VkAccelerationStructureGeometryMicromapDataKHR

When building micromaps with vkCmdBuildAccelerationStructuresKHR, a VkAccelerationStructureGeometryMicromapDataKHR structure can be added to the pNext chain of VkAccelerationStructureGeometryKHR to specify the data used to build the micromap.

The VkAccelerationStructureGeometryMicromapDataKHR structure is defined as:

typedef struct VkAccelerationStructureGeometryMicromapDataKHR {
    VkStructureType sType;
    const void* pNext;
    uint32_t usageCountsCount;
    const VkMicromapUsageKHR* pUsageCounts;
    const VkMicromapUsageKHR* const* ppUsageCounts;
    VkDeviceAddress data;
    VkDeviceAddress triangleArray;
    VkDeviceSize triangleArrayStride;
} VkAccelerationStructureGeometryMicromapDataKHR;

pub struct AccelerationStructureGeometryMicromapDataKHR {
    s_type: vk::StructureType,
    p_next: *const c_void,
    usage_counts_count: u32,
    p_usage_counts: *const vk::MicromapUsageKHR,
    pp_usage_counts: *const *const vk::MicromapUsageKHR,
    data: vk::DeviceAddress,
    triangle_array: vk::DeviceAddress,
    triangle_array_stride: vk::DeviceSize,
}

• sType is a VkStructureType value identifying this structure.
• pNext is NULL or a pointer to a structure extending this structure.
• usageCountsCount specifies the number of usage counts structures that will be used to determine the size of this micromap.
• pUsageCounts is a pointer to an array of VkMicromapUsageKHR structures.
• ppUsageCounts is a pointer to an array of pointers to VkMicromapUsageKHR structures.
• data is the device address to memory which contains the data for the micromap.
• triangleArray is the device address to memory containing the VkMicromapTriangleKHR data
• triangleArrayStride is the stride in bytes between each element of triangleArray

Only one of pUsageCounts or ppUsageCounts can be a valid pointer, the other must be NULL. The elements of the non-NULL array describe the total counts used to build each micromap. Each element contains a count which is the number of micromap triangles of that format and subdivisionLevel contained in the micromap. Multiple elements with the same format and subdivisionLevel are allowed and the total count for that format and subdivisionLevel is the sum of the count for each element.

Each micromap triangle refers to one element in triangleArray which contains the format and subdivisionLevel for that particular triangle as well as a dataOffset in bytes which is the location relative to data where that triangle’s micromap data begins. The data at triangleArray is laid out as a 4 byte unsigned integer for the dataOffset followed by a 2 byte unsigned integer for the subdivision level then a 2 byte unsigned integer for the format. In practice, compilers compile VkMicromapTriangleKHR to match this pattern.

For opacity micromaps, the data at data is packed as either one bit per element for VK_OPACITY_MICROMAP_FORMAT_2_STATE_KHR or two bits per element for VK_OPACITY_MICROMAP_FORMAT_4_STATE_KHR and is packed from LSB to MSB in each byte. The data at each index in those bytes is interpreted as discussed in Ray Opacity Micromap.