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mm_opacity

README.md

Opacity Micro-Map in Vulkan

Opacity Micro-Map Visualization

Overview

This sample demonstrates the implementation of Opacity Micro-Maps in Vulkan for efficient ray tracing.

Key Concepts

  • Encodes visibility for triangle sub-regions
  • Triangle classification: opaque (blue), transparent unknown (red), transparent (invisible)
  • Selective AnyHit shader invocation based on opacity state

Implementation Details

Micro-Triangle Generation

  • 4^subdivision triangles per subdivision level
  • Barycentric coordinate mapping using BirdCurveHelper::micro2bary
  • World position projection for radius-based classification

Data Structures

  1. Value Buffer: 2-state (1-bit) or 4-state (2-bit) opacity encoding
  2. VkMicromapTriangleKHR Buffer: Maps triangles to value buffer data
  3. Index Buffer: Continuous array for VkMicromapTriangleKHR indexing
  4. Micromap object: VkMicromapEXT (EXT path) or VkAccelerationStructureKHR (KHR path)

BLAS Integration

  • EXT path: VkAccelerationStructureTrianglesOpacityMicromapEXT chained via pNext
  • KHR path: VkAccelerationStructureTrianglesOpacityMicromapKHR chained via pNext

VK_KHR_opacity_micromap Support

The sample supports both VK_EXT_opacity_micromap (original NV extension) and VK_KHR_opacity_micromap (the promoted KHR standard). At startup the sample requests both; KHR is preferred when available, with EXT as a fallback for drivers that do not yet expose the KHR extension.

Architectural difference

The two extensions differ fundamentally in how the micromap object is built and referenced:

VK_EXT_opacity_micromap VK_KHR_opacity_micromap
Micromap handle VkMicromapEXT VkAccelerationStructureKHR
Create function vkCreateMicromapEXT vkCreateAccelerationStructure2KHR
Build function vkCmdBuildMicromapsEXT vkCmdBuildAccelerationStructuresKHR
Geometry type (dedicated API) VK_GEOMETRY_TYPE_MICROMAP_KHR
AS type (dedicated API) VK_ACCELERATION_STRUCTURE_TYPE_OPACITY_MICROMAP_KHR
Geometry data VkMicromapBuildInfoEXT VkAccelerationStructureGeometryMicromapDataKHR via pNext
BLAS attachment VkAccelerationStructureTrianglesOpacityMicromapEXT VkAccelerationStructureTrianglesOpacityMicromapKHR

KHR-specific spec rules (validation gotchas)

When using the KHR build path, several rules differ from a regular BLAS build:

  • VK_KHR_device_address_commands must also be requested — it is a required dependency of VK_KHR_opacity_micromap.
  • vkGetAccelerationStructureBuildSizesKHR: pMaxPrimitiveCounts must be NULL (size is derived from the usage counts on the geometry pNext chain).
  • vkCreateAccelerationStructure2KHR (not vkCreateAccelerationStructureKHR) is required for VK_ACCELERATION_STRUCTURE_TYPE_OPACITY_MICROMAP_KHR; it takes a VkDeviceAddressRangeKHR instead of a VkBuffer.
  • vkCmdBuildAccelerationStructuresKHR: ppBuildRangeInfos must be a valid pointer, but ppBuildRangeInfos[i] must be NULL for micromap geometry entries.
  • Input buffers (data, triangleArray) must be created with VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_BUILD_INPUT_READ_ONLY_BIT_KHR, not only the EXT micromap bit.

Opacity-MicroMap-SDK

Features

  • Automated asset conversion for ray tracing optimization
  • Texture and geometry analysis
  • Runtime or offline processing capabilities
  • CPU and GPU conversion options

Integration

  • Compatible with various ray tracing applications
  • Flexible API for pipeline integration
  • Supports diverse hardware and software configurations

SDK Repository