New documentation

docs/vkrt_tuto_anyhit.md.htm

@@ -235,6 +235,174 @@ As mentioned earlier, for the effect to work, we need to accumulate frames over
 * [Storing or Updating](vkrt_tuto_jitter_cam.md.htm#toc1.4)
 * [Application Frame Update](vkrt_tuto_jitter_cam.md.htm#toc2)
 
+
+# Fixing Pipeline
+
+The above code works, but might not work in the future. The reason is, the shadow ray `traceRayEXT` call in the Closest Hit shader, uses payload 1 
+and when intersecting the object, the any hit shader will be executed using payload 0. In the time of writing those lines, the driver add
+padding and there are no side effect, but this is not how thing should be done.
+
+Each `traceRayEXT` invocation should have as many Hit Groups as there are trace calls with different payload. For the other examples, it is still fine,
+because we are using the `gl_RayFlagsSkipClosestHitShaderNV` flag and the closest hit shader (payload 0) will not be called and there were not 
+any hit or intersection shaders in the Hit Group. But in this example, the closest hit will be skiped, but not the any hit.
+
+**To fix this**, we need to add another hit group. 
+
+This is how the current SBT looks like.
+
+![](Images/anyhit_0.png)
+
+And we need to add the following to the ray tracing pipeline, a copy of the previous Hit Group, with a new AnyHit using the proper payload.
+
+![](Images/anyhit_01.png)
+
+
+## New shaders 
+
+Create two new files `raytrace_0.ahit` and `raytrace_1.ahit`, and rename `raytrace.ahit` to `raytrace_ahit.glsl`
+
+!!! WARNING CMake
+    Cmake need to be re-run to add the new files to the project.
+
+In `raytrace_0.ahit` add the following code 
+
+~~~~ C 
+#version 460
+#extension GL_GOOGLE_include_directive : enable
+
+#define PAYLOAD_0
+#include "raytrace_rahit.glsl"
+~~~~ 
+
+and in `raytrace_1.ahit`, replace `PAYLOAD_0` by `PAYLOAD_1`
+
+Then in `raytrace_ahit.glsl` remove the `#version 460`  and add the following code, so that we have the right layout.
+
+~~~~ C 
+#ifdef PAYLOAD_0
+layout(location = 0) rayPayloadInNV hitPayload prd;
+#elif defined(PAYLOAD_1)
+layout(location = 1) rayPayloadInNV shadowPayload prd;
+#endif
+~~~~ 
+  
+## New Payload 
+
+We cannot simply have a bool for our shadow ray payload. We also need the `seed` for the random function. 
+
+In the `raycommon.glsl` file, add the following structure
+
+~~~~ C
+struct shadowPayload
+{
+  bool isHit;
+  uint seed;
+};
+~~~~
+
+The usage of the shadow payload is done in the closest hit and shadow miss shader. First, let's modify `raytraceShadow.rmiss` to look like this
+
+~~~~ C
+#version 460
+#extension GL_NV_ray_tracing : require
+#extension GL_GOOGLE_include_directive : enable
+
+#include "raycommon.glsl"
+
+layout(location = 1) rayPayloadInNV shadowPayload prd;
+
+void main()
+{
+  prd.isHit = false;
+}
+~~~~ 
+
+The the change in the closest hit shader `raytrace.rchit`, need to change the usage of the payload, but also the call to `traceRayEXT`
+
+Replace the payload to 
+
+~~~~ C 
+layout(location = 1) rayPayloadNV shadowPayload prdShadow;
+~~~~ 
+
+Then just before the call to `traceRayEXT`, initialize the values to 
+
+~~~~ C 
+prdShadow.isHit = true;
+prdShadow.seed  = prd.seed;
+~~~~
+
+and after the trace, set the seed value back to the main payload 
+
+~~~~ C
+prd.seed = prdShadow.seed; 
+~~~~ 
+
+And check if the trace shadow hit an object of not 
+
+~~~~ C 
+if(prdShadow.isHit) 
+~~~~ 
+
+### traceRayEXT 
+
+When we call `traceRayEXT`, since we are using the payload 1 (last argument), we also 
+need the trace to hit the alternative hit group, the one using the payload 1. 
+To do this, we need to set the sbtRecordOffset to 1 
+
+~~~~ C 
+traceRayEXT(topLevelAS,  // acceleration structure
+  flags,       // rayFlags
+  0xFF,        // cullMask
+  1,           // sbtRecordOffset
+  0,           // sbtRecordStride
+  1,           // missIndex
+  origin,      // ray origin
+  tMin,        // ray min range
+  rayDir,      // ray direction
+  tMax,        // ray max range
+  1            // payload (location = 1)
+  );
+~~~~
+
+
+
+
+## Ray tracing Pipeline
+
+The final step is to add the new Hit Group. This is a change in `HelloVulkan::createRtPipeline()`. 
+We need to load the new any hit shader and create a new Hit Group.
+
+Replace the `"shaders/raytrace.rahit.spv"` for `"shaders/raytrace_0.rahit.spv"`
+
+Then, after the creating of the first Hit Group, create a new one, where only the any hit using payload 1 
+is added. We are skipping the closest hit shader in the trace call, so we can ignore it in the Hit Group.
+
+~~~~ C 
+// Payload 1
+vk::ShaderModule ahit1SM =
+    nvvk::createShaderModule(m_device,  //
+                             nvh::loadFile("shaders/raytrace_1.rahit.spv", true, paths));
+hg.setClosestHitShader(VK_SHADER_UNUSED_NV);  // Not used by shadow (skipped)
+stages.push_back({{}, vk::ShaderStageFlagBits::eAnyHitNV, ahit1SM, "main"});
+hg.setAnyHitShader(static_cast<uint32_t>(stages.size() - 1));
+m_rtShaderGroups.push_back(hg);
+~~~~ 
+
+At the end of the function, delete the shader module `ahit1SM`.
+
+
+!!! NOTE Re-Run
+    Everything should work as before, but now it does it right.
+
+
+
+
+
+
+
+
+
 # Final Code
 
 You can find the final code in the folder [ray_tracing_anyhit](https://github.com/nvpro-samples/vk_raytracing_tutorial_KHR/tree/master/ray_tracing_anyhit)