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DeviceLink support — implementation notes

Shipped 2026-08-15. This page describes how DeviceLink profiles are supported as built. Tests: __tests__/transform_devicelink.tests.js, with two families of self-validating fixtures:

  • testbed/profiles/lcms_testbed/ (LittleCMS-testbed generated): linlcms2.icc — curves-only linearization link (parametric gamma 3.0, verified as out = in³ exactly); limitlcms2.icc — CMYK→CMYK 150% ink-limit link (lcms's algorithm reproduced: K preserved, CMY scaled into the remaining budget).
  • testbed/profiles/devicelink/ (Serendipity Software samples): null CMYK→CMYK and RGB→RGB identities, plus asymmetric simple-cmyk-to-rgb (4→3) and simple-rgb-to-cmyk (3→4, K-only grey axis).

A DeviceLink (pClass: 'link') is a complete device→device transform with no PCS in the middle — the single A2B tag carries the whole conversion.

Usage

const dl = new Profile();
dl.load('MyDeviceLink.icc', () => {
    const t = new Transform({ dataFormat: 'int8', buildLut: true });
    t.create(dl);                       // the DeviceLink alone — nothing else
    const out = t.array(pixels, false, false);
});
  • The rendering intent comes from the profile header (the spec says the single A2B tag serves whichever intent is declared there); any intent argument is ignored.
  • Passing further profiles alongside a DeviceLink throws — there is no PCS to link through.
  • buildLut: true works (the baked LUT carries a single-profile chain); identity detection never collapses a link.

How it works

Profile loading (src/Profile.js)

  • The 'link' class passes the profile-class gate.
  • The header PCS field is not a PCS — for links it carries the output device space ('CMYK', 'RGB ', '5CLR', …). The Lab/XYZ PCS gate is skipped for links; the output space resolves to profile.outputChannels, while the header space field (the input side, as usual) resolves to profile.deviceLinkInputChannels. Asymmetric links (CMYK→RGB, RGB→CMYK) work.
  • Links have no wtpt tag — the absolute-adaptation precompute is guarded on mediaWhitePoint being present (absolute intent doesn't apply without a PCS anyway).
  • profile.type reflects the input space (e.g. CMYK); Transform branches on header.pClass === 'link', not on type.

Pipeline (src/Transform.js)

createMultiStage detects the link, strips the padded intent/output slots that create(dl) produces, sets input/output channels from the two header fields, and skips identity detection. createPipeline then calls createPipeline_DeviceLink for the single-profile chain (the normal profile-pair linking loop no-ops for a length-1 chain), bracketed by the standard input/output format conversion stages.

createPipeline_DeviceLink walks the full LUT element structure, device→device throughout — a bare CLUT stage is not enough in practice:

V2 mft1/mft2:  inputCurve → CLUT → outputCurve
V4 mAB:        aCurves → CLUT → mCurves → matrix → bCurves

Any element may be absent. Two spec subtleties handled here:

  • A curves-only mAB (B-curves, no CLUT) is legal and is exactly what an lcms linearization DeviceLink is. The LUT decoder no longer crashes on CLUT-less tags (stride precompute is guarded), and the builder just emits the curve stage.
  • mft1/mft2 tags always contain a 3×3 matrix field, but per ICC spec it applies only when the input side is PCSXYZ — never true for a link — so it is skipped for mft tags. Only mAB's M-matrix is a real stage.

A curve-evaluation gap this exposed

stage_curves_v4 could evaluate parametric curves with a curveFn and sampled tables, but not pure-gamma curves (para function type 0, or curv with count 1) — the decoder stores those as an inline gamma exponent for speed, and no evaluator handled it (NaN). It now applies Math.pow(x, gamma) when there is no table (sampled curves also carry a midpoint gamma hint, so the table branch keeps priority).

What did NOT need changing

Area Reason
LUT decoders (mft1, mft2, mAB, mBA) A2B decodes normally (plus the CLUT-less guard)
Interpolation Same tuned paths as any other LUT
Input/output format conversion Link type reflects its device spaces — existing stages apply
detectBlackpoint / BPC / chromatic adaptation Already guarded on pClass 'link' / PCS presence
Smoke test / validate, LUT bake Work as-is once the pipeline is built