fix #426: extract solid-colour (NO_POS_UVS) faces; skip untextured subsets only on building shells and cells like retail

The Holtburg windmill axle (GfxObj 0x010010CE, 8 polygons, all
Stippling.NoPos + SurfaceType.Base1Solid) extracted to a 0-vertex mesh.
NoPos ("NO_POS_UVS", acclient.h:7380-7388) means "this side has no
texture coordinates" — true of every solid-colour polygon, since
nothing samples them — not "there is no positive face". Extraction read
it as the latter and dropped the polygon entirely, client-wide, for
every untextured polygon on every object.

Retail's D3DPolyRender::DrawMesh (@0x0059d4a0, named-retail decomp
~line 426048) draws an untextured subset on an ordinary object exactly
like a textured one; the only retail cases that skip an untextured
subset are a building shell (RenderDeviceD3D::DrawBuilding @0x0059f2a0
sets ObjBuildingOrBuildingPart=1) or an EnvCell interior
(RenderDeviceD3D::DrawEnvCell @0x0059f170, arg4=1). The #119
investigation's "retail's skipNoTexture never draws them either"
conclusion was itself wrong as a general rule.

- MeshExtractor.PrepareGfxObjMeshData / GfxObjMesh.Build: emit the
  positive side whenever PosSurface is a valid index, regardless of
  NoPos; the existing UV-index-0 fallback already produces zero
  texcoords for a NoPos polygon with no UVs on the wire.
- RetailUntexturedSurfacePolicy.IsUntextured(SurfaceType): the one
  place that answers "is this surface textured"
  ((type & (Base1Image|Base1ClipMap)) == 0), replacing the old
  `isSolid = NoPos || Base1Solid` (which also mis-classified a NEG-side
  batch by the POS-side's NoPos flag).
- RetailUntexturedSubsetPolicy.Draws(isBuildingShell, isUntextured):
  the shared draw-time gate wired into WbDrawDispatcher.ClassifyBatches,
  .PackedOracle.ClassifyPackedBatches, and
  .DirectionalShadows.AddDirectionalShadowBatches — one predicate so the
  three walks cannot drift (Campaign VM VM6 lesson).
- CellMesh.cs / MeshExtractor.PrepareCellStructMeshData deliberately
  KEEP their NoPos-gated skip for cell-wall geometry — retail's
  DrawEnvCell really does skip untextured subsets there; register row
  AP-234 documents the NoPos-vs-Surface.Type approximation.
- PakFormat.CurrentBakeToolVersion 4->5 (LauncherInstallRecordStore in
  lockstep): a pak baked by an older tool is missing every untextured
  face. No bake was run as part of this commit.

Also fixed: WorldBuilder's own upstream ObjectMeshManager.cs has the
identical NoPos bug (ObjectMeshManager.cs:959,984) — our port had
faithfully carried it over, and our own conformance test
(Build_NoPosFlag_OnlyEmitsNegSide) asserted the bug as correct WB
conformance. Renamed/reworded to Build_NoPosFlag_EmitsBothPosAndNegSide
with a citation for why retail decomp overrides WB here.

Issue119UpNullGfxObjDumpTests re-run against the installed DAT:
#119's own two objects (0x010002B4 9/9 polys, 0x010008A8 1/1 poly) now
gate DRAWS on every polygon instead of extracting to nothing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/Wb/ObjectMeshManager.cs

