MeshPainter: Rückseiten-Abweisung über Flächen-Normale bei großem Brush

Zusätzlich zum Kugel-Test wird die Oberflächen-Normale jedes Texels gegen
die Brush-Normale geprüft (CPU/GPU/Preview). Neues Inspector-Feld
"Brush Max Surface Angle" (90-180°, Default 120°). Bei fehlenden Mesh-Normalen
Degradation zur reinen Kugel.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/Components/MeshPainter/MeshPainter.cs

@@ -266,6 +266,21 @@ public partial class MeshPainter : MonoBehaviour
     [Range(0.01f, 1f)]
     public float brushSizeScrollFactor = 0.15f;
 
+    [Tooltip("Max angle (deg) between a texel's surface normal and the clicked surface for the brush to " +
+             "still paint it. Prevents a large brush from bleeding onto the opposite side of thin geometry " +
+             "(that side faces ~180deg away). Lower = stricter separation; 180 = disabled (paint everything " +
+             "inside the sphere, old behavior). ~120 keeps corners/rounded edges while cutting the back face.")]
+    [Range(90f, 180f)]
+    public float brushMaxSurfaceAngle = 120f;
+
+    /// <summary>
+    /// cos(brushMaxSurfaceAngle): a texel is painted only if dot(texelNormal, brushNormal) >= this.
+    /// At 180deg the cutoff is -1 (always true) -> the normal test is effectively disabled. Returns -1
+    /// when the mesh has no normals (test not possible) so the brush degrades to the plain sphere.
+    /// </summary>
+    private float BrushNormalCutoff =>
+        _normals != null ? Mathf.Cos(brushMaxSurfaceAngle * Mathf.Deg2Rad) : -1f;
+
     /// <summary>
     /// Raised after Shift+wheel changes <see cref="brushSize"/> so the in-game HUD can re-sync its
     /// brush-size slider (the slider then re-clamps the value to its own configured range).
@@ -372,6 +387,7 @@ public partial class MeshPainter : MonoBehaviour
 
     // Mesh caches (in local/collider space).
     private Vector3[] _verts;
+    private Vector3[] _normals;     // per-vertex normals (brush back-face rejection); null if unavailable
     private int[] _tris;
     private Vector2[] _uvs;
     private Vector3[] _triCenter;   // centroid per triangle (CPU culling)
@@ -464,6 +480,7 @@ public partial class MeshPainter : MonoBehaviour
     private static readonly int BrushUId = Shader.PropertyToID("_BrushU");
     private static readonly int BrushVId = Shader.PropertyToID("_BrushV");
     private static readonly int BrushNormalId = Shader.PropertyToID("_BrushNormal");
+    private static readonly int BrushNormalCutoffId = Shader.PropertyToID("_BrushNormalCutoff");
     private static readonly int MaskTexId = Shader.PropertyToID("_MaskTex");
     private static readonly int PadId = Shader.PropertyToID("_Pad");
     private static readonly int PreviewOpacityId = Shader.PropertyToID("_PreviewOpacity");
@@ -773,6 +790,7 @@ public partial class MeshPainter : MonoBehaviour
             _previewMat.SetVector(BrushUId, u);
             _previewMat.SetVector(BrushVId, v);
             _previewMat.SetVector(BrushNormalId, n);
+            _previewMat.SetFloat(BrushNormalCutoffId, BrushNormalCutoff);
 
             // Re-draw the collider mesh; the shader clips it to the brush box. The collider mesh is in
             // the same local space as the raycast hit and valid for skinned (baked) and static meshes.