A Survey of Color for Computer Graphics

A Survey of Color for Computer Graphics SIGGRAPH 2001
brightness contrast, which is why black text on white paper (or white text on dark slides)
is so popular. In fact, an edge defined by only hue difference cannot be seen. Such an example
is difficult to construct, but I can assure you, it is very disconcerting. You can see
that two adjacent regions are differently colored, but it is impossible to find the edge between
them. Another issue for text legibility is the distribution and focus of the short
wavelength or blue cones as compared to the red and green cones. The blue cones are
much more sparse, and focus on a slightly different plane than the other two. This is why
some colors of bright blue text appear blurry—they are stimulating primarily the blue
cones, which do not provide the same acuity as the others. The slides show a diagram
from Mark Fairchild that shows the relative distribution of the three cone types. Note that
there are no blue cones in the very center of the retina.
Another type of cross media problem has to do with grayscale and color-mapped displays.
Grayscale systems, of course, must map all colors to gray values. Care must be
taken that these grays will function as replacements for the colors. Many desktop systems
display a maximum of 256 colors, using dithering to approximate the rest. Dithered text
and other fine features are often illegible, so it is important to chose colors that will remain
solid. This is also true for printers, especially for low-resolution digital desktop
systems. Text and fine features are much more crisp when they contain at least one solid
separation, even in offset printing. A rather nice feature of the Netscape web browsers is
that they automatically map text to solid colors on dithered displays. I wish more applications
did this.
All viewers are not the same either. As noted in the vision section, a significant part of
the population has atypical color vision. As the red-green anomalies are the most common,
one rule of thumb is to avoid encoding purely with a red-green distinction (an issue
that apparently never occurred to the designers of stoplights). However, to ensure that
important information can be distinguished by everyone, color should be combined with
shape or other ways of encoding the information (like a stop sign). Another viewer issue
is poor acuity, or lack of focus. Not all acuity problems can be fully corrected with
lenses. Also, the eye gradually looses its ability to change focus as people age, which is
why most people need some form of “reading glasses” by the time they are fifty. Avoiding
small, low-contrast text and other features is important to accommodate this problem.
Pill-bottle labels with small colored letters on a colored background should be banned!
In summary, when in doubt, use color conservatively. Duplicate the information carried
by the color whenever possible. Know your target media and audience well, and be sure
to test your designs thoroughly.
Maureen C. Stone 24
StoneSoup Consulting