Now we lower our sights a bit from the global issues of Net commerce to
consider two vexing and unrelated problems with color images on the Web.
In a thread on the Apple Internet Authoring mailing list, writers were
complaining that images they had painstakingly constructed to look good
on Macintosh computers looked muddy, dithered, or worse on Windows machines.
There are two causes behind most of these problems. Here's the ugly truth
of what's going on, how to work around it, and a new proposed standard that
offers some hope for relief.
The first problem is that Macintosh and Windows use color tables in funda-
mentally different ways. eblawler@iglou.com (Elizabeth Lawler) writes:
>> Is there a standard set of colors that display the same in Mac or Win-
>> dows? I had created a background color... that made text unreadable by
>> Windows users, and it had to be turned back to gray.
The best answer came from David Holzgang (for whom I could not find an email
address), who has written a book about HTML publishing using the Macintosh,
forthcoming from Ventana Press: _HTML Publishing on the Internet for Macin-
tosh_ (ISBN 1-56604-228-3).
> This is a common problem on the Net... Windows uses an 8-bit (256) color
> palette for its displays, no matter what the screen device may be. It
> reserves 20 colors in its palette for the system, and changes the other
> colors based on the currently active window... The Mac only reserves two
> colors (Black and White)... The trick to making your colors come out OK
> across all platforms is to use ju-jitsu on Windows -- by sticking to the
> base 20 colors. Here is a list of these 20 colors, including their RGB
> values in both decimal and hexadecimal (used by Netscape's color HTML
> extensions).
[For the list see 20colors.html -- ed.]
Gamma refers to a characteristic of CRT displays: they don't produce light
in proportion to their input voltage, but in proportion to that voltage
raised to some power (called gamma). For TV screens the value of gamma is
about 2.5. The input voltages have to be corrected with this gamma in mind
or the picture will look too bright or too dim.
Macintosh displays want their input gamma-corrected to a value of 1.8.
Silicon Graphics workstations want 1.4. PCs and Sun workstations want
2.5. Worse yet, Mac and SGI machines actively try to correct for gamma
effects in their video drivers, while PCs and Suns do not. The result,
as posted by mhadams@bentley.UnivNorthCo.edu (Michael H Adams):
> A brilliant, saturated photo on a Mac looks dark and muddy on a PC. A
> good-looking photo on a PC looks overexposed and thin on a Mac. There
> isn't a satisfactory answer to this, except compromise. Make the Mac
> photo a little light, so it's a little dark on a PC.
He points out several online resources relating to monitor gamma:
<http://www.cs.cmu.edu/afs/cs.cmu.edu/user/rwb/www/gamma.html>
<http://www.inforamp.net/~poynton/Poynton-colour.html>
<http://www.cgsd.com/papers/gamma.html>
I found the best practical advice, with instructions for using Photoshop
on Mac and PC to achieve the desired results, at
<http://www.cgsd.com/papers/gamma.web.html>:
> Making a web image with the right amount of brightness means giving it
> the average amount of brightness for the WWW (making it look average on
> all platforms)... Since Mac's and PC's are most common I try to average
> between the two at 2.2. Anticipate your viewers' platforms and adjust
> accordingly.
The 8-bit GIF format, most widely used for graphics on the Web, makes no
allowance for gamma; nor does JPEG, the 24-bit format supported by Net-
scape and some other browsers. (The TIFF format does allow for gamma in-
formation, but it is not used on the Web.) The PNG (Portable Network
Graphics) format proposed as a replacement for GIF -- spec at
<http://sunsite.unc.edu/boutell/png.html> -- handles the gamma problem in
the most general way. It allows for encoding the gamma of the recording
device (camera or scanner) and insists that complient applications correct
for gamma on output.
An update from Brooks Talley's "Satire Online" project. For those of you
who weren't tuned in to TBTF for 1995-09-03, Brooks is
the perpetrator of a
number of bogus and highly amusing political Web pages. Here is Brooks
recounting how his spoof has recently, inadvertently, ensnared America Online:
> Since September 25th, AOL has had a link somewhere encouraging members to
> send suggestions about the federal budget crisis to Bill Clinton, Newt
> Gingrich, and Bob Dole. AOL even provided a convenient form for members
> to do so. Clinton's mail went to Bill's real email address, as did Newt's.
> Dole's, however, went to aiken@dole96.org, our spoof address.
> When we started getting 4 or 5 "Federal Budget Suggestions" a day, we
> set up an auto-reply, which sent back a brief but twisted response
> (appended below).
> This went on until October 25th, when AOL moved this "Federal Budget
> Suggestion" link to their news "Top Story". Since Wednesday we've
> received over 1000 of these suggestions, and mailed back over a 1000 of
> these responses.
The auto reply follows.
> Thank you for your suggestions regarding the Federal Budget. As an
> important senator, and a candidate for President, it is important that I
> appear to care about your opinion.
> Unfortunately, there are a lot of suggestions coming in every day. It
> is not possible to review each one personally, but a select few are
> brought to my attention.
> The main problem that I have with most people's suggestions are that
> they involve cutting expenditures. Let's face it: if we cut the
> Democrats' pet projects, then they'll go after ours. If not today, then
> sometime soon.
> The only way the Federal Budget process can work is if both sides
> support each other and avoid partisan bickering over what projects to
> cut, by mutually agreeing not to cut anything substantive.
> Again, thank you for your input.
>>Apple Internet Authoring mailing list -- mail listproc@abs.apple.com without
> subject and with message: subscribe apple-internet-authoring Your Name .
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