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A renderer-independent OpaqueSurface (colour, metallic fraction, roughness, dielectric IOR) with bounded inputs, an original GGX/Smith/Schlick reference response with Lambert diffuse, and exact piecewise sRGB8 decoding. Numerical contract only: no document format, command or UI change.
70 lines
4.2 KiB
Markdown
70 lines
4.2 KiB
Markdown
# Opaque surface numerical contract
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First numerical slice of [#1004](https://github.com/bherbruck/solidcraft/issues/1004),
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under #813/#620. `solidcraft_render::surface` is an original, device-independent
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reference for the upcoming integrated consumer. It does not change existing looks,
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commands, document formats, snapshots or viewport rendering. No backend advertises
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PBR because this module exists. No external code, dataset or file asset is added.
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## Admission and color
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`OpaqueSurface::new` accepts linear-sRGB reflectance channels in [0, 1], metallic
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fraction in [0, 1], perceptual roughness in [0.045, 1] and dielectric relative IOR
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in [1, 3]. All values must be finite. Private fields prevent bypassing admission;
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future command/deserialization adapters must call this constructor. The lower
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roughness bound is an explicit preview limit, not a silently clamped perfect mirror.
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These are rendering bounds, not a claim that all real materials fit this model.
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`srgb8_to_linear` decodes existing color channels with the piecewise sRGB transfer,
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including its linear toe. Texture roles, other source primaries, HDR radiance,
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exposure, tone mapping and output encoding are separate responsibilities. Mechanical
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density, elastic constants, condition-dependent measurements and material axes are
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not inputs to this visual contract.
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## Response and limitations
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`brdf(normal, view, light)` returns RGB response in inverse steradians. Directions
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point away from the surface and are normalized using max-component scaling, allowing
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finite extreme input magnitudes without overflow. Zero/nonfinite directions fail;
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directions tangent to or below the surface return zero. Consumers apply incident
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radiance and the light cosine exactly once, then exposure/display transfer separately.
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The specular term uses isotropic GGX, separable Smith masking/shadowing and Schlick
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Fresnel. Alpha is perceptual roughness squared. Dielectric normal reflectance is
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`((ior - 1)/(ior + 1))²`; metallic fraction interpolates it towards base color and
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removes the diffuse substrate. The algebra evaluates `G1/cos(theta)` directly to
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avoid unstable division at grazing angles without clamping the cosine to an epsilon.
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Diffuse is Lambert weighted by `(1-metallic) * (1-F(n·view)) * (1-F(n·light))` per
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channel. This symmetric entry/exit attenuation preserves reciprocity and avoids
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adding unattenuated diffuse to grazing reflection. It is a conservative preview
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approximation; there is no internal reflection or microfacet multiple-scattering
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compensation. The model can lose energy, particularly rough metals. It is not an
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exact layered BSDF, spectral conductor model or measurement-derived material fit.
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No transmission, refraction, absorption, thin film, anisotropy, subsurface scattering,
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texture mapping, IBL, caustics or path integration is provided. In particular, this
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opaque model must not be used as an opacity-based substitute for glass.
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## Evidence and remaining gates
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Tests use independent normal-incidence analytic values, reciprocal direction pairs,
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grazing/extreme-scale inputs, invalid admission and sRGB reference values. Deterministic
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256×256 hemisphere quadrature checks white reflectance at metallic fractions
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0/0.5/1, roughness 0.25/0.5/1, IOR 1/1.5/3 and view cosines 0.2/1, with a 0.025
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numerical margin. This finite grid is not a universal energy-conservation proof and
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does not resolve the narrowest admitted lobes. Narrow-lobe importance quadrature
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and device parity remain consumer acceptance gates.
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An initial half-vector-only diffuse attenuation failed the grazing white dielectric
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test (integrated response 1.2324 at roughness 0.25, view cosine 0.2). The symmetric
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entry/exit formulation replaces it; the test tolerance was not relaxed to accept
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that result.
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Next: connect this meaning through design-owned appearance definitions, existing
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assignment commands and product scenes; test persistence, undo and unchanged mass;
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install the actual integrated GPU/CPU consumers and record original native examples.
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Only then advertise executor capabilities. Unsupported executors must refuse the
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new response atomically rather than silently render the legacy scalar approximation.
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No screenshots are claimed for this numerical-only slice.
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