Concept: VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT

• Parent: PathTracer Learning - Phase 3 - GPU and Vulkan

What It Is

• A Vulkan buffer usage flag that enables GPU-side pointer access
• Allows shaders to access buffer data via a raw 64-bit GPU address
• Required for: Shader Binding Table, AS input buffers, AS scratch buffers
• Extension: VK_KHR_buffer_device_address (core in Vulkan 1.2)

Why It’s Needed for Ray Tracing

• Acceleration structure build inputs must be specified as device addresses
• SBT entries contain shader handles accessed via device address
• Allows the GPU to follow pointers — enables complex data structures on GPU
• Without it: can’t build BLAS/TLAS or create SBT

How to Use

• Buffer creation

  VkBufferCreateInfo bufferInfo{};
  bufferInfo.usage = VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT
                   | VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_BUILD_INPUT_READ_ONLY_BIT_KHR;
  // ... create buffer ...

• Memory allocation — must enable device address feature

  VkMemoryAllocateFlagsInfo flagsInfo{};
  flagsInfo.sType = VK_STRUCTURE_TYPE_MEMORY_ALLOCATE_FLAGS_INFO;
  flagsInfo.flags = VK_MEMORY_ALLOCATE_DEVICE_ADDRESS_BIT;
  // chain into VkMemoryAllocateInfo.pNext

• Getting the device address

  VkBufferDeviceAddressInfo addressInfo{};
  addressInfo.sType = VK_STRUCTURE_TYPE_BUFFER_DEVICE_ADDRESS_INFO;
  addressInfo.buffer = myBuffer;
  VkDeviceAddress address = vkGetBufferDeviceAddress(device, &addressInfo);

• Using in AS build

  geometry.geometry.triangles.vertexData.deviceAddress = vertexBufferAddress;
  geometry.geometry.triangles.indexData.deviceAddress  = indexBufferAddress;

Device Address in Shaders

• GLSL: use uint64_t or buffer reference extension
• GL_EXT_buffer_reference — typed pointer to buffer

  #extension GL_EXT_buffer_reference : require
  layout(buffer_reference, scalar) buffer VertexBuffer {
      Vertex vertices[];
  };
  // In shader:
  VertexBuffer vb = VertexBuffer(instanceData[gl_InstanceCustomIndexEXT].vertexAddress);
  Vertex v = vb.vertices[gl_PrimitiveID * 3 + 0];

• This is how NVPathtracer accesses per-instance vertex data in closest-hit shader

Common Mistakes

• Forgetting VK_MEMORY_ALLOCATE_DEVICE_ADDRESS_BIT on memory allocation
  • Buffer creation succeeds but vkGetBufferDeviceAddress returns 0
• Not enabling bufferDeviceAddress feature in VkPhysicalDeviceVulkan12Features

  VkPhysicalDeviceVulkan12Features features12{};
  features12.bufferDeviceAddress = VK_TRUE;

• Using device address after buffer is destroyed
  • Address becomes invalid — GPU will read garbage or crash

Related

• PathTracer Learning - BLAS and TLAS
• PathTracer Learning - Vulkan RT Pipeline