C Specification

Acceleration structure instances can be built into top-level acceleration structures. Each acceleration structure instance is a separate entry in the top-level acceleration structure which includes all the geometry of a bottom-level acceleration structure at a transformed location. Multiple instances can point to the same bottom level acceleration structure.

An acceleration structure instance is defined by the structure:

// Provided by VK_KHR_acceleration_structure
typedef struct VkAccelerationStructureInstanceKHR {
    VkTransformMatrixKHR          transform;
    uint32_t                      instanceCustomIndex:24;
    uint32_t                      mask:8;
    uint32_t                      instanceShaderBindingTableRecordOffset:24;
    VkGeometryInstanceFlagsKHR    flags:8;
    uint64_t                      accelerationStructureReference;
} VkAccelerationStructureInstanceKHR;

or the equivalent

// Provided by VK_NV_ray_tracing
typedef VkAccelerationStructureInstanceKHR VkAccelerationStructureInstanceNV;

Members

  • transform is a VkTransformMatrixKHR structure describing a transformation to be applied to the acceleration structure.

  • instanceCustomIndex is a 24-bit user-specified index value accessible to ray shaders in the InstanceCustomIndexKHR built-in.

  • mask is an 8-bit visibility mask for the geometry. The instance may only be hit if rayMask & instance.mask != 0

  • instanceShaderBindingTableRecordOffset is a 24-bit offset used in calculating the hit shader binding table index.

  • flags is an 8-bit mask of VkGeometryInstanceFlagBitsKHR values to apply to this instance.

  • accelerationStructureReference is either:

Description

The C language spec does not define the ordering of bit-fields, but in practice, this struct produces the correct layout with existing compilers. The intended bit pattern is for the following:

  • instanceCustomIndex and mask occupy the same memory as if a single int32_t was specified in their place

    • instanceCustomIndex occupies the 24 least significant bits of that memory

    • mask occupies the 8 most significant bits of that memory

  • instanceShaderBindingTableRecordOffset and flags occupy the same memory as if a single int32_t was specified in their place

    • instanceShaderBindingTableRecordOffset occupies the 24 least significant bits of that memory

    • flags occupies the 8 most significant bits of that memory

If a compiler produces code that diverges from that pattern, applications must employ another method to set values according to the correct bit pattern.

Valid Usage (Implicit)

See Also

Document Notes

For more information, see the Vulkan Specification

This page is extracted from the Vulkan Specification. Fixes and changes should be made to the Specification, not directly.

Copyright 2014-2021 The Khronos Group Inc.

SPDX-License-Identifier: CC-BY-4.0