Fix #19: Verify sun traces with a reverse occlusion trace

KeepersCompound.Lightmapper/LightMapper.cs

@@ -564,8 +564,8 @@ public class LightMapper
             }
 
             var visibleSet = settings.FastPvs switch {
-                true => pvs.ComputeVisibilityFast(lightCellMap[i]),
-                false => pvs.ComputeVisibilityExact(_lights[i].Position, lightCellMap[i], _lights[i].Radius)
+                true => pvs.ComputeVisibilityFast(cellIdx),
+                false => pvs.ComputeVisibilityExact(_lights[i].Position, cellIdx, _lights[i].Radius)
             };
             
             // Log.Information("Light {i} sees {c} cells", i, visibleSet.Count);
@@ -1068,16 +1068,17 @@ public class LightMapper
     // TODO: direction should already be normalised here
     private bool TraceSunRay(Vector3 origin, Vector3 direction)
     {
-        // Avoid self intersection
-        origin += direction * MathUtils.Epsilon;
-        
         var hitResult = _scene.Trace(new Ray
         {
             Origin = origin,
             Direction = Vector3.Normalize(direction),
         });
 
-        if (hitResult)
+        // If origin is very close to a wall, the initial trace to the sun sometimes misses the wall. Now that we have
+        // backface culling enabled in Embree, this can result in reaching a sky when we shouldn't.
+        // By doing another occlusion trace in the reverse direction we fix this. Any backfaces we passed through in
+        // the initial trace become frontfaces to be occluded by.
+        if (hitResult && !TraceOcclusion(_scene, hitResult.Position + hitResult.ErrorOffset * hitResult.Normal, origin))
         {
             return _triangleTypeMap[(int)hitResult.PrimId] == SurfaceType.Sky;
         }

KeepersCompound.Lightmapper/PotentiallyVisibleSet.cs

@@ -154,7 +154,7 @@ public class PotentiallyVisibleSet
             return [];
         }
 
-        var visibleCells = new HashSet<int>();
+        var visibleCells = new HashSet<int> { cellIdx };
         foreach (var edgeIdx in _graph[cellIdx].EdgeIndices)
         {
             var edge = _edges[edgeIdx];