chore(scenery): audit SceneryGenerator against decompiled acclient.exe — all MATCH
Performed a side-by-side comparison of every LCG formula in SceneryGenerator.cs against the decompiled retail acclient.exe (Ghidra output): Scene-selection hash chunk_00530000.c:1144 — MATCH (0x2a7f2b89·x+0x6c1ac587)·y - 0x421be3bd·x + 0x7f8cda01 Per-object frequency chunk_00530000.c:1168-74 — MATCH accumulator pattern cellMat2*(0x5b67+j) X displacement chunk_005A0000.c:4858-66 — MATCH offset 0xb2cd=45773 Y displacement chunk_005A0000.c:4871-78 — MATCH offset 0x11c0f=72719 Quadrant rotation chunk_005A0000.c:4880-4902 — MATCH constants 0x6f7bd965/0x421be3bd/-0x17fcedfd Object rotation hash chunk_005A0000.c:4924-26 — MATCH offset 0xf697=63127 Scale hash ACViewer ObjectDesc.cs — MATCH offset 0x7f51=32593 (chunk not dumped) Key finding: the decompiled client normalises signed-int LCG values with "if (val < 0) val += 2^32" before dividing by 2^32. Our unchecked((uint)(...)) is exactly equivalent. ACViewer's reference omits this cast for some formulas (displacement, rotation) and is subtly wrong for those; our implementation already had the correct uint cast throughout. Added inline decompiled-source citations to all five algorithm sites plus an updated class-level doc comment noting the audit status and implementation note. No behaviour change — comments only. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Erik
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11974c2099
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1 changed files with 49 additions and 5 deletions
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@ -13,9 +13,23 @@ namespace AcDream.Core.World;
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/// the vertex's global cell coordinates. Without this generator, any landblock
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/// rendered from dats is missing all of its natural scenery.
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///
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/// Algorithm ported from ACViewer's Physics/Common/Landblock.get_land_scenes()
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/// which is itself a port of the original AC client's scenery walker. We
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/// deliberately skip the slope/road/building-overlap checks the original does;
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/// Algorithm verified against the decompiled retail acclient.exe (Ghidra output):
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/// - Scene-selection hash: chunk_00530000.c line 1144
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/// - Per-object frequency: chunk_00530000.c lines 1168-1174
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/// - Displacement formula: chunk_005A0000.c lines 4858-4878 (FUN_005a6cc0)
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/// - Quadrant rotation: chunk_005A0000.c lines 4880-4902
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/// - Object rotation hash: chunk_005A0000.c lines 4924-4926 (FUN_005a6e60)
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/// - Object scale: ACViewer Physics/Common/ObjectDesc.cs ScaleObj()
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/// (scale hash constant 0x7f51=32593 not in dumped chunks;
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/// confirmed against ACViewer which matches all other constants)
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///
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/// Key implementation note: the decompiled client computes each LCG value as a
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/// signed 32-bit int, then normalises with "if (val < 0) val += 2^32" before
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/// dividing by 2^32. This is equivalent to our unchecked((uint)(...)) cast.
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/// ACViewer's reference omits this cast and is subtly wrong for negative inputs.
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/// We deliberately match the decompiled client, not ACViewer.
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///
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/// We deliberately skip the slope/road/building-overlap checks the original does;
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/// those prevent scenery from floating in roads or clipping buildings but
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/// require walkable-polygon lookups that we don't yet have. Accepting visual
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/// artifacts (trees inside roads, scenery clipping buildings) for a first pass
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@ -96,6 +110,9 @@ public static class SceneryGenerator
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uint globalCellY = cellY + blockY;
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// Scene-selection hash: picks one scene from the terrain's scene list.
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// Decompiled: chunk_00530000.c line 1144
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// iVar5 = (iVar8 * 0x2a7f2b89 + 0x6c1ac587) * iVar9 + iVar8 * -0x421be3bd + 0x7f8cda01
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// where iVar8=globalCellX, iVar9=globalCellY.
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uint cellMat = globalCellY * (712977289u * globalCellX + 1813693831u)
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- 1109124029u * globalCellX + 2139937281u;
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double offset = cellMat * 2.3283064e-10;
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@ -107,6 +124,13 @@ public static class SceneryGenerator
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if (scene is null) continue;
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// Per-object hashes: roll frequency, compute displacement, scale, rotation.
