A Perceptual linear or radial gradient was always written as an image,
so a gradient of spot ink tints lost its plate, and no warning named the
spot ink.
PDF export now writes a Perceptual gradient whose stops are tints of one
spot color (and paper white) as a Separation shading in that ink. Its
tints follow the gradient's curve: at each stop the renderer places on
the curve, the tint with the OKLab lightness of the color there.
Dithered fills, freeform gradients and Perceptual gradients that mix a
spot color with other colors are still images, and the export now warns
when such a gradient has stops linked to a spot swatch that its spot
colors are written in process colors.
- A radial gradient from a spot swatch to a CMYK color is written as a
DeviceN shading (ShadingType 3) of Yellow and the spot ink.
- A Lab spot color used as a flat fill and in a mixed gradient is
written as one Separation space with the Lab alternate, which the
DeviceN space names under /Colorants.
- docs/roadmap.md lists the gradients that still export without the
spot ink: mixed spot/process gradients in SVG and PDF gradients with
RGB or Lab stops. docs/target-app-parity.md also notes that a
Perceptual gradient is written to PDF as an image.
A linear or radial gradient whose stops mix a spot color with process
colors, or two spot colors, was written in process colors with a
warning. It is now written as a DeviceN shading of the process inks
its stops use and its spot inks, so each spot ink separates onto its
own plate. An off-center midpoint becomes a stop with the inks halfway
between its two stops. A gradient that mixes a spot color with RGB or
Lab colors is still written without the spot ink, with a warning that
says so.
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