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/Shaders/MeshPaintBrush.shader

@@ -51,18 +51,21 @@ Shader "Hidden/MeshPaintBrush"
             float3 _BrushV;        // stamp up axis (unit, local space)
             float3 _BrushNormal;   // surface normal at the hit (unit, local space)
             float  _BrushRadius;   // stamp half-extent in local mesh space
+            float  _BrushNormalCutoff; // cos(maxAngle): paint only if dot(texelN, brushN) >= this (-1 = off)
             sampler2D _MaskTex;
 
             struct appdata
             {
                 float4 vertex : POSITION;   // local 3D position
+                float3 normal : NORMAL;     // local surface normal
                 float2 uv     : TEXCOORD0;  // UV (target in the texture layout)
             };
 
             struct v2f
             {
-                float4 pos   : SV_POSITION;
-                float3 local : TEXCOORD0;   // interpolated local 3D position
+                float4 pos    : SV_POSITION;
+                float3 local  : TEXCOORD0;   // interpolated local 3D position
+                float3 normal : TEXCOORD1;   // interpolated local surface normal
             };
 
             v2f vert (appdata v)
@@ -76,6 +79,7 @@ Shader "Hidden/MeshPaintBrush"
                 #endif
                 o.pos = float4(p, 0.0, 1.0);
                 o.local = v.vertex.xyz;
+                o.normal = v.normal;
                 return o;
             }
 
@@ -91,6 +95,11 @@ Shader "Hidden/MeshPaintBrush"
                 // (both sides of a 3D edge) yet never bleeds onto far/adjacent surfaces past r.
                 clip(_BrushRadius * _BrushRadius - (du * du + dv * dv + dn * dn));
 
+                // Back-face rejection: the opposite side of thin geometry faces ~180deg away,
+                // so drop texels whose normal turns away from the clicked surface past the cutoff.
+                float nl = dot(normalize(i.normal), _BrushNormal);
+                clip(nl - _BrushNormalCutoff);
+
                 // Mask -> opacity. The sphere guarantees |du|,|dv| <= r -> uv in [0,1].
                 float2 muv = float2(du, dv) / _BrushRadius * 0.5 + 0.5;
                 fixed4 m = tex2D(_MaskTex, muv);

Assets/Shaders/MeshPaintPreview.shader

@@ -43,6 +43,7 @@ Shader "Hidden/MeshPaintPreview"
             float3 _BrushV;         // stamp up axis (unit, local space)
             float3 _BrushNormal;    // surface normal at the hit (unit, local space)
             float  _BrushRadius;    // stamp half-extent in local mesh space
+            float  _BrushNormalCutoff; // cos(maxAngle): show only if dot(texelN, brushN) >= this (-1 = off)
             float  _PreviewOpacity; // overall preview alpha multiplier (>1 saturates to fully opaque)
             float  _PreviewMode;    // 0 = on-surface (mesh), 1 = flat decal quad
             sampler2D _MaskTex;
@@ -50,14 +51,16 @@ Shader "Hidden/MeshPaintPreview"
             struct appdata
             {
                 float4 vertex : POSITION;
+                float3 normal : NORMAL;
                 float2 uv     : TEXCOORD0;
             };
 
             struct v2f
             {
-                float4 pos   : SV_POSITION;
-                float3 local : TEXCOORD0;  // object-space position (on-surface box test)
-                float2 uv    : TEXCOORD1;  // quad uv (decal sampling)
+                float4 pos    : SV_POSITION;
+                float3 local  : TEXCOORD0;  // object-space position (on-surface box test)
+                float2 uv     : TEXCOORD1;  // quad uv (decal sampling)
+                float3 normal : TEXCOORD2;  // object-space normal (on-surface back-face test)
             };
 
             v2f vert (appdata v)
@@ -66,6 +69,7 @@ Shader "Hidden/MeshPaintPreview"
                 o.pos = UnityObjectToClipPos(v.vertex);
                 o.local = v.vertex.xyz;
                 o.uv = v.uv;
+                o.normal = v.normal;
                 return o;
             }
 
@@ -80,6 +84,8 @@ Shader "Hidden/MeshPaintPreview"
                     float dv = dot(d, _BrushV);
                     float dn = dot(d, _BrushNormal);
                     clip(_BrushRadius * _BrushRadius - (du * du + dv * dv + dn * dn));
+                    // Back-face rejection: hide the preview on the opposite side of thin geometry.
+                    clip(dot(normalize(i.normal), _BrushNormal) - _BrushNormalCutoff);
                     muv = float2(du, dv) / _BrushRadius * 0.5 + 0.5;
                 }
                 else