Charles Poynton
-Color technology
If you've been framed, escape!
Charles Poynton
-I have written answers to "Frequently Asked Questions"
about Gamma and Color in video, computer graphics, and print.
The documents are available in several formats: PDF, PostScript, and in some cases,
ASCII text. I encourage you to use the PDF format. If you access
the PDF format, configure your browser or ftp client to transfer
in binary mode. If you transfer in text mode, you are likely
to corrupt the file.
Permanent, easy solutions to many of the problems in tone and
color reproduction in computing require assistance - even leadership
- from the developers and manufacturers of hardware and software.
Solving that problem is the primary goal of the Gamma FAQ
and Color FAQ documents.
I dedicated many weeks to developing the Gamma and Color FAQs,
and I make efforts to keep them up to date and free of errors.
The popularity of the documents is gratifying - they average several
hundred accesses a day. However, I receive one or two unsolicited
color or video queries each day. That may not seem like a lot,
but that's 350 or 700 in a year. Owing to the requirement for
me to work to pay my rent, I regret that I can't promise a response
to unsolicited queries, particularly those related to specific
combinations of devices. (I don't even own a Wintel machine!)
For color science issues, you can try the newsgroup sci.engr.color;
for computer graphics issues, try comp.graphics.algorithms;
for video, try sci.engr.television.broadcast
(for broadcast video) or rec.video
(for consumer video).
I would appreciate comments, criticisms, corrections, and contributions. In particular, I am looking for a qualified volunteer to contribute a section on color management systems, and a qualified volunteer to write about UCR, GCR, and RGB-to-CMYK separation.
You may be interested in the companion Frequently-questioned answers about gamma, the GammaFQA.
James M. Palmer is upset, as I am, concerning misuse of the term intensity. See his note, Getting Intense on Intensity; also, you might be interested in his FAQ on Radiometry and Photometry. The most important thing for me to point out to the computer community is that intensity is, by definiton, proportional - that is, linearly related - to a physical measure of light power. A quantity isn't necessarily an intensity value just because you get more intensity as the quantity increases!
On 1998-02-25, Timo Autiokari published, in an open letter in a Usenet posting, some inflammatory remarks concerning my FAQs. These remarks are sufficiently grave that I feel obliged to respond. You can read about the Timo incident.
This note introduces the two main user adjustments of a video monitor, Black Level and Picture (sometimes called Brightness and Contrast). I explain the effect that these controls have on picture reproduction, and I explain how to set them. You can read the note online, or you can obtain a typographic-quality PDF version (107975 bytes).
This note is available online, or you can obtain a typographic-quality
PDF version (101879 bytes).
I presented this paper at the SPIE/IS&T Conference in San Jose, Calif., Jan. 26 - 30, 1998. The paper is published in the Proceedings of that conference, B. E. Rogowitz and T. N. Pappas (eds.), Proceedings of SPIE 3299, 232-249 (Bellingham, Wash.: SPIE, 1998).
This article describes the theory of color reproduction in video, and some of the engineering compromises necessary to make practical cameras and practical coding systems. I presented this paper at the SMPTE Advanced Television and Electronic Imaging Conference, San Francisco, Feb. 1995. This is an edited version of the paper published in the proceedings of that conference, New Foundations for Video Technology (pages 167-180).
Proceedings of International Broadcasting Convention, 1994 (Amsterdam, 16-20 September 1994), IEE Conference Publication No. 397, pages 218-222.
The following two articles are dated now, but indicate the state of color management in 1992:
Color Management Technology for Workstations, Sun Expo '92, Manchester, U.K., Sepember 10, 1992.
RISC/UNIX Workstations in Desktop Color Prepress, Youngblood/IBEC's DeskTop PrePress Today - Colour Conference 92, Toronto, March 26, 1992.
The International Color Consortium has agreed on a standard for color device characterization. The standard, designated ICC.1:1998-09 File Format for Color Profiles is available from the ICC web site, in Acrobat PDF format (815769 bytes).
A previous version, 3.4, is available from SGI's ftp site, in PostScript format (5046196 bytes) or Acrobat PDF format (744484 bytes).
As far as I know, the current version is not available on the
web. The latest on the web seems to be version
3.0a (dated February 15, 1995), at SGI's web site. That page
is about 200 KB, however - though it probably works really well
on a UNIX workstation permanently connected via Ethernet to the
Internet backbone, for those of us at the wrong end of a modem
link it is a long wait. So with the permission of the Chairman
of the ICC, I "sliced and diced" the HTML into its constituent
chapters and appendices.
Start with the Table
of Contents.
Paul Haeberli has written an interesting set of notes, Graphica Obscura. In his Viewing Notes, Paul points out some problems with gamma on the web. Paul has written a note Matrix Operations for Image Processing that explains how certain image processing operations that people think have to use HSB, HSL and so on can actually be accomplished more efficiently in RGB space.
If you're interested in gamma, you might want to read the article
It
may be Gamma to you, but it's Greek to me, by Maureen
Stone, Todd Newman and Mark Callow. The paper is written for VRML
developers, so it assumes synthetic images and is mostly focused
on output devices.