Hyperion

Vulkan path-tracer for OpenPBR.

Hyperion — Titan of heavenly light, father of Helios, Selene and Eos.

Hyperion is a GPU path tracer built entirely on Vulkan 1.4 KHR ray tracing.
It implements the OpenPBR Surface v1.1 material model and outputs a physically correct, linearly encoded HDR frame every render call.

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Screenshots

Cornell Box	Spheres	Suzanne

Metals	Dielectrics	Coat

Fuzz	Specular	Organics

Thin-film	Meadow IBL	Textured Cube

A Beautiful Game (IBL)	Dragon & Teapot (IBL)	Advanced Transmission

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Features

Rendering

• Vulkan 1.4 KHR ray tracing pipeline (raygen / closest-hit / miss / intersection shaders)
• Unidirectional path tracing with configurable bounce depth and samples per pixel
• Next Event Estimation (NEE) with Multiple Importance Sampling (MIS) — balance heuristic combining BSDF pdf and light pdf; eliminates black-dot noise and halves required SPP
• Emissive mesh area lights — per-triangle direct sampling using Shirley's sqrt-folding barycentric coordinates; area-to-solid-angle PDF conversion
• Environment map importance sampling — 2D separable CDF (256×128) built from panorama luminance × sin(θ); MIS-weighted against BSDF paths in the miss shader; eliminates fireflies from bright suns and skies
• Analytic spheres via VK_KHR_ray_tracing_pipeline intersection shaders
• Image-based lighting (IBL) — equirectangular HDR panorama via env_map
• Firefly suppression (channel-average clamping with NaN guard)
• À trous wavelet denoiser pass
• Headless render mode with PNG + EXR output

Material model — OpenPBR Surface v1.1

All parameters follow the OpenPBR spec naming:

Layer	Parameters
Base	base_weight, base_color, base_diffuse_roughness, base_metalness
Specular	specular_weight, specular_color, specular_ior, specular_roughness, specular_roughness_anisotropy
Coat	coat_weight, coat_color, coat_ior, coat_roughness, coat_darkening
Fuzz	fuzz_weight, fuzz_color, fuzz_roughness
Emission	emission_luminance, emission_color
Thin-film	thin_film_weight, thin_film_thickness, thin_film_ior
Transmission	transmission_weight, transmission_color, transmission_depth
Subsurface	subsurface_weight, subsurface_color, subsurface_radius, subsurface_scale
Geometry	geometry_opacity

Conductor reflectance uses the OpenPBR generalized-Schlick F82-tint model (base_color = F0, specular_color = the 82° tint); specular/coat microfacets use GGX with the spec's anisotropy remapping.

Color pipeline

• All internal calculations in linear Rec.2020
• Physical camera exposure via EV100 (ev100 scene keyword)
• Physical environment scale via env_unit_nits (cd/m² per EXR unit)
• Tone mapping: switchable per scene — AgX (Troy Sobotka, wide DR, natural sun highlight rolloff), ACES RRT+ODT (Stephen Hill fit), Reinhard luminance, Hable / Uncharted-2 filmic
• Display output: SDR (sRGB), HDR10 (PQ/ST2084), scRGB — runtime negotiated with the swapchain

Bindless textures

• Descriptor set 1, binding 4: COMBINED_IMAGE_SAMPLER array (up to 1024 entries)
• NonUniformResourceIndex for correct divergent access
• Per-material texture maps, each converted to the render color space at load: map_base_color, map_normal, map_orm (packed occlusion/roughness/metalness, G=roughness/B=metalness), map_emission_color

Scene format

Line-based text format (.scene) inspired by — but distinct from — Wavefront OBJ/MTL. Material libraries use the companion .mtlx format (OpenPBR keyword names; not Wavefront MTL and not MaterialX XML):

mtllib cornell.mtlx         # load material library (.mtlx)

camera
  translate  278  273  -800
  look_at    278  273   279
  vfov       39.1

ev100        7.0            # physical camera exposure
spp          64             # samples per pixel
max_depth    8

instance cornell.obj        # instantiate geometry (OBJ is geometry-only — its
  material Floor     WhiteWall   # own materials are never imported; assign here)
  material LeftWall  RedWall
  material RightWall GreenWall

sphere  60.0
  usemtl Glass
  translate  430  60  200

env_map       meadow_2_4k.exr
env_unit_nits 10000
tonemapper    agx             # aces (default) | agx | reinhard | hable

OBJ files contribute only geometry — material import from OBJ/MTL is disabled by design; all material assignments are declared in the .scene file.

