Monograph · 08 · Hybrid RT

Pipelines · full depth

Hybrid ray tracing

Chapter contract

Document RT shadow and RT GI as first-class techniques: how they init (pipeline + SBT), what they bind each frame, and exactly where deferred inserts them. Not a third world model.

Injection model

Deferred still owns beauty. Hybrid answers two questions with rays on the same TLAS the path tracer uses: “is this lit?” and “what one-bounce indirect arrives?”

Fig. HY-1

Dashed arrow: results feed lighting; no second scene upload.

Technique init lifecycle

Both RTShadowTechnique and RTGITechnique follow the same skeleton (see their init):

init(device, physicalDevice, …)

Load KHR ray-tracing function pointers
createOutputImage — full-res storage image for visibility or GI
GI only: material albedo buffer for secondary shading
createDescriptorResources — TLAS + GBuffer views + outputs
createRTPipeline — compact raygen/miss/(hit) group
createShaderBindingTable
resize when framebuffer size changes

Why separate techniques (not PathTracer)

Full PT carries 30+ AOVs and multi-bounce state. Hybrid needs thin pipelines that only answer V or Lᵢ¹ and fit inside the deferred graph’s time budget.

Shadow render workflow

RTShadowTechnique::render(cmd, ShadowInput)

Fill ShadowInput: position/normal/depth views, view/proj, camera pos, size, light direction/type
Update descriptors if views changed
Push constants / uniforms for light
traceRays — primary work is visibility rays from GBuffer world pos
Output: soft/hard visibility factor image
Lighting samples it as Vₖ

V(x\!\to\!\ell)=\begin{cases}0&\text{any hit before light}\\1&\text{miss}\end{cases}

Ray flags typically prioritize TerminateOnFirstHit and skip shading — pure occlusion. Soft shadows average multiple samples toward an area/sphere light (same idea as PT NEE, cheaper continuation).

GI render workflow

RTGITechnique::render(cmd, GIInput)

Inputs: position, normal, albedo, depth + camera + TLAS
Optional material albedo SSBO for hit shading
For each pixel: sample hemisphere at normal; trace one secondary ray
On hit: shade with direct / emissive / simple lighting; on miss: env if bound
Weight by BRDF×cosine/pdf; write indirect RGB
Lighting adds as bounce fill (biased vs full path trace)

L_i^{(1)}(x)\approx\frac{f_r\,L(y\!\to\!x)\,(n\!\cdot\!\omega)}{p(\omega)}\,V

Why one bounce only

Realtime. Multi-bounce color bleeding is PathTracer’s job (Ch. 06). Hybrid targets contact GI and soft fill for deferred iteration.

Placement in deferred graph

DeferredRenderer::render (relevant slice)

CSM (raster cascades)
GBuffer MRT
SSAO → bind into lighting
if AS supported && instanceCount > 0: addComputePass RTShadow
if same: RT GI pass
Deferred lighting consumes V and Lᵢ¹
Sky / post

Graph declares texture reads so barriers place GBuffer in SHADER_READ before RT techniques sample it.

Host enable path

render_dispatch.cpp · conceptual

// each deferred frame
if (m_rtAccel && m_rtAccel->isSupported()) {
  m_deferredRenderer->setAccelerationStructure(m_rtAccel.get());
  m_deferredRenderer->setRTShadowsEnabled(true);
}
// techniques only run if instanceCount > 0 inside deferred graph

Limits

Honest scope

Static BLAS in trunk (animation removed). Hybrid ≠ PT parity. Noise depends on spp/filter — often combined with temporal post, not offline OIDN.

rt_shadow_technique.cppinit · SBT · render

rt_gi_technique.cppinit · materials · render

deferred_renderer.cppgraph inject

render_dispatch.cpphost enable

rt_acceleration_structure.*shared TLAS

Design units in this module

Each card is a focused design page (what / how / why + sources). Full tree: Sitemap.

shadow-technique

Hybrid ray tracing

Injection model

Technique init lifecycle

Shadow render workflow

GI render workflow

Placement in deferred graph

Host enable path

Limits

Design units in this module

RT shadow technique

RT GI technique

Visibility helpers