(v13) Named colors in transparency groups

This page applies to Harlequin v13.1r0 and later; both Harlequin Core and Harlequin MultiRIP.

When named colorants are present in a group, the RIP composites each colorant independently without converting to process colorants, as recommended in the PDF specification [PDF1.4]. This independent compositing of named colors continues down through the transparency group stack until the page group, when it is then necessary to convert colors to the output device. The process is illustrated in Figure: PDF Transparency Group Stack which shows the independent path for spots.

As a first step when converting colors from the page group to the output device, the RIP internally changes the blend space to a /DeviceN space constructed from the original blend space and the set of named colorants. The tint transform is simple, and derived from CMYK equivalent values for each named colorant, which may result in different color from that using the original tint transform supplied by the job. This is the search order used to find a CMYK equivalent values for each named color:

1. Named color database (NCD) resources in the gstate, (v13) Named color management.
2. Colorants dictionary. (PDF only) color spaces in PDF allow an optional 5th element of a Colorants dictionary which may contain a Separation color space for the current colorant. If that color space is present, it is used. A color value of 1 is evaluated.
3. Color space in the job. (All PDLs) the first color space in the job that includes the colorant is used to evaluate the tint transform. A color value of 1 is used for the colorant, if other colorants are present in a /DeviceN color space, their color values are set to zero and the color space is evaluated to obtain the CMYK equivalent values for the solid color.
4. Custom color comments. [PostScript language only] %%CMYKCustomColor: comments contain CMYK equivalent values. 

The constructed space is then processed in the same way as normal /DeviceN spaces in the RIP, as described in (v13) Decomposing a DeviceN colorant list. The important point being that NCDs will intercept individual named colorants, while device colorants are rendered. The constructed tint transform is only used for other colorants.

The above description is correct for all the allowed blend spaces of CMYK, RGB, and Gray.

Since RGB and Gray are additive spaces, named colors are not normally used alongside them, but some applications do this.