DIGITAL COLOR: THEORY AND PRACTICE

Report on course "A Field Guide to Digital Color" organized by
Maureen Stone and Pauline Ts'o

by Ben Wyrick
24 July 2002

Color is a powerful, subtle and sometimes fickle tool. Add a little too much red to your blue, and suddenly you're looking at something entirely different than what you started with. How does color work, and what can be done to keep it consistent through digital and mechanical processes? Maureen Stone and Pauline Ts'o shared their knowledge of color in a course entitled "A Field Guide to Digital Color."

Stone began the course by explaining how the human visual system reads color. Light enters the eye, strikes the retina, and is absorbed by cone cells. The light is then transmitted to the brain via the optic nerve, where it is interpreted. An important lesson to draw from this model is that an observer may not actually see the true color of an object--the observer's cones may not respond to the exact wavelength. Furthermore, the brain interprets the signal received from the cones, and that interpretation could be misleading, hence optical illusions.

Stone explained the three types of cones: long, medium and short. Long cones are sensitive to red colors (which have long wavelengths), medium cones are sensitive to green, and short cones are sensitive to the shortest visible wavelengths, blue colors.

Cone cells are found mostly at the center of the eye and are unevenly distributed. Blue cones are rarer than red and green. Rod cells, which are not sensitive to color, are located around the periphery of the eye, and are responsible for night vision. According to Stone, rods and cones usually do not function at the same time.

The eye has a blind spot where the optic nerve meets the retina. However, this blind spot is seldom noticed since our eyes are continuously scanning the world and integrating between snapshots.

Understanding the theory behind the human visual system is useful in developing practical color models and standards for computer graphics. For example, the phosphors inside televisions and computer monitors are red, green and blue--just the colors our cones are looking for.

Humans are capable of detecting millions of colors--more than desktop printers, monitors, projectors and printing presses are capable of producing. To complicate matters, different devices produce different ranges of colors. Monitors mix light and printers mix ink, and results often vary. For example, your monitor might be capable of displaying a color like "fire engine red," but to your printer it looks more like "dry rust red."

To combat the problem of accurate color reproduction, sets of device-independent standards are being created. One such standard was created by the International Color Consortium (ICC) and has been widely accepted, Stone explained. The ICC standard is based on experiments conducted by the Commission Internationale de l'Eclairage (CIE), which tested how people perceive color.

Once a standard color space like ICC is agreed upon, devices can convert their colors to colors within the space. Problems arise when a device's colors are exotic flavors of the standard set. These color mismatches are resolved in a process called gamut mapping, which uses algorithms to convert extreme colors to standard colors. When everything works, what you see on your monitor is what you get in print, thanks to an understanding of the human visual system and an agreed-upon color space.

* * *

A thorough knowledge and exacting use of color is critical to a special effects production house like Rhythm & Hues Studios. Pauline Ts'o of Rhythm & Hues talked about how color can lead the eye and strike a mood. Preserving the richness and subtlety of color is taken very seriously at special effects houses, which use high-end hardware and software to get the right look. Ts'o spoke on the particulars of Rhythm & Hues' color pipeline for both video/broadcast work and film work.

First, live action footage is scanned into a proprietary file format called Rll, which uses 16-bit color channels. Most consumer image editing devices use 8-bit color channels and as a result are not able to store and manipulate a rich spectrum of colors. 8-bit schemes can also introduce banding artifacts due to their limitations. The Rll format stores colors on a logarithmic scale as opposed to the more commonly used linear scale. According to Ts'o, a logarithmic representation of color allows more gradations in dark colors, which film is more sensitive to. Rendered frames are created in a linear space and then special software is used to correctly map them to the logarithmic color space used by monitors.

Calibrating monitors is a routine task at Rhythm & Hues and Ts'o joked about users being severely warned not to change the settings. Employees work in darkened rooms to avoid bright light reflecting on monitor glass. Ts'o admitted to calibrating her home television set by watching Rhythm & Hues commercials when they are broadcast.

Working with film stock requires extra steps to ensure color correctness, Ts'o explained. Positive match clips and wedge clips are special referencing tests made to keep color consistent between the client's live-action footage and the studio's special effects footage. Ts'o also says her studio works closely with the film processing lab to make sure chemicals are fresh and their equipment is properly calibrated.

When everything works, the end result of understanding the human visual system and maintaining a pure digital color through the many steps of a production pipeline is a faithful representation of the director's vision.Links: The International Color Consortium: www.color.org