Correct Alpha handling in RT Pipeline example, also work around microsoft/DirectXShaderCompiler#6464

Author: devsh

71_RayTracingPipeline/app_resources/common.hlsl

@@ -3,6 +3,7 @@
 
 #include "nbl/builtin/hlsl/cpp_compat.hlsl"
 #include "nbl/builtin/hlsl/cpp_compat/basic.h"
+#include "nbl/builtin/hlsl/random/pcg.hlsl"
 
 NBL_CONSTEXPR uint32_t WorkgroupSize = 16;
 NBL_CONSTEXPR uint32_t MAX_UNORM_10 = 1023;
@@ -196,10 +197,20 @@ struct MaterialId
 
 struct [raypayload] PrimaryPayload
 {
-    float32_t3 worldNormal : read(caller) : write(closesthit);
-    float32_t rayDistance : read(caller) : write(closesthit, miss);
-    float32_t alphaThreshold : read(closesthit, anyhit) : write(caller);
-    MaterialId materialId : read(caller) : write(closesthit);
+    using generator_t = nbl::hlsl::random::Pcg;
+/* bugged out by https://github.com/microsoft/DirectXShaderCompiler/issues/6464
+    bool nextDiscard(const float32_t alpha)
+    {
+        const uint32_t bitpattern = pcg();
+        const float32_t xi = (float32_t(bitpattern)+0.5f)/float32_t(0xFFFFFFFF);
+        return xi > alpha;
+    }
+*/
+
+    float32_t3  worldNormal : read(caller) : write(closesthit);
+    float32_t   rayDistance : read(caller) : write(closesthit, miss);
+    generator_t pcg         : read(anyhit) : write(caller,anyhit);
+    MaterialId  materialId  : read(caller) : write(closesthit);
 
 };
 

71_RayTracingPipeline/app_resources/raytrace.rahit.hlsl

@@ -7,10 +7,10 @@ void main(inout PrimaryPayload payload, in BuiltInTriangleIntersectionAttributes
 {
     const int instID = InstanceID();
     const STriangleGeomInfo geom = vk::RawBufferLoad < STriangleGeomInfo > (pc.triangleGeomInfoBuffer + instID * sizeof(STriangleGeomInfo));
-    const Material material = nbl::hlsl::_static_cast<Material>(geom.material);
-    
-    if (material.alpha > payload.alphaThreshold)
-    {
+
+    // Should have been a method of the payload but https://github.com/microsoft/DirectXShaderCompiler/issues/6464 stops it
+    // alpha is quantized to 10 bits
+    const uint32_t bitpattern = payload.pcg()>>22;
+    if (bitpattern > geom.material.alpha)
         IgnoreHit();
-    }
 }

71_RayTracingPipeline/app_resources/raytrace.rgen.hlsl

@@ -2,7 +2,6 @@
 
 #include "nbl/builtin/hlsl/jit/device_capabilities.hlsl"
 #include "nbl/builtin/hlsl/random/xoroshiro.hlsl"
-#include "nbl/builtin/hlsl/random/pcg.hlsl"
 
 #include "nbl/builtin/hlsl/glsl_compat/core.hlsl"
 #include "nbl/builtin/hlsl/spirv_intrinsics/raytracing.hlsl"
@@ -28,8 +27,8 @@ void main()
     const uint32_t3 launchSize = DispatchRaysDimensions();
     const uint32_t2 coords = launchID.xy;
 
-    const uint32_t seed1 = nbl::hlsl::Pcg::construct(pc.frameCounter)();
-    const uint32_t seed2 = nbl::hlsl::Pcg::construct(launchID.y * launchSize.x + launchID.x)();
+    const uint32_t seed1 = nbl::hlsl::random::Pcg::create(pc.frameCounter)();
+    const uint32_t seed2 = nbl::hlsl::random::Pcg::create(launchID.y * launchSize.x + launchID.x)();
     nbl::hlsl::Xoroshiro64StarStar rnd = nbl::hlsl::Xoroshiro64StarStar::construct(uint32_t2(seed1, seed2));
 
     float32_t3 hitValues = float32_t3(0, 0, 0);
@@ -55,7 +54,7 @@ void main()
         rayDesc.TMax = 10000.0;
 
         PrimaryPayload payload;
-        payload.alphaThreshold = nextRandomUnorm(rnd);
+        payload.pcg = PrimaryPayload::generator_t::create(rnd());
         TraceRay(topLevelAS, RAY_FLAG_NONE, 0xff, ERT_PRIMARY, 0, EMT_PRIMARY, rayDesc, payload);
 
         const float32_t rayDistance = payload.rayDistance;

71_RayTracingPipeline/app_resources/raytrace_shadow.rahit.hlsl

@@ -9,6 +9,9 @@ void main(inout OcclusionPayload payload, in BuiltInTriangleIntersectionAttribut
     const STriangleGeomInfo geom = vk::RawBufferLoad < STriangleGeomInfo > (pc.triangleGeomInfoBuffer + instID * sizeof(STriangleGeomInfo));
     const Material material = nbl::hlsl::_static_cast<Material>(geom.material);
 
-    payload.attenuation = material.alpha * payload.attenuation;
+    payload.attenuation = (1.f-material.alpha) * payload.attenuation;
+    // arbitrary constant
+//    if (payload.attenuation < 1.f/1024.f)
+//        TerminateRay();
     IgnoreHit();
 }

71_RayTracingPipeline/main.cpp

@@ -1130,7 +1130,7 @@ class RaytracingPipelineApp final : public examples::SimpleWindowedApplication,
           .diffuse = {0.2, 0.8, 0.2},
           .specular = {0.8, 0.8, 0.8},
           .shininess = 1.0f,
-          .alpha = 0.8,
+          .alpha = 0.2,
         },
         .transform = getTranslationMatrix(5.0f, 1.0f, 0),
       },