on a fine grid (~1500 points) covering all 64 cells and
-        // crossing every cell's diagonal boundary. A triangle-pick bug
-        // would show up as a >0.5 m Z mismatch on the diagonal-spanning
-        // cells (the corner heights vary by ~10 bytes = 10 Z each cell).
-        for (float lx = 0.5f; lx < 192f; lx += 5f)
-            for (float ly = 0.5f; ly < 192f; ly += 5f)
-            {
-                float instanceZ = instance.SampleZ(lx, ly);
-                float staticZ = TerrainSurface.SampleZFromHeightmap(
-                    heights, hTable, lbX, lbY, lx, ly);
-                Assert.True(
-                    Math.Abs(instanceZ - staticZ) < 0.0001f,
-                    $"Z mismatch at ({lx:F1},{ly:F1}) lb=(0x{lbX:X},0x{lbY:X}): instance={instanceZ:F4} static={staticZ:F4}");
-            }
-    }
-
-    [Fact]
-    public void SampleZFromHeightmap_RejectsBadInputs()
-    {
-        var goodHeights = new byte[81];
-        var goodTable = LinearHeightTable();
-
-        Assert.Throws<ArgumentNullException>(() =>
-            TerrainSurface.SampleZFromHeightmap(null!, goodTable, 0, 0, 0f, 0f));
-        Assert.Throws<ArgumentNullException>(() =>
-            TerrainSurface.SampleZFromHeightmap(goodHeights, null!, 0, 0, 0f, 0f));
-        Assert.Throws<ArgumentException>(() =>
-            TerrainSurface.SampleZFromHeightmap(new byte[80], goodTable, 0, 0, 0f, 0f));
-        Assert.Throws<ArgumentException>(() =>
-            TerrainSurface.SampleZFromHeightmap(goodHeights, new float[255], 0, 0, 0f, 0f));
-    }
-
     [Fact]
     public void SampleSurfacePolygon_ReturnsContainingTriangleVertices()
     {