@@ -1661,16 +1661,36 @@ namespace AcDream.App.Rendering.Wb
                 var renderData = UploadGfxObjMeshData(meshData);
                 if (renderData == null)
                 {
-                    // 0-vertex mesh: every polygon was gated out at extraction. #119
-                    // (2026-06-11) dat-verified this is LEGITIMATE for all-no-draw
-                    // models (all polys NoPos + Base1Solid surfaces — retail's
-                    // skipNoTexture never draws them either; 0x010002B4/0x010008A8
-                    // are this class, Issue119UpNullGfxObjDumpTests). The empty
-                    // cache is the correct terminal state for those. The line stays
-                    // as a tripwire for the OTHER way to get here (extraction
-                    // dropped textured polys — a real defect; dat-verify with the
-                    // dump test before treating as one).
-                    Console.WriteLine($"[up-null] 0x{meshData.ObjectId:X10} produced a 0-vertex mesh — caching empty render data (legitimate for all-no-draw models; dat-verify via Issue119UpNullGfxObjDumpTests)");
+                    // 0-vertex mesh: every polygon was gated out at extraction.
+                    // #119 (2026-06-11) ORIGINALLY reasoned this is LEGITIMATE
+                    // for every all-NoPos+Base1Solid ("all-no-draw") model,
+                    // claiming retail's skipNoTexture never draws untextured
+                    // subsets at all. #426 (2026-08-23, the Holtburg windmill
+                    // axle 0x010010CE) corrected that: retail's skipNoTexture
+                    // only suppresses untextured subsets on a BUILDING SHELL
+                    // (RenderDeviceD3D::DrawBuilding @0x0059f2a0 sets
+                    // ObjBuildingOrBuildingPart=1) or inside an EnvCell
+                    // interior (DrawEnvCell @0x0059f170, arg4=1) — an
+                    // ORDINARY object's untextured (solid-colour) polygons DO
+                    // draw (D3DPolyRender::DrawMesh(..., arg4=0)). Extraction
+                    // now emits the positive side for every polygon with a
+                    // valid PosSurface regardless of NoPos, so a 0-vertex
+                    // mesh here is legitimate ONLY for a model whose every
+                    // polygon is degenerate (fewer than 3 vertices) or
+                    // references no valid Surface index at all.
+                    // Issue119UpNullGfxObjDumpTests' own dump against the
+                    // installed DAT confirms #119's original two objects
+                    // (0x010002B4, 9 polys; 0x010008A8, 1 poly — both
+                    // all-NoPos+Base1Solid) now gate DRAWS on every polygon
+                    // and are NOT examples of the legitimate 0-vertex case
+                    // any more; they were never actually all-degenerate or
+                    // all-invalid-surface, they were all-solid, which #426
+                    // now extracts. The line stays as a tripwire for the
+                    // OTHER way to get here (extraction dropped textured
+                    // polys — a real defect;
+                    // dat-verify with the dump test before treating a hit as
+                    // legitimate).
+                    Console.WriteLine($"[up-null] 0x{meshData.ObjectId:X10} produced a 0-vertex mesh — caching empty render data (legitimate only for degenerate/no-valid-surface models post-#426; dat-verify via Issue119UpNullGfxObjDumpTests)");
                     renderData = new ObjectRenderData();
                 }
 

src/AcDream.App/Rendering/Wb/WbDrawDispatcher.DirectionalShadows.cs

@@ -1183,6 +1183,15 @@ public sealed partial class WbDrawDispatcher
              batchIndex++)
         {
             ObjectRenderBatch batch = renderData.Batches[batchIndex];
+
+            // #426: a batch retail never draws for this entity casts no
+            // shadow either — same gate as ClassifyBatches/
+            // ClassifyPackedBatches (RetailUntexturedSubsetPolicy), so the
+            // caster and receiver agree by construction (mirrors the
+            // FoliageWindClassification comment below).
+            if (!RetailUntexturedSubsetPolicy.Draws(candidate.IsBuildingShell, batch.Key.IsSolid))
+                continue;
+
             sourceBatches++;
             if (!DirectionalShadowPreparedDraws.TryClassifyMaterial(
                     batch.Translucency,

src/AcDream.App/Rendering/Wb/WbDrawDispatcher.PackedOracle.cs

@@ -631,6 +631,14 @@ public sealed unsafe partial class WbDrawDispatcher
         {
             ObjectRenderBatch batch =
                 renderData.Batches[batchIndex];
+
+            // #426: mirrors the classic ClassifyBatches gate exactly — see
+            // RetailUntexturedSubsetPolicy for the retail citation. ONE
+            // shared predicate so the classic and packed classifiers cannot
+            // drift (Campaign VM VM6).
+            if (!RetailUntexturedSubsetPolicy.Draws(entity.IsBuildingShell, batch.Key.IsSolid))
+                continue;
+
             TranslucencyKind translucency = batch.Translucency;
             if (opacity < 1f && IsOpaque(translucency))
                 translucency = TranslucencyKind.AlphaBlend;

src/AcDream.App/Rendering/Wb/WbDrawDispatcher.cs

@@ -3421,6 +3421,15 @@ public sealed partial class WbDrawDispatcher : IDisposable
         {
             var batch = renderData.Batches[batchIdx];
 
