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perf(lighting): allocation-free nearest-N light selection (#133 FPS)

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Tick built a new List<>(N) and ran an O(N log N) Sort every frame; in a dungeon
N is thousands of torches, so it allocated a large list per frame (GC pressure ->
FPS). Replace with an insertion partial-select that keeps the nearest maxPoint
directly in the _active window — O(N * maxPoint), maxPoint<=8, zero allocation.
Same selection result (nearest 8); lighting suite 20/20 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
Erik 2026-06-13 21:26:17 +02:00
parent 0fe479ba06
commit 5872bcf075
1 changed files with 51 additions and 38 deletions
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89
src/AcDream.Core/Lighting/LightManager.cs
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@ -93,53 +93,66 @@ public sealed class LightManager
/// </summary>
public void Tick(Vector3 viewerWorldPos)
{
// Pass 1: compute DistSq + filter out lights outside the slack radius.
var candidates = new List<LightSource>(_all.Count);
foreach (var light in _all)
{
if (!light.IsLit) continue;
if (light.Kind == LightKind.Directional)
{
// Directional lights don't participate in this ranking —
// the sun is always slot 0.
continue;
}
Vector3 delta = light.WorldPosition - viewerWorldPos;
light.DistSq = delta.LengthSquared();
// Retail D3D-style fixed-pipeline lighting picks the 8 NEAREST point
// lights and applies each light's hard range-cutoff PER SURFACE in the
// shader (mesh_modern.frag: `if (d < range && range > 1e-3)`). The
// previous viewer-range candidacy filter (skip when DistSq > Range²·slack²)
// was wrong — it dropped a torch whenever the VIEWER stood outside that
// torch's range, so a dungeon room with 2227 registered torches lit only
// the ~1 the player was standing inside (activeLights≈1, the rest of the
// room at flat 0.2 ambient — the "dungeon lighting off" report). Take the
// nearest 8 regardless of viewer range; the shader's per-fragment
// `d < range` does the actual hard cutoff.
candidates.Add(light);
}
// Pass 2: sort by DistSq ascending, take up to 7.
candidates.Sort((a, b) => a.DistSq.CompareTo(b.DistSq));
// Retail D3D-style fixed-pipeline lighting takes the nearest (MaxActiveLights-1)
// point lights (slot 0 is the sun) and applies each light's hard range cutoff
// PER SURFACE in the shader (mesh_modern.frag: `if (d < range && range > 1e-3)`),
// NOT a viewer-range candidacy filter — a torch the viewer stands outside the
// range of must still light the wall it sits on.
//
// Allocation-free partial selection: the old path built `new List<>(N)` and
// ran an O(N log N) Sort EVERY FRAME; in a dungeon N is thousands of torches,
// so that allocated a large list per frame (GC pressure → FPS). Instead keep
// the nearest maxPoint directly in the _active window, maintained sorted by
// insertion. O(N · maxPoint), maxPoint ≤ 8, zero allocation.
Array.Clear(_active);
_activeCount = 0;
// Slot 0 = sun when present.
// Slot 0 = sun when present (directional; never ranked by distance).
int baseSlot = 0;
if (Sun is not null)
{
_active[0] = Sun;
_activeCount = 1;
baseSlot = 1;
}
int maxPoint = MaxActiveLights - _activeCount;
int pointCount = Math.Min(maxPoint, candidates.Count);
for (int i = 0; i < pointCount; i++)
int maxPoint = MaxActiveLights - baseSlot;
int filled = 0;
if (maxPoint > 0)
{
_active[_activeCount + i] = candidates[i];
foreach (var light in _all)
{
if (!light.IsLit || light.Kind == LightKind.Directional) continue;
Vector3 delta = light.WorldPosition - viewerWorldPos;
light.DistSq = delta.LengthSquared();
// Maintain _active[baseSlot .. baseSlot+filled) sorted ascending by
// DistSq. Insert if there's room or this light is nearer than the
// current farthest (then the farthest falls off the end).
if (filled < maxPoint)
{
int j = baseSlot + filled;
while (j > baseSlot && _active[j - 1]!.DistSq > light.DistSq)
{
_active[j] = _active[j - 1];
j--;
}
_active[j] = light;
filled++;
}
else if (light.DistSq < _active[baseSlot + maxPoint - 1]!.DistSq)
{
int j = baseSlot + maxPoint - 1;
while (j > baseSlot && _active[j - 1]!.DistSq > light.DistSq)
{
_active[j] = _active[j - 1];
j--;
}
_active[j] = light;
}
}
}
_activeCount += pointCount;
_activeCount = baseSlot + filled;
}
}