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Frequently Asked Questions about Color

Charles Poynton www.poynton.com poynton@poynton.com

This FAQ is intended to clarify aspects of color that are important to color image coding, computer graphics, image processing, video, and the transfer of digital images to print. I assume that you are familiar with intensity, luminance (CIE Y), lightness (CIE L*), and the nonlinear relationshipbetween CRT voltage and intensity (gamma). To learn more about these topics, please read the companion Frequently Asked Questions about Gamma before starting this. This document is available on the Internet from Toronto at I retain copyright to this note. You have permission to use it, but you may not publish it. Table of Contents 1 What is color? 3 2 What is intensity? 3 3 What is luminance? 3 4What is lightness? 4 5 What is hue? 4 6 What is saturation? 4 7 How is color specified? 4 8 Should I use a color specification system for image data? 5 9 What weighting of red, green and blue corresponds to brightness? 5 10 Can blue be assigned fewer bits than red or green? 6 11 What is “luma”? 6 12 What are CIE XYZ components? 7 13 Does my scanner use the CIE spectral curves? 7 14 What are CIE xand y chromaticity coordinates? 7 15 What is white? 8 16 What is color temperature? 8 17 How can I characterize red, green and blue? 9

© 1997-03-02a Charles A. Poynton. All rights reserved.

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Frequently Asked Questions About Colour

18 How do I transform between CIE XYZ and a particular set of RGB primaries? 9 19 Is RGB always device-dependent? 10 20 How do I transform datafrom one set of RGB primaries to another? 10 21 Should I use RGB or XYZ for image synthesis? 11 22 What is subtractive color? 11 23 Why did my grade three teacher tell me that the primaries are red, yellow and blue? 11 24 Is CMY just one-minus-RGB? 12 25 Why does offset printing use black ink in addition to CMY? 12 26 What are color differences? 13 27 How do I obtain color difference componentsfrom tristimulus values? 14 28 How do I encode Y'PBPR components? 14 29 How do I encode Y'CBCR components from R'G'B' in [0, +1]? 15 30 How do I encode Y'CBCR components from computer R'G'B' ? 15 31 How do I encode Y'CBCR components from studio video? 16 32 How do I decode R'G'B' from PhotoYCC™? 17 33 Will you tell me how to decode Y'UV and Y'IQ? 17 34 How should I test my encoders and decoders?17 35 What is perceptual uniformity? 18 36 What are HSB and HLS? 19 37 What is true color? 19 38 What is indexed color? 20 39 I want to visualize a scalar function of two variables. Should I use RGB values corresponding to the colors of the rainbow? 21 40 What is dithering? 21 41 How does halftoning relate to color? 21 42 What’s a color management system? 22 43 How does a CMS know about particulardevices? 22 44 Is a color management system useful for color specification? 22 45 I’m not a color expert. What parameters should I use to code my images? 23 46 References 23 47 Contributors 24

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What is color?

Color is the perceptual result of light in the visible region of the spectrum, having wavelengths in the region of 400 nm to700 nm, incident upon the retina. Physical power (or radiance) is expressed in a spectral power distribution (SPD), often in 31 components each representing a 10 nm band. The human retina has three types of color photoreceptor cone cells, which respond to incident radiation with somewhat different spectral response curves. A fourth type of photoreceptor cell, the rod, is also present in theretina. Rods are effective only at extremely low light levels (colloquially, night vision), and although important for vision play no role in image reproduction. Because there are exactly three types of color photoreceptor, three numerical components are necessary and sufficient to describe a color, providing that appropriate spectral weighting functions are used. This is the concern of the science of...