ULTRIX Worksystem Software Color Extensions for the Display PostScript® System

This document, titled "Color Extensions for the Display PostScript® System" for ULTRIX Worksystem Software, dated October 25, 1989, details significant enhancements to the POSTSCRIPT language to provide more comprehensive color functionality.

The key extensions and new features described include:

1. CMYK Color Specification: Introduction of Cyan-Magenta-Yellow-Black (CMYK) color support, in addition to existing Red-Green-Blue (RGB) and Hue-Saturation-Brightness (HSB) models, specifically for print output devices.
2. Black Generation and Undercolor Removal: New functions (setblackgeneration, setundercolorremoval) allow for tunable conversion from RGB to CMYK. These functions optimize the amount of black ink used and reduce corresponding CMY inks to improve the fidelity of printed colors, especially blacks and grays.
3. Direct CMYK Color Specification: The setcmykcolor operator and the four-color option of the colorimage operator allow users to directly specify CMYK values, bypassing the black generation and undercolor removal processes for maximum control.
4. Expanded Color Controls: New operators like setcolorscreen and setcolortransfer provide expanded capabilities for defining halftone screens and transfer functions for individual color components (Red, Green, Blue, Gray, or Cyan, Magenta, Yellow, Black).
5. The colorimage Operator: This is a major extension of the image operator, designed to handle sampled images with multiple color components (one for grayscale, three for RGB, or four for CMYK). It supports various data formats, including single-procedure (interleaved color data) and multiple-procedure (separate procedures for each color channel).

The document provides conceptual descriptions of these new features, mathematical definitions for color transformations, and examples of how to use the new operators. It notes that these color operators are either built into newer POSTSCRIPT interpreters or available as an emulation package for older versions.