+            // #426: retail's D3DPolyRender::DrawMesh skips an UNTEXTURED
+            // (solid-colour) subset only on a BUILDING SHELL
+            // (RenderDeviceD3D::DrawBuilding sets ObjBuildingOrBuildingPart);
+            // ordinary statics/scenery/creatures/items draw it same as any
+            // textured subset. ONE shared predicate with ClassifyPackedBatches
+            // and AddDirectionalShadowBatches — see RetailUntexturedSubsetPolicy.
+            if (!RetailUntexturedSubsetPolicy.Draws(entity.IsBuildingShell, batch.Key.IsSolid))
+                continue;
+
             TranslucencyKind translucency = batch.Translucency;
 
             // #188: a mid-fade instance whose surface is otherwise Opaque/ClipMap

src/AcDream.Content/MeshExtractor.cs

@@ -347,9 +347,20 @@ public sealed class MeshExtractor {
             if (poly.VertexIds.Count < 3) continue;
 
             // Handle Positive Surface
-            if (!poly.Stippling.HasFlag(StipplingType.NoPos)) {
-                AddSurfaceToBatch(poly, poly.PosSurface, false);
-            }
+            // #426 (2026-08-23, Holtburg windmill axle 0x010010CE): NoPos
+            // ("NO_POS_UVS", acclient.h:7386) means "this side has no texture
+            // coordinates" — that's true of every SOLID-COLOUR polygon, not
+            // "there is no positive face". Retail's D3DPolyRender::DrawMesh
+            // draws untextured (solid) subsets on ordinary objects same as
+            // textured ones (see RetailUntexturedSurfacePolicy); only a
+            // building shell or an EnvCell interior skips them, and that is
+            // a DRAW-time decision (RetailUntexturedSubsetPolicy, applied in
+            // WbDrawDispatcher), not an extraction-time one. So the positive
+            // side is always emitted when PosSurface is a valid index;
+            // AddSurfaceToBatch already falls back to UV index 0 / zero
+            // texcoords (via BuildPolygonIndices) when NoPos leaves no UVs to
+            // read.
+            AddSurfaceToBatch(poly, poly.PosSurface, false);
 
             // Handle Negative Surface
             // Some objects use Clockwise CullMode to indicate negative surface data is present
@@ -376,7 +387,13 @@ public sealed class MeshExtractor {
                 TextureFormat textureFormat;
                 UploadPixelFormat? uploadPixelFormat = null;
                 UploadPixelType? uploadPixelType = null;
-                bool isSolid = poly.Stippling.HasFlag(StipplingType.NoPos) || surface.Type.HasFlag(SurfaceType.Base1Solid);
+                // #426: "solid" (untextured) is a SURFACE fact, not a
+                // polygon-stippling fact — see RetailUntexturedSurfacePolicy.
+                // The old `NoPos ||` term conflated "this polygon's positive
+                // side has no UVs" with "this surface is untextured"; it also
+                // wrongly classified a NEG-side batch by the POS-side's NoPos
+                // flag, since this method is shared by both sides.
+                bool isSolid = RetailUntexturedSurfacePolicy.IsUntextured(surface.Type);
                 bool isClipMap = surface.Type.HasFlag(SurfaceType.Base1ClipMap);
                 uint paletteId = 0;
                 bool isDxt3or5 = false;
@@ -710,6 +727,20 @@ public sealed class MeshExtractor {
             // GL cull enum: 0 = pos, 1 = pos twice with reversed winding,
             // 2 = pos + neg surface. The DAT-side NoPos/NoNeg flags still
             // suppress hidden portal/cap faces before they reach our mesh.
+            //
+            // #426: unlike PrepareGfxObjMeshData (ordinary objects — fixed to
+            // emit every NoPos-flagged positive side, since NoPos means "no
+            // UVs", not "no face"), this NoPos gate is INTENTIONALLY kept.
+            // Cell-wall geometry draws through retail's
+            // RenderDeviceD3D::DrawEnvCell (@0x0059f170), which calls
+            // D3DPolyRender::DrawMesh with arg4=1 and skips every UNTEXTURED
+            // subset — approximated here by the polygon's own NoPos flag
+            // rather than by resolving Surface.Type
+            // (Base1Image/Base1ClipMap, see RetailUntexturedSurfacePolicy)
+            // before this decision is made. See
+            // docs/architecture/retail-divergence-register.md AP-234. Do NOT
+            // remove this gate to mirror the GfxObj fix — that would draw
+            // solid-colour cell-wall faces retail never shows.
             bool hasPos = !poly.Stippling.HasFlag(StipplingType.NoPos);
             bool hasNeg = !poly.Stippling.HasFlag(StipplingType.NoNeg);
 
