Implemented implicit backface culling of triangle meshes.

This implementation utilises the order of the vertices of each triangle to determine whether the side facing the direction of the ray is the front or back side. The front side is defined as the side where all vertices of the triangle are defined in clockwise order.
The implementation makes use of one of the two integer padding fields of the ray data structure to declare whether the ray needs to cull backfaces or not. Thus to enable backface culling for a ray, the new member variable "int doBackfaceCulling;" needs to be set to true/1 either manually or by using the new method "void SetDoBackfaceCulling(bool _bDoBackfaceCulling)".
If backface culling is disabled via the CMake config switch (RR_ENABLE_BACKFACE_CULL; off by default) all backface culling checks in the intersection kernels are disabled at compile time to make sure that there is no performance impact when not using the feature.

Author: scschaefer

CMakeLists.txt

@@ -11,6 +11,7 @@ option(RR_NO_TESTS "Don't add any unit tests and remove any test functionality f
 option(RR_ENABLE_STATIC "Create static libraries rather than dynamic" OFF)
 option(RR_SHARED_CALC "Link Calc(compute abstraction layer) dynamically" OFF)
 option(RR_ENABLE_RAYMASK "Enable ray masking in intersection kernels" OFF)
+option(RR_ENABLE_BACKFACE_CULL "Enable backface culling in intersection kernels" OFF)
 #option(RR_TUTORIALS "Add tutorials projects" OFF)
 option(RR_SAFE_MATH "use safe math" OFF)
 mark_as_advanced(FORCE RR_USE_VULKAN)

RadeonRays/CMakeLists.txt

@@ -195,6 +195,10 @@ if (RR_ENABLE_RAYMASK)
     target_compile_definitions(RadeonRays PRIVATE RR_RAY_MASK)
 endif (RR_ENABLE_RAYMASK)
 
+if (RR_ENABLE_BACKFACE_CULL)
+    target_compile_definitions(RadeonRays PRIVATE RR_BACKFACE_CULL)
+endif (RR_ENABLE_BACKFACE_CULL)
+
 if (RR_USE_OPENCL)
     target_link_libraries(RadeonRays PUBLIC OpenCL::OpenCL)
     target_compile_definitions(RadeonRays PUBLIC USE_OPENCL=1)

RadeonRays/include/math/ray.h

@@ -42,6 +42,7 @@ namespace RadeonRays
             SetTime(time);
             SetMask(-1);
             SetActive(true);
+            SetDoBackfaceCulling(false);
         }
 
         float3 operator ()(float t) const
@@ -89,9 +90,20 @@ namespace RadeonRays
             return extra.y > 0;
         }
 
+        void SetDoBackfaceCulling(bool _bDoBackfaceCulling)
+        {
+            doBackfaceCulling = _bDoBackfaceCulling ? 1 : 0;
+        }
+
+        bool GetDoBackfaceCulling() const
+        {
+            return doBackfaceCulling > 0;
+        }
+
         float4 o;
         float4 d;
         int2 extra;
-        int2 padding;
+        int doBackfaceCulling;
+        int padding;
     };
 }

RadeonRays/src/intersector/intersector_2level.cpp

@@ -127,13 +127,15 @@ namespace RadeonRays
         , m_gpudata(new GpuData(device))
         , m_cpudata(new CpuData)
     {
-        std::string buildopts =
+        std::string buildopts;
 #ifdef RR_RAY_MASK
-            "-D RR_RAY_MASK ";
-#else
-            "";
+        buildopts.append("-D RR_RAY_MASK ");
 #endif
-        
+
+#ifdef RR_BACKFACE_CULL
+        buildopts.append("-D RR_BACKFACE_CULL ");
+#endif // RR_BACKFACE_CULL
+
 #ifdef USE_SAFE_MATH
         buildopts.append("-D USE_SAFE_MATH ");
 #endif

RadeonRays/src/intersector/intersector_bittrail.cpp

@@ -88,13 +88,15 @@ namespace RadeonRays
         , m_gpudata(new GpuData(device))
         , m_bvh(nullptr)
     {
-        std::string buildopts =
+        std::string buildopts;
 #ifdef RR_RAY_MASK
-            "-D RR_RAY_MASK ";
-#else
-            "";
+        buildopts.append("-D RR_RAY_MASK ");
 #endif
-        
+
+#ifdef RR_BACKFACE_CULL
+        buildopts.append("-D RR_BACKFACE_CULL ");
+#endif // RR_BACKFACE_CULL
+
 #ifdef USE_SAFE_MATH
         buildopts.append("-D USE_SAFE_MATH ");
 #endif

