|
The following is a
brief overview of color computer displays that explains some
of the basic terminology used in the Macintosh and Windows
operating systems. Current color monitors for desktop
microcomputers are based on cathode ray tubes (CRT's). Because
CRT's transmit light, CRT displays use the red-green-blue (RGB)
additive color model. The RGB model is called
"additive" because a combination of the three pure
colors "adds up" to white light.
The computer's operating system (Mac, Windows, etc.) organizes
the display screen into a grid or x,y coordinates, like a
checkerboard. Each little box on the screen is called a
"pixel" (short for "picture element").
Current Macintosh and Windows displays are made up of these
grids of pixels (see screen diagram below).
Pixels and color
To control the color of each pixel on the screen the operating
system must dedicate a small amount of memory to each pixel.
In aggregate this memory dedicated to the display screen is
often referred to as "video RAM" or "VRAM".
In the simplest form of black and white computer displays a
single bit of memory is assigned to each pixel. Since each
memory bit can only be positive or negative (0 or
1), a one-bit display system can only manage two colors (black
or white) for each pixel on the screen:
If we dedicate more bits of memory to each pixel in the
display, we can manage more colors. When eight bits of memory
are dedicated to each pixel, each pixel could be one of 256
colors. (256 = 2 to the eighth power; in other words, 256 is
the maximum number of unique combinations of 0's and 1's you
can make with eight bits). This kind of computer display is
called an "eight-bit" or "256-color"
display, and is very common in current micro computing,
especially on lap-top computers and older desktop machines.
If still more memory is dedicated to each pixel, we can get
nearly photographic color on the computer screen. "True-color"
or "24-bit" color displays can show millions of
unique colors simultaneously on the computer screen. True-color
(24-bit) images are composed by dedicating 24 bits of memory
to each pixel; eight each for the red, green, and blue
components (8+8+8=24).
The amount of VRAM dedicated to each screen pixel in the
display is commonly referred to as the "bit depth"
of the monitor. Most Mac and Windows microcomputers sold in
the last few years are capable of displaying bit depths
greater than eight-bit, in thousands (16-bit) or millions (24
bit) of simultaneous colors.
To check your computer system for the range of bit depths
available to you, use the "Display" control panel
(Windows95) or the "Monitors" control panel (for
Macintosh):
Bit depth and color graphics files The terminology and memory
schemes used in color displays are directly analogous those
used to describe color depth in graphics files. In their
uncompressed states, eight-bit or 256-color image files
dedicate eight bits to each color pixel in the image. In
eight-bit images the 256 colors that make up the image are
referenced to a "palette" or "index" (also
called a color lookup table, or CLUT). The main point for
eight-bit images is that they can never contain more than 256
colors.
True-color or 24-bit images are typically much larger than
eight-bit images in their uncompressed state, because each
pixel in a 24-bit image has 24 bits of memory dedicated to it,
typically in three monochrome layers: red, green and blue:
References
Rizzo, J., and K. D. Clark. 1996. How Macs work.
Emeryville, CA: Ziff Davis Press.
Siegel, D. 1996. Creating killer web sites. Indianapolis:
Hayden Books.
www.killersites.com
Weinman, L. 1996. Designing Web graphics.
Indianapolis: New Riders.
www.lynda.com
But to get total color control,
you'll need to use hexadecimal codes that represent colors.
That's not as confusing as it sounds: there are plenty of
people who've already worked out all the color combinations
and posted their hexadecimal equivalents on the Web. The first
two links contain the 216 colors which appear the same on a
Mac and PC screen. These are the best colors to use for
consistent browsing.
RGB
HEX Color Chart by Hue - http://www.lynda.com/hexh.html
No
Dither Netscape Color Palette - http://www.onr.com/user/lights/netcol.html
Convert
RGB to HEX - http://www.echonyc.com/~xixax/Mediarama/hex.html
|
