Fixing SBT alignment

docs/vkrt_tuto_manyhits.md.htm

@@ -218,9 +218,10 @@ The size of each group can be described as follows:
 
 ~~~~ C++
   // Sizes
-  uint32_t rayGenSize = groupHandleSize;
-  uint32_t missSize   = groupHandleSize;
-  uint32_t hitSize    = groupHandleSize + sizeof(HitRecordBuffer);
+  uint32_t rayGenSize = baseAlignment;
+  uint32_t missSize   = baseAlignment;
+  uint32_t hitSize =
+      ROUND_UP(groupHandleSize + static_cast<int>(sizeof(HitRecordBuffer)), baseAlignment);
   uint32_t newSbtSize = rayGenSize + 2 * missSize + 2 * hitSize;
 ~~~~
 
@@ -238,14 +239,16 @@ Then write the new SBT like this, where only Hit 1 has extra data.
     memcpy(pBuffer, handles[2], groupHandleSize);  // Miss 1
     pBuffer += missSize;
 
-    memcpy(pBuffer, handles[3], groupHandleSize);  // Hit 0
-    pBuffer += groupHandleSize;
-    pBuffer += sizeof(HitRecordBuffer);  // No data
+    uint8_t* pHitBuffer = pBuffer;
+    memcpy(pHitBuffer, handles[3], groupHandleSize);  // Hit 0
+    // No data
+    pBuffer += hitSize;
 
-    memcpy(pBuffer, handles[4], groupHandleSize);  // Hit 1
-    pBuffer += groupHandleSize;
-    memcpy(pBuffer, &m_hitShaderRecord[0], sizeof(HitRecordBuffer));  // Hit 1 data
-    pBuffer += sizeof(HitRecordBuffer);
+    pHitBuffer = pBuffer;
+    memcpy(pHitBuffer, handles[4], groupHandleSize);  // Hit 1
+    pHitBuffer += groupHandleSize;
+    memcpy(pHitBuffer, &m_hitShaderRecord[0], sizeof(HitRecordBuffer));  // Hit 1 data
+    pBuffer += hitSize;
   }
 ~~~~
 
@@ -255,12 +258,21 @@ Then change the call to `m_alloc.createBuffer` to create the SBT buffer from `sb
   m_rtSBTBuffer = m_alloc.createBuffer(cmdBuf, sbtBuffer, vk::BufferUsageFlagBits::eRayTracingKHR);
 ~~~~
 
+Note: we are using this `define` for rounding up to the correct alignment
+~~~~ C++ 
+#ifndef ROUND_UP
+#define ROUND_UP(v, powerOf2Alignment) (((v) + (powerOf2Alignment)-1) & ~((powerOf2Alignment)-1))
+#endif
+~~~~ 
+
+
 ## `raytrace`
 
 Finally, since the size of the hit group is now larger than just the handle, we need to set the new value of the hit group stride in `HelloVulkan::raytrace`.
 
 ~~~~ C++
-vk::DeviceSize hitGroupStride = progSize + sizeof(HitRecordBuffer);
+vk::DeviceSize hitGroupStride =
+ROUND_UP(m_rtProperties.shaderGroupHandleSize + sizeof(HitRecordBuffer), progOffset);
 ~~~~
 
 !!! Note:
@@ -316,10 +328,11 @@ The size of the SBT will now account for its 3 hit groups:
 Finally, we need to add the new entry as well at the end of the buffer, reusing the handle of the second Hit Group and setting a different color.
 
 ~~~~ C++
-    memcpy(pBuffer, handles[4], groupHandleSize);  // Hit 2
-    pBuffer += groupHandleSize;
-    memcpy(pBuffer, &m_hitShaderRecord[1], sizeof(HitRecordBuffer));  // Hit 2 data
-    pBuffer += sizeof(HitRecordBuffer);
+    pHitBuffer = pBuffer;
+    memcpy(pHitBuffer, handles[4], groupHandleSize);  // Hit 2
+    pHitBuffer += groupHandleSize;
+    memcpy(pHitBuffer, &m_hitShaderRecord[1], sizeof(HitRecordBuffer));  // Hit 2 data
+    pBuffer += hitSize;
 ~~~~
 
 !!! Warning

docs/vkrt_tutorial.md.htm

@@ -1238,9 +1238,9 @@ group for that instance.
 