@@ -745,7 +776,13 @@ public sealed class MeshExtractor {
                 TextureFormat textureFormat;
                 UploadPixelFormat? uploadPixelFormat = null;
                 UploadPixelType? uploadPixelType = null;
-                bool isSolid = poly.Stippling.HasFlag(StipplingType.NoPos) || surface.Type.HasFlag(SurfaceType.Base1Solid);
+                // #426: "solid" (untextured) is a SURFACE fact, not a
+                // polygon-stippling fact — see RetailUntexturedSurfacePolicy.
+                // The old `NoPos ||` term conflated "this polygon's positive
+                // side has no UVs" with "this surface is untextured"; it also
+                // wrongly classified a NEG-side batch by the POS-side's NoPos
+                // flag, since this method is shared by both sides.
+                bool isSolid = RetailUntexturedSurfacePolicy.IsUntextured(surface.Type);
                 bool isClipMap = surface.Type.HasFlag(SurfaceType.Base1ClipMap);
                 uint paletteId = 0;
                 bool isDxt3or5 = false;

src/AcDream.Content/Pak/PakFormat.cs

@@ -22,9 +22,15 @@ public static class PakFormat {
     /// version 3 embeds exact render-pass translucency in each texture batch
     /// so production never rebuilds surface metadata from live DAT. Version 4
     /// adds complete immutable flat collision and EnvCell-topology payloads.
-    /// The binary format remains version 1.
+    /// Version 5 (#426, 2026-08-23) extracts untextured (solid-colour)
+    /// positive faces that versions <=4 dropped — every GfxObj polygon
+    /// whose Stippling carries NoPos (NO_POS_UVS) previously extracted to
+    /// zero vertices on its positive side, so any pak baked by an older tool
+    /// is missing those faces (e.g. the Holtburg windmill axle 0x010010CE,
+    /// 8 polygons all NoPos + Base1Solid, extracted to a 0-vertex mesh). The
+    /// binary format remains version 1.
     /// </summary>
-    public const uint CurrentBakeToolVersion = 4;
+    public const uint CurrentBakeToolVersion = 5;
 }
 
 /// <summary>

src/AcDream.Core/Meshing/CellMesh.cs

@@ -41,7 +41,19 @@ public static class CellMesh
             if (poly.VertexIds.Count < 3)
                 continue;  // degenerate polygon
 
-            // Skip if NoPos stippling is set (polygon has no positive surface geometry).
+            // Retail's RenderDeviceD3D::DrawEnvCell (@0x0059f170) calls
+            // D3DPolyRender::DrawMesh with arg4=1, which skips every
+            // UNTEXTURED subset inside an EnvCell interior — unlike ordinary
+            // objects, which draw them (see RetailUntexturedSurfacePolicy /
+            // RetailUntexturedSubsetPolicy, #426). We approximate
+            // "untextured" here with the polygon's own NoPos stippling flag
+            // rather than resolving the Surface's own Type
+            // (Base1Image/Base1ClipMap) before this per-polygon decision —
+            // see docs/architecture/retail-divergence-register.md AP-234. Do
+            // NOT remove this gate the way #426 removed the matching gate in
+            // GfxObjMesh.Build/MeshExtractor.PrepareGfxObjMeshData — retail
+            // genuinely skips untextured cell geometry, unlike ordinary
+            // objects.
             if (poly.Stippling.HasFlag(DatReaderWriter.Enums.StipplingType.NoPos))
                 continue;
 

