With a LUT destination, sRGB white came out as 0.3-0.5 % C/M/Y through
Transform::eval and precise float conversions: the legacy Lab encoding
puts L* 100 at 0.996 and a* = 0 at 0.50001, between the BToA grid
nodes, and fix_white only patched the device-link node, so only the
table paths were right.
Transform: for every intent but absolute colorimetric (and proofs
without paper simulation), a pixel equal to the source's device white
gives exactly the destination's device white. fix_white still sets the
device-link node; the paths that do not read that node exactly (the
exact pipeline, float interpolation, and 8-bit lookups when white is no
node) compare each pixel with white once. No other colour changes.
device_white(Lab) had a* = b* = 0.5; Lab white is 128/255 in the v4
encoding, which is no node of the 33-point grid, so Lab sources take
the per-pixel check there too.
Profile: synth::CmykParams::white_on_node maps no ink to exactly
L* 100, a* = b* = 0 in the AToB and gives the BToA tables input curves
that put that white on a node holding no ink. The Coated FOGRA39
profile is regenerated with it and separates PCS white to 2e-5 ink
(was 0.5 %), so the step into white is as smooth as its neighbours.
The default parameters and the Photocraft profile are unchanged; its
white still falls between nodes, so its last grey step into white is
up to about 0.4 % C larger than the one before, as with lcms's
white-node fix, and the test allows that for this profile.
FOGRA39 accuracy against 16a66e1: AToB against the patches unchanged
(mean 0.07, p95 0.18, max 0.53 ΔE00), reference sRGB values unchanged
(max 0.76), Lab -> CMYK -> Lab p95 1.04 -> 1.07, max 2.54 -> 2.55
(mean 0.24).
Add Builtin::CoatedFogra39 ("Coated FOGRA39 (fitted, craft-cms)",
profiles/craft-coated-fogra39.icc: AToB 11^4, BToA 33^3, 330% TAC,
medium GCR). It is fitted to Fogra's FOGRA39 characterisation data
(tests/data/FOGRA39.txt, unmodified, 1617 patches): a Yule-Nielsen
Neugebauer model with free overprints, per-ink tone curves and n, plus
a Gaussian-kernel ridge correction of the residuals. Against the
patches, relative colorimetric through the AToB: mean dE00 0.07,
p95 0.18, max 0.53. The BToA tables invert the AToB table
(synth::cmyk_profile_from_model); the Photocraft coated profile and its
synth defaults are unchanged.
Builtin::from_profile_name maps the names documents carry for
FOGRA39-based profiles (Coated FOGRA39, ISO Coated v2 (ECI)) to the new
profile and every built-in by its description. Adds math::delta_e2000.
Refs #36
aiongg