 The SBT is an array containing the handles to the shader groups used in the ray tracing pipeline. In our example, we
 will create a buffer for the three groups: raygen, miss and closest hit. The size of the handle is given by the
-`shaderGroupHandleSize` member of the ray tracing properties. We will then allocate a buffer of size `3 *
-shaderGroupHandleSize` and will consecutively write the handle of each shader group. To retrieve all the handles, we
-will call `vkGetRayTracingShaderGroupHandlesKHR`.
+`shaderGroupHandleSize` member of the ray tracing properties, but the offset need to be aligned on `shaderGroupBaseAlignment`.
+ We will then allocate a buffer of size `3 * shaderGroupBaseAlignment` and will consecutively write the handle of each shader group.
+  To retrieve all the handles, we will call `vkGetRayTracingShaderGroupHandlesKHR`.
 
 The buffer will have the following information, which will later be used when calling `vkCmdTraceRaysKHR`:
 
@@ -1279,9 +1279,10 @@ void HelloVulkan::createRtShaderBindingTable()
   auto groupCount =
       static_cast<uint32_t>(m_rtShaderGroups.size());               // 3 shaders: raygen, miss, chit
   uint32_t groupHandleSize = m_rtProperties.shaderGroupHandleSize;  // Size of a program identifier
+  uint32_t baseAlignment   = m_rtProperties.shaderGroupBaseAlignment;  // Size of shader alignment
 
   // Fetch all the shader handles used in the pipeline, so that they can be written in the SBT
-  uint32_t sbtSize = groupCount * groupHandleSize;
+  uint32_t sbtSize = groupCount * baseAlignment;
 ````
 
 We then fetch the handles to the shader groups of the pipeline, and let the allocator allocate the device memory and
@@ -1291,16 +1292,21 @@ copy the handles into the SBT:
   std::vector<uint8_t> shaderHandleStorage(sbtSize);
   m_device.getRayTracingShaderGroupHandlesKHR(m_rtPipeline, 0, groupCount, sbtSize,
                                               shaderHandleStorage.data());
+   // Write the handles in the SBT
+  m_rtSBTBuffer = m_alloc.createBuffer(sbtSize, vk::BufferUsageFlagBits::eTransferSrc,
+                                       vk::MemoryPropertyFlagBits::eHostVisible
+                                           | vk::MemoryPropertyFlagBits::eHostCoherent);
+  m_debug.setObjectName(m_rtSBTBuffer.buffer, std::string("SBT").c_str());
+
   // Write the handles in the SBT
-  nvvk::CommandPool genCmdBuf(m_device, m_graphicsQueueIndex);
-  vk::CommandBuffer cmdBuf = genCmdBuf.createCommandBuffer();
-
-  m_rtSBTBuffer =
-      m_alloc.createBuffer(cmdBuf, shaderHandleStorage, vk::BufferUsageFlagBits::eRayTracingKHR);
-  m_debug.setObjectName(m_rtSBTBuffer.buffer, "SBT");
-
-
-  genCmdBuf.submitAndWait(cmdBuf);
+  void* mapped = m_alloc.map(m_rtSBTBuffer);
+  auto* pData  = reinterpret_cast<uint8_t*>(mapped);
+  for(uint32_t g = 0; g < groupCount; g++)
+  {
+    memcpy(pData, shaderHandleStorage.data() + g * groupHandleSize, groupHandleSize);  // raygen
+    pData += baseAlignment;
+  }
+  m_alloc.unmap(m_rtSBTBuffer);
 
   m_alloc.finalizeAndReleaseStaging();
 }
@@ -1360,7 +1366,7 @@ each shader. In our case the stride is simply the size of a shader group handle,
 shader-group-specific data within the SBT, resulting in a larger stride.
 
 ```` C  
-  vk::DeviceSize progSize = m_rtProperties.shaderGroupHandleSize;  // Size of a program identifier
+  vk::DeviceSize progSize = m_rtProperties.shaderGroupBaseAlignment;  // Size of a program identifier
   vk::DeviceSize rayGenOffset   = 0u * progSize;  // Start at the beginning of m_sbtBuffer
   vk::DeviceSize missOffset     = 1u * progSize;  // Jump over raygen
   vk::DeviceSize missStride     = progSize;