RadeonRays/src/intersector/intersector_hlbvh.cpp

@@ -77,13 +77,15 @@ namespace RadeonRays
         , m_gpudata(new GpuData(device))
         , m_bvh(nullptr)
     {
-        std::string buildopts =
+        std::string buildopts;
 #ifdef RR_RAY_MASK
-            "-D RR_RAY_MASK ";
-#else
-            "";
+        buildopts.append("-D RR_RAY_MASK ");
 #endif
-        
+
+#ifdef RR_BACKFACE_CULL
+        buildopts.append("-D RR_BACKFACE_CULL ");
+#endif // RR_BACKFACE_CULL
+
 #ifdef USE_SAFE_MATH
         buildopts.append("-D USE_SAFE_MATH ");
 #endif

RadeonRays/src/intersector/intersector_lds.cpp

@@ -100,6 +100,11 @@ namespace RadeonRays
 #ifdef RR_RAY_MASK
         buildopts.append("-D RR_RAY_MASK ");
 #endif
+
+#ifdef RR_BACKFACE_CULL
+        buildopts.append("-D RR_BACKFACE_CULL ");
+#endif // RR_BACKFACE_CULL
+
 #ifdef USE_SAFE_MATH
         buildopts.append("-D USE_SAFE_MATH ");
 #endif

RadeonRays/src/intersector/intersector_short_stack.cpp

@@ -83,13 +83,15 @@ namespace RadeonRays
         , m_gpudata(new GpuData(device))
         , m_bvh(nullptr)
     {
-        std::string buildopts =
+        std::string buildopts;
 #ifdef RR_RAY_MASK
-            "-D RR_RAY_MASK ";
-#else
-            "";
+        buildopts.append("-D RR_RAY_MASK ");
 #endif
-        
+
+#ifdef RR_BACKFACE_CULL
+        buildopts.append("-D RR_BACKFACE_CULL ");
+#endif // RR_BACKFACE_CULL
+
 #ifdef USE_SAFE_MATH
         buildopts.append("-D USE_SAFE_MATH ");
 #endif

RadeonRays/src/intersector/intersector_skip_links.cpp

@@ -81,13 +81,15 @@ namespace RadeonRays
         , m_gpudata(new GpuData(device))
         , m_bvh(nullptr)
     {
-        std::string buildopts =
+        std::string buildopts;
 #ifdef RR_RAY_MASK
-            "-D RR_RAY_MASK ";
-#else
-            "";
+        buildopts.append("-D RR_RAY_MASK ");
 #endif
 
+#ifdef RR_BACKFACE_CULL
+        buildopts.append("-D RR_BACKFACE_CULL ");
+#endif // RR_BACKFACE_CULL
+
 #ifdef USE_SAFE_MATH
         buildopts.append("-D USE_SAFE_MATH ");
 #endif

RadeonRays/src/kernels/CL/common.cl

@@ -55,7 +55,8 @@ typedef struct
     float4 o;
     float4 d;
     int2 extra;
-    int2 padding;
+    int doBackfaceCulling;
+    int padding;
 } ray;
 
 // Intersection definition
@@ -96,6 +97,12 @@ float ray_get_time(ray const* r)
     return r->d.w;
 }
 
+INLINE
+int ray_get_doBackfaceCull(ray const* r)
+{
+    return r->doBackfaceCulling;
+}
+
 /*************************************************************************
 FUNCTIONS
 **************************************************************************/
@@ -172,6 +179,14 @@ float fast_intersect_triangle(ray r, float3 v1, float3 v2, float3 v3, float t_ma
 {
     float3 const e1 = v2 - v1;
     float3 const e2 = v3 - v1;
+
+#ifdef RR_BACKFACE_CULL
+    if (ray_get_doBackfaceCull(&r) && dot(cross(e1, e2), r.d.xyz) > 0.f)
+    {
+        return t_max;
+    }
+#endif // RR_BACKFACE_CULL
+
     float3 const s1 = cross(r.d.xyz, e2);
 
     float denom = dot(s1, e1);