In order to get a feel for the quality of the images on these pages, you need
to be using 24-bit video hardware. 24-bit color uses three bytes per pixel: one
for each of the three video color channels (red, green, and blue). A total of
16.7 million colors can be represented with 24-bit color.
The number of bits used to represent a single pixel in video memory is
commonly known as "color depth." Frequently-used color depths are 24-bit, 16-bit,
and 8-bit.
Many systems use 8-bit color, in which each pixel uses up one byte of video
memory. Each pixel selects one of 256 colors; these colors are defined using
either 18 or 24 bits per pixel; this allows 256 colors chosen from a palette of
262,144 or 16.7 million to be displayed at one time. By using a palette, image
quality can be improved despite the limited color resolution; colors can be
chosen based on the image content. If the image is an outdoor scene rich in
greens and browns, the palette can include a large number of shades of green and
brown. For a portrait, the palette might be rich in skin tones instead.
This scheme works fairly well until one tries to display multiple images with
different palettes at the same time. Consider the outdoor scene with its
mostly-green-and-brown palette when displayed on the same Web page as a portrait
with a mostly-skintone palette. One image or the other is going to look really
horrible since only one palette can be used at a time.
Web browsers like Netscape address this problem by installing their own fixed
palette, then adjusting incoming image data to match up with this palette as well
as it can. This is why photographic images on the Web look really grainy on
256-color systems. If 8-bit color is all you have available, you can often really
improve the quality of a displayed image by saving it to disk, then loading it in
a tool like LView (on the PC) which can install its own color palette based on
the particular single image you're viewing.
Some systems (usually PCs) also support an intermediate mode, 16-bit color, in
which each pixel takes up two bytes. Five bits are used for the red and blue
color value; six bits are used for green (the human eye is most sensitive to
colors in the yellow-green portion of the visible spectrum, so the extra
information is most valuable here). Image quality is, predictably, better than
that available on 8-bit color video systems--but not nearly so good as that
attainable on 24-bit color video systems.
The bottom line? If you're serious about imaging, 24-bit color is the only
reasonable choice.
As if color depth weren't enough of a problem, there's monitor gamma to
contend with.
Simply put, monitor gamma describes the relationship between the strength of a
monitor's input signal and the brightness of the displayed image. Some systems do
some or all of this translation in hardware (Macs, SGIs); others don't (PCs).
This causes major headaches when you're trying to come up with images which look
good on all platforms; it's particularly nettlesome when looking at images such
as these, where highlight and shadow detail is important.
You'll find that images from this review look a bit dark if viewed on a PC or
most Unix workstations; on a Mac or Silicon Graphics machine, they'll be a bit
light instead.
Since the JPEG format in which these files are stored relies on perceptual
coding to achieve its compression, it's not even possible to tweak the image
brightness up or down in hard/software in order to get maximum highlight or
shadow detail; you'll discover that the JPEG algorithm has, for your convenience,
discarded the information already since you couldn't see it before the brightness
change (the theory: why store for display what you can't see?) The only solution
right now is to rely on a lossless storage format such as TIFF; these images tend
to be really big.
Here's a copy of the first slide image
from this review in uncompressed form; be warned, though: at 780,084 bytes, it's
really big.
Computer Graphics Systems Development has put together a much more comprehensive overview of
monitor gamma and gamma correction; take a look at it if you'd like more
information on these topics.
