Fix horizon culling causing terrain tiles to disappear

src/rocky/Horizon.cpp

@@ -4,6 +4,7 @@
  * MIT License
  */
 #include "Horizon.h"
+#include <cmath>
 
 using namespace ROCKY_NAMESPACE;
 
@@ -123,9 +124,21 @@ Horizon::isVisible(double x, double y, double z, double radius) const
     VT = target - _eye;
     double a = glm::dot(VT, -_eyeUnit);
     double b = a * _coneTan;
-    double c = sqrt(glm::dot(VT, VT) - a * a);
+    double VTdotVT = glm::dot(VT, VT);
+    double cSquared = VTdotVT - a * a;
+
+    // Guard against numerical precision issues that could produce negative value
+    if (cSquared < 0.0)
+        cSquared = 0.0;
+
+    double c = sqrt(cSquared);
     double d = c - b;
     double e = d * _coneCos;
+
+    // Check for NaN in the calculation (indicates numerical instability)
+    if (std::isnan(e))
+        return true; // Default to visible if we can't determine
+
     if (e > -radius)
         return true;
 

src/rocky/vsg/terrain/SurfaceNode.h

@@ -113,16 +113,19 @@ namespace ROCKY_NAMESPACE
             auto viewID = rv.getCommandBuffer()->viewID;
             auto& horizon = (*_horizon)[viewID];
 
+            // Use the bounding sphere radius as margin for more conservative culling
+            double cullRadius = worldBoundingSphere.radius;
+
             if (_horizonCullingPoint_valid)
             {
-                return horizon.isVisible(_horizonCullingPoint);
+                return horizon.isVisible(_horizonCullingPoint, cullRadius);
             }
             else
             {
                 for (int p = 0; p < 4; ++p)
                 {
                     auto& wp = _worldPoints[p];
-                    if (horizon.isVisible(wp.x, wp.y, wp.z))
+                    if (horizon.isVisible(wp.x, wp.y, wp.z, cullRadius))
                         return true;
                 }
                 return false;