src/AcDream.Core/Meshing/GfxObjMesh.cs

@@ -28,8 +28,16 @@ public static class GfxObjMesh
     /// The rule for emitting a polygon side:
     /// </para>
     /// <list type="bullet">
-    ///   <item><b>Pos side:</b> emit whenever <c>!Stippling.NoPos</c> and
-    ///         <c>PosSurface</c> is a valid index.</item>
+    ///   <item><b>Pos side:</b> emit whenever <c>PosSurface</c> is a valid
+    ///         index, REGARDLESS of <c>Stippling.NoPos</c>. #426
+    ///         (2026-08-23): NoPos ("NO_POS_UVS", acclient.h:7386) means
+    ///         "this side has no texture coordinates" — every solid-colour
+    ///         polygon carries it, since it has no UVs to carry — not "there
+    ///         is no positive face". Retail's <c>D3DPolyRender::DrawMesh</c>
+    ///         draws untextured (solid) subsets on ordinary objects the same
+    ///         as textured ones (see <see cref="RetailUntexturedSurfacePolicy"/>);
+    ///         a NoPos polygon with no UVs to read falls back to UV index 0 /
+    ///         zero texcoords below.</item>
     ///   <item><b>Neg side:</b> emit when
     ///         <c>Stippling.Negative</c>, <c>Stippling.Both</c>, or
     ///         <c>(!Stippling.NoNeg &amp;&amp; SidesType == CullMode.Clockwise)</c>.
@@ -69,9 +77,10 @@ public static class GfxObjMesh
                 continue;  // degenerate — can't form a triangle
 
             // --- Positive side ---
-            bool hasPos = !poly.Stippling.HasFlag(StipplingType.NoPos);
-            if (hasPos)
-                EmitSide(poly, poly.PosSurface, isNeg: false);
+            // #426: always emit — NoPos only means "no positive UVs", not
+            // "no positive face" (see the class doc). EmitSide's own
+            // surfaceIdx-validity guard is the only real gate.
+            EmitSide(poly, poly.PosSurface, isNeg: false);
 
             // --- Negative side ---
             // Three ways AC flags a polygon as double-sided:

