solvecraft/docs/render-hdr-reflections-integration.md

HDR radiance in scene reflections (#1693 + #1658)

This document records the shared CPU HDR/reflection contract and its recovered follow-ups. The personal rendering branch now ports that work onto the combined CPU/GPU2 snapshot described in advanced CPU integration. VNDF, spatial AA, surface effects, visible emission and the bounded dielectric path are integrated source. The combined CPU renderer has since executed the corresponding numerical, bounded-work and image-consumer checks; current combined engine/UI and native visual qualification remain pending. Historical checks do not qualify those remaining gates.

Disjoint lighting paths

A camera ray that misses geometry still writes the requested background exactly. It does not see the environment emitter and does not pass that background through exposure. A reflected secondary ray that misses geometry reads the existing owned environment through radiance_linear_srgb(direction) and adds throughput * Li to its linear path radiance. Reflection sampling already put specular_BRDF * incident_cosine / matching_PDF into that throughput; do not divide by a second environment PDF at the miss. Mirror is an explicit delta event with its existing Fresnel weight, independent of minimum roughness.

Selected mirror/rough reflectors receive no environment next-event estimate. Nonselected opaque hits retain the existing environment direct-light estimator and visibility. This partitions the environment contribution at each hit, without MIS or double counting the same specular environment paths. Existing directional direct lighting remains separate: there is no emissive geometry or infinite directional-light intersection on a reflected miss. At the fixed bounce cap, untraced continuation is discarded, as in the original finite-depth preview.

The first HDR+rough combination admits only fully metallic (metallic == 1) selected bodies and every effective face override. Dielectric, partly metallic or scalar face responses refuse in engine preflight, host admission and direct task construction. This conservative body-wide check also refuses a mixed body whose unvisited face is nonmetallic. Without this restriction, omitting the environment next-event term would silently lose diffuse illumination. Supporting those responses requires an explicit diffuse/specular partition or validated MIS. Explicit mirrors retain their declared ideal-reflection/Fresnel intent.

The foundation retained the pinned reflection unit's projected-NDF GGX proposal. The separate VNDF follow-up replaces its production proposal and outgoing PDF together, with the same BSDF and null accounting; its separate numerical checks do not transfer foundation validation. The integrated planar anisotropic adjunct replaces only the selected face's matched rough sampler and directional direct BRDF. There is no retry, PDF renormalization or diffuse-over- specular-PDF mixture. Both samplers preserve the same HDR miss contract.

Reflection sequence and deterministic primary reuse

The reflection variate generator receives the actual requested count R in 1..64 and rejects ordinals outside that count. It visits each radial stratum once using a shared 32-bit midpoint jitter, with a pixel/bounce digital shift of a base-two radical-inverse azimuth. Both coordinates remain strictly open. Exact dyadic azimuth coverage applies only to power-of-two R; other counts retain radial coverage only. Changing R changes the radial sequence, so there is no progressive prefix, independent-draw, unbiasedness or universal variance-reduction guarantee.

The variate sequence itself changes no mapping, PDF or specular weight; the matched VNDF follow-up is separate. Null-event accounting remains unchanged. Every null attempt still contributes zero to the original R divisor. R-attempt and bounce bounds are unchanged; actual outgoing paths, null outcomes and work can differ with the directions. Five new source tests exercise the production generator, including the white-conductor mean 1-ln(2), second moment 3-4ln(2) and a derived 1/R quadrature bound for that specific monotone fixture. Existing tests that retain the test-only hash control do not certify the new generator. The production image's independent weight check uses the new sequence.

The primary generator uses a fixed center for one sample, or a fixed 2x2/4x4 grid for spatial AA. Within one spatial sample, a completed nonselected primary surface's direct/environment shading can be reused across reflection samples, while retaining the original ordered scalar sums, per-add radiance bound, divisor and single display transform. See primary AA. Selected reflectors and secondary paths retain their full sampling. Future jitter, film or time-varying inputs must revoke that repeatability proof. No antialiasing or film capability follows from this optimization. Seven source tests cover its color, work and refusal contracts; none has executed on this central candidate. Equal-R visible quality and actual-work comparisons remain required for the new reflection sequence.

