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