The SampleZ method had the two triangle-boundary conditions swapped relative
to what the mesh index buffer actually renders, causing the physics Z to
sample from the wrong triangle on roughly half of all terrain cells. The
error could be up to 7.5 units on steep 24×24 cells, which manifested as
feet clipping into rising terrain on slopes.
Root cause (discovered via WorldBuilder-ACME-Edition exhaustive analysis):
"SWtoNE cut" means BL and TR are the *isolated* vertices; the shared
hypotenuse runs TL(0,1) → BR(1,0), so the correct dividing test is
tx+ty=1, NOT ty=tx. The old code used ty≤tx for the SWtoNE branch
(the BL→TR diagonal), which matches the SEtoNW mesh layout instead.
Fix: swap the boundary conditions for the two split cases so they match
LandblockMesh.cs's actual index buffer layout:
SWtoNE: tx+ty ≤ 1 → BL+BR+TL, else TR+TL+BR
SEtoNW: ty ≤ tx → BL+BR+TR, else BL+TR+TL
Verified by running all three formulas (acdream, ACME GetHeight, ACME
HeightSampler) against the mesh index buffer at 50 interior points across
both split types: fixed version matches 0 errors, old version had 50.
All 283 tests still pass.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
The triangle-aware Z sampling from the previous commit produces exact
terrain-surface Z values, but a point-sampled Z on a tilted surface
places the character's center on the surface while their feet (which
extend horizontally) clip into the rising terrain ahead/behind.
Fix: in PlayerMovementController, sample Z 1 unit ahead in the walk
direction and add 40% of the gradient as an upward bias. This
compensates for the character's collision cylinder radius on slopes
while producing zero bias on flat ground. The bias is applied in the
movement controller (gameplay concern) not in TerrainSurface.SampleZ
(which stays exact for physics/tests).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Two fundamental terrain fixes based on the AC2D + holtburger deep dive:
1. Terrain split formula: replaced WorldBuilder's physics-path formula
(214614067/1813693831) with AC2D's render-path formula (0x0CCAC033,
0x421BE3BD, 0x6C1AC587, 0x519B8F25). The two produce different splits
for some cells. Since the render mesh uses this formula, the physics
Z sampler must match it to avoid misalignment on slopes.
2. Triangle-aware Z: replaced bilinear interpolation in TerrainSurface
with per-triangle barycentric interpolation. Each cell is split into
two triangles (using the same AC2D formula). SampleZ determines which
triangle the query point falls in, then interpolates within that
triangle. This produces Z values that exactly match the visual terrain
mesh — no more slope clipping.
Removes the multi-point Z sampling hack from PlayerMovementController
(no longer needed with exact triangle Z).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Extracts the bilinear heightmap interpolation from GameWindow's
inlined SampleTerrainZ into a reusable class. Also adds outdoor
cell ID computation (8×8 grid of 24-unit cells, 0x0001..0x0040).
First component of the physics collision engine.
6 new tests, all green.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
