refactor(render): cull terrain by walk in-view cells

src/AcDream.App/Rendering/RetailPViewPassExecutor.WalkLeaf.cs

@@ -10,6 +10,11 @@ namespace AcDream.App.Rendering;
 /// methods — the cutover changes ORDER only.</summary>
 internal sealed partial class RetailPViewPassExecutor
 {
+    private IReadOnlySet<uint>? _walkTerrainInViewLandcells;
+
+    internal void SetWalkTerrainInViewLandcells(IReadOnlySet<uint>? cells) =>
+        _walkTerrainInViewLandcells = cells;
+
     /// <summary>The walk's single sky turn: retail draws GameSky once
     /// inside <c>LScape::draw</c>, clipped by the active views; looping
     /// today's per-slice scissor + terrain-clip block reproduces today's
@@ -105,7 +110,8 @@ internal sealed partial class RetailPViewPassExecutor
                 frame.Frustum,
                 neverCullLandblockId: frame.PlayerLandblockId,
                 clipPlanes: default,
-                ndcClipAabb: new Vector4(-1f, -1f, 1f, 1f));
+                ndcClipAabb: new Vector4(-1f, -1f, 1f, 1f),
+                inViewLandcells: _walkTerrainInViewLandcells);
             _terrainDiagnostics.Complete();
             return;
         }
@@ -120,7 +126,8 @@ internal sealed partial class RetailPViewPassExecutor
             frame.Frustum,
             neverCullLandblockId: frame.PlayerLandblockId,
             clipPlanes: slice.Planes,
-            ndcClipAabb: slice.NdcAabb);
+            ndcClipAabb: slice.NdcAabb,
+            inViewLandcells: _walkTerrainInViewLandcells);
         _terrainDiagnostics.Complete();
         DisableClipDistances();
         if (scissor)

src/AcDream.App/Rendering/RetailPViewRenderer.cs

@@ -1349,7 +1349,15 @@ public sealed class RetailPViewRenderer
         Walk.WalkFrameDriver driver)
     {
         var (frame, encoder) = passes.RequireWalkSubmission();
+        passes.SetWalkTerrainInViewLandcells(driver.VisitedLandscapeCellIds);
+        try
+        {
             driver.Replay(frame, encoder);
+        }
+        finally
+        {
+            passes.SetWalkTerrainInViewLandcells(null);
+        }
 
         // Landscape-stage static-owner particles (candles, the cathedral
         // falls) submit AT THEIR OWN WALK TURNS inside Replay

src/AcDream.App/Rendering/TerrainModernRenderer.cs

@@ -68,6 +68,7 @@ public sealed partial class TerrainModernRenderer : IDisposable
     // Reusable per-frame buffers.
     private readonly List<int> _visibleSlots = new();
     private readonly HashSet<uint> _visibleCellIds = new();
+    private readonly HashSet<uint> _walkVisibleLandblocks = new();
     private DrawElementsIndirectCommand[] _deicScratch = Array.Empty<DrawElementsIndirectCommand>();
 
     // Diag.
@@ -204,24 +205,47 @@ public sealed partial class TerrainModernRenderer : IDisposable
         FrustumPlanes? frustum = null,
         uint? neverCullLandblockId = null,
         ReadOnlySpan<Vector4> clipPlanes = default,
-        Vector4? ndcClipAabb = null)
+        Vector4? ndcClipAabb = null,
+        IReadOnlySet<uint>? inViewLandcells = null)
     {
         if (_alloc.LoadedCount == 0) return;
 
         Matrix4x4 viewProjection = camera.View * camera.Projection;
 
+        // Campaign FW4: RetailFrameWalk is the sole visibility authority.
+        // Terrain remains one full-landblock MDI draw, but only landblocks
+        // containing a walk-admitted landcell participate, and the published
+        // landcell set is the walk's exact in_view set rather than a second
+        // frustum-derived approximation.
+        _walkVisibleLandblocks.Clear();
+        if (inViewLandcells is not null)
+        {
+            foreach (uint cellId in inViewLandcells)
+            {
+                _walkVisibleLandblocks.Add(cellId & 0xFFFF0000u);
+                _visibleCellIds.Add(cellId);
+            }
+        }
+
         // Build visible slot list with per-slot frustum cull.
         _visibleSlots.Clear();
         for (int slot = 0; slot < _slots.Length; slot++)
         {
             var data = _slots[slot];
             if (data is null) continue;
+            if (inViewLandcells is not null
+                && !_walkVisibleLandblocks.Contains(data.LandblockId & 0xFFFF0000u))
+            {
+                continue;
+            }
             if (frustum is not null && data.LandblockId != neverCullLandblockId)
             {
                 if (!FrustumCuller.IsAabbVisible(frustum.Value, data.AabbMin, data.AabbMax))
                     continue;
             }
             _visibleSlots.Add(slot);
+            if (inViewLandcells is null)
+            {
                 CollectVisibleCells(
                     _visibleCellIds,
                     data.LandblockId,
@@ -233,6 +257,7 @@ public sealed partial class TerrainModernRenderer : IDisposable
                     clipPlanes,
                     ndcClipAabb);
             }
+        }
         if (_visibleSlots.Count == 0) return;
 
         BuildIndirectCommands();