92 3D: Colour space and final output

9.3

Racket

92 3D: Colour space and final output🔗ℹ

Semantic rgba-color values are sRGB: RGB channels use the ordinary 0 through 255 display encoding and alpha is linear coverage. Before interpolation, lighting, or source-over composition, an opaque 3D renderer converts RGB into linear light. It keeps a separate linear-rgba3d value internally because material emission or specular terms may exceed one without being clipped. At the final output boundary RGB is tone-mapped, converted back to sRGB, and stored; alpha is preserved unchanged.

struct
(struct linear-rgba3d (red green blue alpha)
    #:transparent)
  red : nonnegative-real?
  green : nonnegative-real?
  blue : nonnegative-real?
  alpha : (and/c real? (between/c 0 1))

An internal-style straight-alpha linear-light colour. RGB is nonnegative and may exceed 1 before tone mapping.

procedure
(srgb-channel->linear channel) → real?
channel : (and/c real? (between/c 0 1))

Applies the IEC 61966-2-1 sRGB transfer curve to one normalized channel.

procedure
(linear-channel->srgb channel) → real?
channel : (and/c real? (between/c 0 1))

Applies the inverse IEC 61966-2-1 transfer curve to one normalized channel.

procedure
(rgba-srgb->linear color) → linear-rgba3d?
color : rgba-color?

Converts a semantic sRGB colour to straight linear light; alpha is unchanged.

procedure
(rgba-linear->srgb color) → rgba-color?
color : linear-rgba3d?

Converts a unit-range linear-light value to semantic sRGB. Values above one must first be passed through tone-map3d-apply.

procedure
(linear-rgba3d-over source destination) → linear-rgba3d?
source : linear-rgba3d?
destination : linear-rgba3d?

Performs straight-alpha source-over in linear light.

struct
(struct tone-map3d (mode exposure white-point)
    #:transparent)
  mode : (or/c 'clamp 'reinhard)
  exposure : nonnegative-real?
  white-point : positive-real?

The immutable final RGB output policy. Exposure multiplies linear RGB before the selected operator. 'clamp clamps the exposed value to one; 'reinhard maps an exposed channel x to x/(x + white-point). The white point is retained for 'clamp too, so a later mode switch preserves the authored policy.

value
default-tone-map3d : tone-map3d?

The default (tone-map3d 'clamp 1 1).

procedure
(tone-map3d-apply policy color) → linear-rgba3d?
policy : tone-map3d?
color : linear-rgba3d?

Applies policy to RGB only and preserves alpha exactly.

Limitations. The colour contract covers opaque 3D rendering and its 3D strokes, markers, and billboards. It does not provide ICC profiles, wide-gamut or display-HDR export, texture colour-space metadata, or guaranteed linear composition between a rendered view3d and arbitrary outer two-dimensional Picts. Transparency remains order-sorted rather than order-independent.

The runnable comparison is "examples/3d/tone-map-emission.rkt"; it uses strong emission to show the visible difference between the default clamp policy and Reinhard output.

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II	Cookbook
III	Guide
IV	Complete Example Programs
V	Reference

52	Find a task or a name
53	Modules to require
54	Scenes and sampling
55	Scene states
56	Animation requests
57	Native model details
58	Visuals
59	Geometry
60	Number Lines and Coordinate Decorations
61	Point Markers, Scatter Plots, and Filled Areas
62	Probability and Statistical Diagrams
63	Calculus lessons
64	Colors
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67	Source Programs and Authored Timelines
68	Interactive Preview and Inspection
69	Preview controls and diagnostics
70	Projects and Final Rendering
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72	Pict Renderer Protocol
73	3D reference map
74	3D: Objects, groups, and paths
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76	3D: Views inside a Scene
77	3D: Animating 3D objects
78	3D: Animating the 3D camera
79	3D: Cuts, caps, and section measurements
80	3D: Supplementary surface, section, and annotation bindings
81	3D: Cuts, sections, and numerical volume
82	3D: Vectors
83	3D: Linear maps and rotations
84	3D: Affine transforms
85	3D: Bounds, rays, and planes
86	3D: Spatial maps and homotopies
87	3D: Materials and lights
88	3D: Lighting inspection
89	3D: Shadow descriptors and stable bounds
90	3D: Finite-light animation
91	3D: Software finite-light evaluation
92	3D: Colour space and final output
93	3D: Relations and geometric annotations
94	3D: Projected labels
95	3D: Textured billboards
96	3D: Spatial curves and vector diagrams
97	3D: Spatial anchors and label layout
98	3D: Anchors and projected labels
99	3D: Parametric surfaces and calculus
100	3D: Adaptive, trimmed, and implicit surfaces
101	3D: Constructive solids
102	3D: Surface constructors and queries
103	3D: Meshes
104	3D: Triangle topology
105	3D: Polygonal faces
106	3D: Deterministic convex hulls
107	3D: Polyhedral duals
108	3D: Schlegel diagrams and polyhedral nets
109	3D: Conservative mesh correspondence
110	3D: Topology-safe mesh matching
111	3D: Checking and repairing mesh orientation
112	3D: Clipping, sections, and transparency
113	3D: Prepared spatial ODE trajectories and vector fields
114	3D: Seeds and streamline sets
115	3D: Poincare sections and return maps
116	3D: Equilibria and local linearization
117	3D: Flow maps and volume change
118	3D: Drawing and inspecting trajectories
119	3D: Reading prepared trajectories
120	3D: Drawing vector fields and particles
121	3D: Trajectory display
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123	3D: Retained renderer backends
124	3D: Compiled geometry and instances
125	3D: Frame requests and results
126	3D: Software rendering and statistics
127	3D: Optional Racket/ Open GL backend
128	Experimental Derived Visuals
129	Source Package Hygiene