# Threejs Exposure Color Grading > Meter and grade scene-linear Three.js WebGPU images. Use for choosing fixed or automatic exposure; adapting EV on the GPU; assigning tone-map and output conversion; or placing and validating 3D LUTs. - Skill: `linegel/threejs-exposure-color-grading` (Agent Skill, multi-file: 5 files) - Install (CLI): `npx skillmds@latest add linegel/threejs-exposure-color-grading` - Raw SKILL.md: https://api.skillmd.com/api/skills/linegel/threejs-exposure-color-grading/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: linegel (https://skillmd.com/u/linegel) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/linegel/threejs-exposure-color-grading --- # Exposure And Color Grading Keep the photographed signal scene-linear until the final image chain. One declared owner controls each exposure group, tone map, and output conversion. ## 1. Lock the color contract Name the scene-linear working primaries, radiance scale, alpha convention, and every producer that enters the photographed signal. Convert irradiance through the material/lighting model before metering radiance. Apply one shared physical or perceptual radiance scale to lights, environment, atmosphere, emissive materials, bloom sources, and optical effects. Partition targets or views into exposure-control groups. A group may share GPU state only when its radiance basis, exposure policy, and reset history are identical; an automatic group also requires the same meter mask, key, and sample schedule. Assign exactly one exposure owner, one tone-map owner, and one output-conversion owner per group. **Complete when:** every photographed input has one basis and scale, and every group names its members, exposure/tone-map/output owners, and state-sharing policy. ## 2. Choose the cheapest meter that meets the image requirement Choose in dependency order: 1. fixed EV for a controlled or calibrated view; 2. a stratified grid or tile sampler for ordinary global auto exposure; 3. exact full-pixel hierarchical reduction when every pixel or exact mask must contribute; 4. a log-luminance pyramid only when another feature consumes its levels or spatial statistics; 5. a histogram only when percentile clipping fixes a demonstrated outlier or bimodal-lighting failure. Tap resolved, pre-bloom HDR by default. This keeps temporal noise out of the meter and avoids bloom/exposure feedback. A different tap is an authored image policy with a regression fixture. Read [the color-pipeline reference](references/scene-referred-color-pipeline.md#meter-implementations) when implementing sampled, exact, pyramid, or histogram metering; it contains the weighted-log equations, traffic model, and small-emitter failure tests. **Complete when:** fixed EV names its value/calibration and requires zero meter source reads; otherwise one meter is selected, its source and mask are named, and each cheaper rejected option has a concrete correctness failure. ## 3. Build the selected exposure controller For fixed EV, bind the authored or calibrated value directly and allocate no meter, reduction, target-publication, or adaptation state. For automatic exposure, keep `targetEV`, `currentEV`, validity, and frame indices in GPU state. Advance adaptation every rendered frame toward the last valid target, even when the meter runs less often. Keep CPU readback diagnostic-only. Use this producer schedule only for automatic exposure: ```text adapt currentEV from the last completed target -> render and present with currentEV -> reduce the new meter source -> publish targetEV for a later frame ``` Bind the source texture as a real node dependency and expose both the source frame and state frame. Initialize or clear the source before the first reduction. Read [GPU exposure state](references/scene-referred-color-pipeline.md#gpu-exposure-state) for reduction state, EV adaptation, invalid aggregates, and r185 compute semantics. **Complete when:** fixed EV has zero metering/adaptation work, or an automatic frame trace proves which source produced each target, adaptation remains GPU-resident, and an invalid aggregate holds the prior valid target without a CPU substitute. ## 4. Handle discontinuities before presentation Give cuts an authored `hold`, `reseed`, or fixed-EV policy. Treat a radiance basis, working-primary, quantity, nonlinear-normalization, or exposure-key change as a new exposure epoch. For a pure positive scale change `L_new = k * L_old` with otherwise identical semantics, preserve the displayed product by shifting the fixed EV, or both automatic states, by `-log2(k)`: ```text currentEV_new = currentEV_old - log2(k) targetEV_new = targetEV_old - log2(k) ``` Every other incompatible change starts a new exposure epoch. Automatic exposure resets meter accumulation and reseeds adapted state; fixed exposure rebinds its authored value before the new signal is presented. Resize or DPR changes rebuild only admitted resolution-dependent meter resources and sampling coordinates. Device loss recreates and reseeds only admitted GPU state under the new resource generation. **Complete when:** every cut, invalid input, basis/scale change, resize, and device-loss event maps to one conversion, hold, rebuild, or reseed action that finishes before the affected frame is admitted, or to an explicit no-op because the selected branch owns no affected state. ## 5. Compose one final-image chain Use this domain order unless the LUT declares another complete contract: ```text scene-linear HDR -> exposure -> tone map -> tone-mapped-linear LUT, when admitted -> alpha restoration -> output conversion ``` Unpremultiply before nonlinear RGB operations and premultiply afterward; exposure preserves alpha. A scene-linear LUT needs a declared shaper. A display-encoded LUT owns the exact output primaries and transfer function and therefore sits after `renderOutput()`. With explicit `renderOutput()`, set `RenderPipeline.outputColorTransform = false`. Mark `renderPipeline.needsUpdate = true` after changing the output node or output ownership. Read [tone mapping and LUTs](references/scene-referred-color-pipeline.md#tone-mapping-and-luts) only when loading, authoring, or placing a cube. When a tone-mapped-linear cube is admitted, read [the identity 3D-LUT example](examples/identity-3d-lut.mjs) for voxel ordering and `Data3DTexture` configuration. It is a correctness fixture, not a look or performance bypass. **Complete when:** the graph contains one exposure multiply, one tone map, one working-to-output conversion, and—only when admitted—one LUT placement in its declared domain. ## 6. Prove the selected branches Capture deterministic fixtures: - for fixed exposure, a calibration card proving the authored EV and multiplier with zero meter source reads or adaptation state; - for automatic exposure, a key-gray card with the expected target EV and a bright source entering and leaving frame with monotone target/current EV trajectories; - for a sampled automatic meter, its mask, small-emitter, and cadence cases; - for a histogram, its underflow, overflow, and accepted percentile interval; - when a LUT is admitted, an identity LUT with ramps and saturated swatches in its declared domain; - output isolation showing exactly one tone map and one output conversion. Measure each admitted meter or LUT as a paired graph delta after warmup on the target. GPU time is available only after post-init timestamp-query support is proven; otherwise report the timing as unavailable. **Complete when:** all applicable fixtures pass, the final image is inspected, and each failed fixture identifies the meter, adaptation, LUT-domain, alpha, or output-ownership cause. ## Routing Use `$threejs-image-pipeline` for shared MRT, temporal history, adaptive DPR, and transient lifetime; `$threejs-bloom` for glare source ownership; and `$threejs-visual-validation` for fixed-view image evidence.