Ownership, budgets and colour

The same captured source, decoder, decoded pixels, host pool and work budget are used. RaytraceOptions::retained_bytes now accounts for original light capacities, reflection selector slots and selector string capacities in one calculation used by engine preflight and host admission. Fixed HDR capture metadata keeps the Record lease; captured image storage stays under the work lease and is released before PNG publication. The combined 384 MiB job limit and 512 MiB pool remain.

One existing decoder state is held in a fallibly allocated capacity-one vector, with a checked 4096-byte state bound, covered by the existing 8192-byte HDR state allowance. This keeps its scanline-scale array out of the image task's inline metadata as the reflection cursor state grows. It is not another decoder, cache, loader, worker or pool. Actual vector capacity must be one. Completion drops that slot; decoded/scanline capacities and source capacities retain their prior checks. A source regression checks the combined image task fits its 4096-byte host slot; that renderer layout test has executed in the combined CPU-feature build.

Preparation, traversal and reflection continuation charge the shared cumulative work budget and observe cancellation. Cursor-to-shading transitions yield before another debit. HDR misses remain linear; finite radiance caps apply before the sample average and single exposure/Reinhard/sRGB output transform. Primary background is independent. Only explicit sRGB/D65 HDR primaries are admitted; default/unknown Radiance primaries and unsupported optics remain refused.

Added source regressions and remaining validation

Five source tests cover: a white ideal mirror receiving constant HDR exactly once; unchanged camera background; rough-metal output from independent normal-view GGX sample weights and deterministic variates; a broad bright cell rotating in world space; rejection of mixed/diffuse rough responses; retained selector capacities/task layout; and cancellation/work exhaustion during secondary transport. All previous HDR and reflection test sources are retained unchanged. These renderer tests have executed; their decoded PNG checks are not native screenshots or visual acceptance.

Before publication, integrate real prerequisite history, run the combined default/feature checks, Clippy/wasm and required project gates, then produce and inspect exact-build native images. The original constant and bright-cell HDR fixtures carry MIT OR Apache-2.0 attribution; no external assets are added here.

Planar anisotropic hooks (source only)

The Kernel-authored core and PlanarTarget::sample bridge are adopted from pinned #1694 source; its reference ImageTask is not copied. One image task captures an authoritative optional mesh face tag per primitive, binds at most two targets by captured body ordinal AND exact body name AND face tag, and caches their validated planar frames. A missing tag or identity mismatch refuses instead of retargeting by plane geometry. Strict planarity can conservatively refuse imported/rotated roundoff. Face appearance surface: None replaces body PBR with the scalar response; it never inherits a metal response for admission.

At a rough selected hit, a bound target supplies the existing matched anisotropic specular direction/PDF/weight. Equal axes delegate the existing isotropic sampler. Other faces use the original rough sampler. Directional direct light uses the bound anisotropic BRDF. HDR next-event lighting is still isotropic, so HDR plus anisotropy on a non-reflector is rejected in preflight/host/task. The supported HDR combination requires a rough reflection selection and the same fully metallic body/ALL-face rule. A delta mirror plus anisotropic rough axes is rejected even without HDR. No anisotropic HDR direct estimator is advertised.

Original target-name capacities join the same retained request calculation and Record lease. Fixed cached targets and request clones fit the conservative bounded state allowance; primitive face tags are included in actual primitive size. Task construction repeats at most two 4096-triangle target validations, debiting the existing cumulative work/cancellation callback. Pixel traversal and sampling keep the existing one-unit yields, including secondary hits and final shadows.

The prior five HDR/reflection tests remain, with three added combination tests: equal-axis HDR reflection matches isotropic PNG bytes; unsupported direct/mirror combinations refuse; and an actual anisotropic secondary hit into another body yields within one unit (four per public quantum) and observes cancellation. Kernel's core/face tests and v4 continuation regressions are also retained from pinned source. The corresponding CPU renderer combination tests have executed; this does not transfer validation to kernel changes or qualify current combined engine/UI/native visual gates.