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.Cpu.cs

@@ -167,6 +167,11 @@ public partial class MeshPainter
 
         float invR = 1f / radiusLocal;
         float r2 = radiusLocal * radiusLocal;
+        // Back-face rejection: skip texels whose surface normal faces away from the clicked
+        // surface (the opposite side of thin geometry faces ~180deg away). cos(maxAngle); at
+        // 180deg it is -1 -> test disabled. n is unit, so dot(normalize(texelN), n) >= cutoff.
+        float nCutoff = BrushNormalCutoff;
+        bool doNormalTest = _normals != null && nCutoff > -0.9999f;
         int triCount = _tris.Length / 3;
         float ts = textureSize;
 
@@ -226,6 +231,18 @@ public partial class MeshPainter
                     // bleed across a seam / around a corner. Also guarantees |du|,|dv| <= r -> mu,mv in [0,1].
                     if (du * du + dv * dv + dn * dn > r2) continue;
 
+                    // Reject the opposite side of thin geometry (interpolated texel normal vs. brush normal).
+                    if (doNormalTest)
+                    {
+                        Vector3 tn0 = _normals[i0], tn1 = _normals[i1], tn2 = _normals[i2];
+                        float nx = a * tn0.x + b * tn1.x + c * tn2.x;
+                        float ny = a * tn0.y + b * tn1.y + c * tn2.y;
+                        float nz = a * tn0.z + b * tn1.z + c * tn2.z;
+                        float nl = nx * n.x + ny * n.y + nz * n.z;
+                        float nmag = Mathf.Sqrt(nx * nx + ny * ny + nz * nz);
+                        if (nmag > 1e-6f && nl < nCutoff * nmag) continue;
+                    }
+
                     float mu = du * invR * 0.5f + 0.5f;
                     float mv = dv * invR * 0.5f + 0.5f;
                     float maskOp = SampleMaskOpacity(mu, mv);