Fix eyedropper sampling base map at wrong UV on convex/primitive colliders

hit.textureCoord is only filled for non-convex MeshColliders; on a convex
MeshCollider (or Box/Sphere/Capsule) it returns Vector2.zero, so the pick
sampled the foreign material's base map at texel (0,0) - a constant, wrong
color ("base map ignored"). Now the UV is recovered from the renderer mesh
itself (camera ray vs triangles, Moeller-Trumbore, barycentric UV) for
convex/primitive colliders, with the screen-pixel pick as fallback when the
mesh is not readable. The fast non-convex path is unchanged.

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

Assets/Scripts/MeshPainter.Pick.cs

@@ -17,8 +17,10 @@ public partial class MeshPainter
     //     same canonical layout, so the pick matches what is stored bit-for-bit.
     //   - Any foreign object: read texel-exact from its material maps at the hit UV
     //     (BaseMap/MetallicGloss/Occlusion/Emission) combined with the material scalars, per
-    //     the URP/Lit conventions, falling back to scalar properties where a map is absent or
-    //     the collider carries no UV (non-MeshCollider).
+    //     the URP/Lit conventions. The UV comes from hit.textureCoord (non-convex MeshCollider)
+    //     or, when the collider can't supply one (convex MeshCollider / primitive collider), is
+    //     recovered from the renderer's mesh; it falls back to scalars where a map is absent and
+    //     to the on-screen pixel where a base map exists but no UV can be resolved.
 
     [Header("Brush - pipette (eyedropper)")]
     [Tooltip("Pipette mode: a click samples ALL channel values at the hit point on ANY surface " +
@@ -141,7 +143,7 @@ public partial class MeshPainter
 
     /// <summary>
     /// Picks from a foreign object's material at the hit UV: texel-exact texture samples
-    /// (where the surface has a MeshCollider and the map exists) combined with the material
+    /// (where a UV at the hit can be resolved and the map exists) combined with the material
     /// scalars per the URP/Lit conventions, with scalar fallbacks everywhere else.
     /// </summary>
     private bool PickFromForeignMaterial(RaycastHit hit)
@@ -154,10 +156,27 @@ public partial class MeshPainter
         Material mat = rend.sharedMaterial;
         if (!mat) return false;
 
-        // textureCoord is only meaningful for a MeshCollider; other collider shapes carry no UV,
-        // so we fall back to the material's uniform scalar values for those.
-        bool hasUv = col is MeshCollider;
-        Vector2 uv = hasUv ? hit.textureCoord : Vector2.zero;
+        // A UV at the hit is required to sample any material map texel-exact. hit.textureCoord is
+        // only populated for a NON-convex MeshCollider; convex MeshColliders and primitive colliders
+        // (Box/Sphere/Capsule/Terrain) return Vector2.zero. In that case recover the UV from the
+        // renderer's own mesh (ray vs. triangles), which works for any solid collider whose mesh is
+        // readable.
+        Vector2 uv;
+        bool hasUv;
+        if (col is MeshCollider mc && !mc.convex && hit.triangleIndex >= 0)
+        {
+            uv = hit.textureCoord;
+            hasUv = true;
+        }
+        else
+        {
+            hasUv = TryRecoverMeshUv(rend, hit, out uv);
+        }
+
+        // A base map exists but no UV could be resolved (e.g. a non-readable mesh behind a convex
+        // collider): a texel-exact read is impossible, so defer to the rendered on-screen pixel,
+        // which at least reflects the texture - rather than silently picking the flat _BaseColor.
+        if (!hasUv && GetTexProp(mat, "_BaseMap", "_MainTex")) return false;
 