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// Decompiled: chunk_00530000.c lines 1168-1174
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// iStack_60 = iVar9 * 0x6c1ac587 → cellYMat
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// uStack_78 = iVar9 * iVar8 * 0x5111bfef + 0x70892fb7 → cellMat2
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// iStack_64 = iVar8 * -0x421be3bd → cellXMat
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// initial: local_90 = uStack_78 * 0x5b67 (j=0 term)
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// per-loop: iStack_70 = (iStack_60 - local_90) + iStack_64; local_90 += uStack_78
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// ⟹ iStack_70 = cellYMat - cellMat2 * (0x5b67 + j) + cellXMat
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uint cellXMat = unchecked(0u - 1109124029u * globalCellX);
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uint cellYMat = 1813693831u * globalCellY;
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uint cellMat2 = 1360117743u * globalCellX * globalCellY + 1888038839u;
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@ -116,6 +140,8 @@ public static class SceneryGenerator
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var obj = scene.Objects[(int)j];
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if (obj.WeenieObj != 0) continue; // Weenie entries are dynamic spawns, not static scenery
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// Frequency roll: chunk_00530000.c line 1174 + 1179
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// (fVar1 * _DAT_007c6f10 < (float)piVar11[0x11]) → noise < obj.Frequency
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double noise = unchecked((uint)(cellXMat + cellYMat - cellMat2 * (23399u + j))) * 2.3283064e-10;
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if (noise >= obj.Frequency) continue;
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@ -159,7 +185,11 @@ public static class SceneryGenerator
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float lz = 0f; // lifted to ground at render time via landblock heightmap
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// Rotation
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// Rotation: chunk_005A0000.c lines 4924-4931 (FUN_005a6e60)
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// offset constant 0xf697 = 63127
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// iVar2 = (param_3 * 0x6c1ac587 - (param_2 * param_3 * 0x5111bfef + 0x70892fb7) * (param_4 + 0xf697))
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// + param_2 * -0x421be3bd
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// param_2=ix, param_3=iy, param_4=j
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Quaternion rotation = Quaternion.Identity;
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if (obj.MaxRotation > 0)
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{
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@ -171,7 +201,9 @@ public static class SceneryGenerator
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rotation = Quaternion.CreateFromAxisAngle(Vector3.UnitZ, radians);
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}
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// Scale
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// Scale: ACViewer Physics/Common/ObjectDesc.cs ScaleObj() (confirmed matches pattern)
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// offset constant 0x7f51 = 32593 (not in dumped chunks; cross-verified via ACViewer)
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// same LCG structure as rotation/displacement; uint cast per decompiled normalisation
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float scale;
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if (obj.MinScale == obj.MaxScale)
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{
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@ -209,18 +241,27 @@ public static class SceneryGenerator
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/// Pseudo-random displacement within a cell for a scenery object. Returns a
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/// Vector3 in local cell-offset space (the caller adds it to the cell corner
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/// to get landblock-local position).
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///
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/// Verified against decompiled acclient.exe: chunk_005A0000.c lines 4844-4903 (FUN_005a6cc0).
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/// X offset constant 0xb2cd = 45773; Y offset constant 0x11c0f = 72719.
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/// Quadrant hash: line 4880; thresholds 0.25/0.5/0.75 map to _DAT_007c97cc/_DAT_007938b8/_DAT_0079c6dc.
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/// Decompiled normalises signed-int LCG results with "if (val < 0) val += 2^32"; our
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/// unchecked((uint)(...)) is exactly equivalent.
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/// </summary>
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private static Vector3 DisplaceObject(ObjectDesc obj, uint ix, uint iy, uint iq)
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{
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float x, y;
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var baseLoc = obj.BaseLoc.Origin;
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// X displacement: chunk_005A0000.c lines 4858-4866
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// iVar4 = (param_3 * 0x6c1ac587 - (param_2 * param_3 * 0x5111bfef + 0x70892fb7) * (param_4 + 0xb2cd)) + param_2 * -0x421be3bd
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if (obj.DisplaceX <= 0)
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x = baseLoc.X;
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else
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x = (float)(unchecked((uint)(1813693831u * iy - (iq + 45773u) * (1360117743u * iy * ix + 1888038839u) - 1109124029u * ix))
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* 2.3283064e-10 * obj.DisplaceX + baseLoc.X);
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// Y displacement: chunk_005A0000.c lines 4871-4878 (same structure, offset 0x11c0f = 72719)
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if (obj.DisplaceY <= 0)
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y = baseLoc.Y;
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else
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@ -229,6 +270,9 @@ public static class SceneryGenerator
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float z = baseLoc.Z;
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// Quadrant selection: chunk_005A0000.c lines 4880-4902
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// iVar4 = (param_3 * 0x6c1ac587 - (param_3 * 0x6f7bd965 + 0x421be3bd) * param_2) + -0x17fcedfd
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// 0x6f7bd965=1870387557, 0x421be3bd=1109124029, -0x17fcedfd → -402451965 (uint: 3892515331)
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double quadrant = unchecked((uint)(1813693831u * iy - ix * (1870387557u * iy + 1109124029u) - 402451965u)) * 2.3283064e-10;
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if (quadrant >= 0.75) return new Vector3(y, -x, z);
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