Vaibhav Rathod's Notes

PathTracer Learning - Concept - Device Address Bit

1 min read

  • 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
    • Memory allocation — must enable device address feature
    • Getting the device address
    • Using in AS build

  • Device Address in Shaders

    • GLSL: use uint64_t or buffer reference extension
    • GL_EXT_buffer_reference — typed pointer to buffer
    • 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
    • Using device address after buffer is destroyed
      • Address becomes invalid — GPU will read garbage or crash

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