         // --- Albedo: BaseMap (sRGB) * BaseColor ---
         Color baseCol = GetColorProp(mat, "_BaseColor", "_Color", Color.white);
@@ -405,4 +424,85 @@ public partial class MeshPainter
         if (Application.isPlaying) Destroy(tmp); else DestroyImmediate(tmp);
         return c;
     }
+
+    /// <summary>
+    /// Recovers the UV at a hit point by intersecting the camera ray with the renderer's own mesh
+    /// (Moller-Trumbore against each triangle, nearest hit wins). Needed when the collider that was
+    /// hit cannot supply hit.textureCoord - a convex MeshCollider or any primitive collider. Requires
+    /// a MeshFilter whose mesh is readable and UV-mapped; returns false otherwise so the caller can
+    /// fall back. A one-shot click cost, so the O(triangles) sweep is acceptable.
+    /// </summary>
+    private bool TryRecoverMeshUv(Renderer rend, RaycastHit hit, out Vector2 uv)
+    {
+        uv = default;
+        if (!rend) return false;
+        MeshFilter mf = rend.GetComponent<MeshFilter>();
+        Mesh mesh = mf ? mf.sharedMesh : null;
+        if (!mesh || !mesh.isReadable) return false;
+
+        Vector2[] uvs = mesh.uv;
+        if (uvs == null || uvs.Length == 0) return false;
+        Vector3[] verts = mesh.vertices;
+        int[] tris = mesh.triangles;
+
+        // Ray from the paint camera through the surface point (its UV is what we want). Fall back to
+        // shooting along the inverted hit normal when no camera is bound.
+        Vector3 ro, rd;
+        if (paintCamera)
+        {
+            ro = paintCamera.transform.position;
+            rd = (hit.point - ro).normalized;
+        }
+        else
+        {
+            rd = -hit.normal;
+            ro = hit.point - rd * 0.01f;
+        }
+
+        Matrix4x4 l2w = rend.transform.localToWorldMatrix;
+        float bestT = float.MaxValue;
+        Vector3 bestBary = default;
+        int bestTri = -1;
+        for (int i = 0; i + 2 < tris.Length; i += 3)
+        {
+            Vector3 a = l2w.MultiplyPoint3x4(verts[tris[i]]);
+            Vector3 b = l2w.MultiplyPoint3x4(verts[tris[i + 1]]);
+            Vector3 c = l2w.MultiplyPoint3x4(verts[tris[i + 2]]);
+            if (RayTriangle(ro, rd, a, b, c, out float t, out float u, out float v) && t < bestT)
+            {
+                bestT = t;
+                bestBary = new Vector3(1f - u - v, u, v);
+                bestTri = i;
+            }
+        }
+        if (bestTri < 0) return false;
+
+        uv = uvs[tris[bestTri]] * bestBary.x + uvs[tris[bestTri + 1]] * bestBary.y + uvs[tris[bestTri + 2]] * bestBary.z;
+        return true;
+    }
+
+    /// <summary>
+    /// Moller-Trumbore ray/triangle intersection. On a hit, <paramref name="t"/> is the ray distance
+    /// and (<paramref name="u"/>, <paramref name="v"/>) are the barycentric weights of vertices b and
+    /// c (vertex a's weight is 1 - u - v). Small epsilons admit edge hits.
+    /// </summary>
+    private static bool RayTriangle(Vector3 ro, Vector3 rd, Vector3 a, Vector3 b, Vector3 c,
+        out float t, out float u, out float v)
+    {
+        t = u = v = 0f;
+        const float eps = 1e-7f;
+        Vector3 e1 = b - a, e2 = c - a;
+        Vector3 p = Vector3.Cross(rd, e2);
+        float det = Vector3.Dot(e1, p);
+        if (det > -eps && det < eps) return false; // ray parallel to the triangle
+        float inv = 1f / det;
+        Vector3 tvec = ro - a;
+        u = Vector3.Dot(tvec, p) * inv;
+        if (u < -1e-4f || u > 1f + 1e-4f) return false;
+        Vector3 q = Vector3.Cross(tvec, e1);
+        v = Vector3.Dot(rd, q) * inv;
+        if (v < -1e-4f || u + v > 1f + 1e-4f) return false;
+        t = Vector3.Dot(e2, q) * inv;
+        return t > eps;
+    }
 }