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Building

Requirements: Vulkan SDK 1.4, CMake 3.28+, Ninja, clang-cl, vcpkg.

cmake -S . -B build -G Ninja \
      -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_C_COMPILER=clang-cl \
      -DCMAKE_CXX_COMPILER=clang-cl \
      -DCMAKE_TOOLCHAIN_FILE="<vcpkg-root>/scripts/buildsystems/vcpkg.cmake"

cmake --build build

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Running

# Headless render → PNG + EXR  (window is hidden)
build/hyperion.exe assets/cornell_classic.scene --output screenshots/cornell_classic.png

# Interactive window (default scene)
build/hyperion.exe assets/meadow_scene.scene

Command-line flags

Flag	Default	Description
<scene>	assets/cornell_classic.scene	Path to .scene file (first positional argument)
--output <file>	—	Headless mode: accumulate and save PNG + EXR, then exit
--spp <n>	scene value / 4	Override samples per pixel
--depth <n>	scene value / 8	Override maximum bounce depth
--width <n>	1920	Override render width in pixels
--height <n>	1080	Override render height in pixels
--no-validation	—	Disable Vulkan validation layers

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Tests

107 tests across unit, component, module and integration suites:

cd build && ctest --output-on-failure

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Dependencies

Library	Purpose
Vulkan SDK	Ray tracing API
volk	Vulkan loader
VMA	GPU memory allocation
SDL3	Window & surface
GLM	Math
stb_image	PNG/JPEG load
tinyexr	EXR load/save
Slang	Shader language
Google Test	Testing

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References

The following specifications, textbooks, and learning resources informed the design of Hyperion:

Rendering & Path Tracing

Resource	Relevance
Physically Based Rendering: From Theory To Implementation, 4th ed. (Pharr, Jakob, Humphreys)	Path tracing, BSDF sampling, MIS balance heuristic (§13.4.3), emissive area light NEE (§12.4), env map importance sampling via 2D separable CDF (§12.5)
Veach — "Robust Monte Carlo Methods for Light Transport Simulation" (1997)	Multiple Importance Sampling (MIS) — balance and power heuristics (§9.2); theoretical foundation for combining BSDF and NEE pdf estimates
Shirley, Wang & Zimmerman — "Monte Carlo Techniques for Direct Lighting Calculations" (1996)	Uniform area sampling of triangles via sqrt-folding barycentric coordinates; area-to-solid-angle PDF conversion
Ray Tracing Gems I & II (Haines et al., Marrs et al.)	Shadow ray precision, NEE techniques, ray tracing best practices
Ray Tracing in One Weekend series (Shirley et al.)	Introductory path-tracer architecture

Vulkan & Ray Tracing API

Resource	Relevance
Vulkan Specification 1.4	VK_KHR_ray_tracing_pipeline, VK_KHR_acceleration_structure, descriptor indexing
Khronos — Ray Tracing in Vulkan	Pipeline setup, SBT layout, shader stages
NVIDIA — Ray Tracing Learning Library	Algorithm-level ray tracing techniques (implementation uses Khronos extensions only — no vendor-specific extensions)
Slang Shading Language	[raypayload] semantic, TraceRay, Vulkan binding annotations

Material Model

Resource	Relevance
OpenPBR Surface Specification v1.1	Material layer stack, parameter naming (base/specular/coat/fuzz/emission/transmission)
MaterialX Standard Surface	Cross-reference for PBR parameter vocabulary
Blender Principled BSDF	Cross-reference for PBR parameter vocabulary

Color Science

Resource	Relevance
OpenColorIO	Color space transforms, ACES RRT/ODT, tonemapping nomenclature
AgX by Troy Sobotka	AgX tone-mapping matrices and S-curve (MIT)
ITU-R BT.2100	PQ/ST2084 and HLG OETF for HDR display output
IEC 61966-2-1 (sRGB)	sRGB EOTF for SDR display output

Scene & Asset Formats

Resource	Relevance
OpenUSD	Naming conventions: Prim, Xform, Mesh, Material, Light, Camera, Instance
glTF 2.0 Specification	PBR material and scene graph conventions
Wavefront OBJ	Geometry-only OBJ import (no MTL — materials are assigned in the .scene file)