src/AcDream.Core/Meshing/RetailUntexturedSurfacePolicy.cs

@@ -0,0 +1,76 @@
+using DatReaderWriter.Enums;
+
+namespace AcDream.Core.Meshing;
+
+/// <summary>
+/// Retail's textured-vs-untextured surface classification, ported from
+/// <c>D3DPolyRender::DrawMesh</c> @0x0059d4a0 (named-retail decomp,
+/// docs/research/named-retail/acclient_2013_pseudo_c.txt ~line 426048): a
+/// surface subset is TEXTURED — and therefore never subject to either of
+/// retail's "skip untextured" gates (see
+/// <see cref="RetailUntexturedSubsetPolicy"/>) — when
+/// <c>(surface-&gt;type &amp; 6) != 0</c>, i.e. when
+/// <see cref="SurfaceType.Base1Image"/> (<c>BASE1_IMAGE</c>, 0x2) or
+/// <see cref="SurfaceType.Base1ClipMap"/> (<c>BASE1_CLIPMAP</c>, 0x4) is set
+/// (docs/research/named-retail/acclient.h:5822-5824). Every other surface —
+/// including <see cref="SurfaceType.Base1Solid"/> and any surface whose type
+/// carries neither bit — is UNTEXTURED: a flat-colour ("solid") subset filled
+/// from <c>Surface.ColorValue</c> rather than a decoded texture.
+/// </summary>
+/// <remarks>
+/// #426 (2026-08-23, the Holtburg windmill axle 0x010010CE): the polygon-side
+/// <c>StipplingType.NoPos</c> flag ("this side has no texture coordinates",
+/// acclient.h:7386) is NOT the same fact as "this surface is untextured" —
+/// every solid-colour polygon carries NoPos (it has no UVs to carry), but
+/// NoPos says nothing about whether the surface itself is textured. The old
+/// extraction conflated the two (<c>isSolid = NoPos || Base1Solid</c>) and,
+/// worse, used NoPos to decide whether to emit the polygon's positive side AT
+/// ALL — dropping every solid-colour polygon on every object. This type is
+/// the ONE place that answers "is this surface textured", built from the
+/// Surface's own Type flags so extraction (isSolid / TextureKey.IsSolid) and
+/// draw-time skip policy agree by construction.
+/// </remarks>
+public static class RetailUntexturedSurfacePolicy
+{
+    public static bool IsUntextured(SurfaceType type) =>
+        (type & (SurfaceType.Base1Image | SurfaceType.Base1ClipMap)) == 0;
+}
+
+/// <summary>
+/// Retail's draw-time policy for an UNTEXTURED (solid-colour) mesh subset on
+/// an ordinary <see cref="AcDream.Core.World.WorldEntity"/>, ported from the
+/// same <c>D3DPolyRender::DrawMesh</c> untextured branch: the subset draws
+/// unless <c>skipNoTexture != 0 &amp;&amp;
+/// RenderDeviceD3D::ObjBuildingOrBuildingPart != 0</c>. <c>skipNoTexture</c>
+/// @0x00820e30 is a global initialised to 1 and never cleared, so in
+/// practice the gate reduces to
+/// <c>RenderDeviceD3D::ObjBuildingOrBuildingPart == 0</c>.
+/// <c>RenderDeviceD3D::DrawBuilding</c> (@0x0059f2a0) sets that flag around
+/// the building-shell draw, so a building shell's own untextured subsets are
+/// the ONE case where an ordinary WorldEntity skips them — statics, scenery,
+/// creatures, and items (<c>DrawMeshInternal</c> @0x0059f360 →
+/// <c>DrawMesh(gfxobj, mesh, arg4: 0)</c>) always draw their untextured
+/// subsets.
+/// </summary>
+/// <remarks>
+/// EnvCell interiors are retail's OTHER "skip untextured" case
+/// (<c>RenderDeviceD3D::DrawEnvCell</c> @0x0059f170 calls
+/// <c>DrawMesh(..., arg4: 1)</c>), but EnvCell/CellStruct geometry never
+/// reaches this predicate — it draws through <c>EnvCellRenderer</c> /
+/// <c>MeshExtractor.PrepareCellStructMeshData</c>, which keeps its own
+/// NoPos-based approximation of the same rule (see
+/// <c>docs/architecture/retail-divergence-register.md</c> AP-234 and
+/// <c>CellMesh.cs</c>'s matching gate).
+/// <para>
+/// ONE shared predicate for <c>WbDrawDispatcher</c>'s classic classifier
+/// (<c>ClassifyBatches</c>), packed classifier (<c>ClassifyPackedBatches</c>),
+/// and the directional-shadow caster walk (<c>AddDirectionalShadowBatches</c>)
+/// — Campaign VM VM6 showed that two hand-maintained classifiers computing
+/// the "same" fact independently drift apart.
+/// </para>
+/// </remarks>
+public static class RetailUntexturedSubsetPolicy
+{
+    public static bool Draws(bool isBuildingShell, bool isUntextured) =>
+        !isUntextured || !isBuildingShell;
+}

src/AcDream.Launcher.Core/Installation/LauncherInstallRecordStore.cs

@@ -29,7 +29,10 @@ public sealed record InstallRecordVerification(
 /// </summary>
 public sealed class LauncherInstallRecordStore
 {
-    public const uint CurrentBakeToolVersion = 4;
+    // Kept in lockstep with AcDream.Content.Pak.PakFormat.CurrentBakeToolVersion
+    // (#426, 2026-08-23: version 5 extracts untextured/solid-colour positive
+    // faces that versions <=4 dropped).
+    public const uint CurrentBakeToolVersion = 5;
 
     private static readonly JsonSerializerOptions SerializerOptions = new()
     {