Color

1
Color

• Color is one of the most interesting aspects of
  both human perception and computer graphics.
• In principle, a display needs only three primary
  colors to produce any color needed for a human
  observer.
• We vary the intensity of each primary to produce
  a color.
• We can look at how color is handled in a graphics
  system from the programmers perspective-that is,
  through the API.
• There are two different approaches.

2
RGB and Indexed color models

• We will stress the RGB-color model because an
  understanding of it will be crucial for our later
  discussion of shading.
• Historically, the indexed-color model was easier
  to support in hardware because of its lower
  memory requirements and the limited colors
  available on displays, but in modern systems RGB
  color has become the norm.

3
RGB color

• Each color component is stored separately in the
  frame buffer
• Usually 8 bits per component in buffer
• Note in glColor3f the color values range from
  0. 0 (none) to 1. 0 (all), whereas in glColor3ub
  the values range from0 to 255

4
Indexed Color

• Colors are indices into tables of RGB values
• Requires less memory
• indices usually 8 bits
• not as important now
• Memory inexpensive
• Need more colors for shading

5
RGB color

• Any color can be constructed by "adding" certain
  amounts of 3 primary colors.
• C rR gG bB
• where r, g and b indicate the "amounts" of each
  primary color.
• Many images have a color depth of eight. Each
  pixel then has one of 256 possible colors.
• A 24 bit system, True-Color systems are common.
  Each color component given 8 bits

6
Example Color Depth6

• Assuming Color Depth 6 bits
• Thus, the composite dot can be made to glow in a
  total of 22 X 22 X 22 64 different colors.
• Some systems have a frame buffer that supports 24
  bits color depth. Each of the DACs has eight
  input bits, so there are 256 levels of red, 256
  of green, and 256 of blue, for a total of 16
  million colors.
• The CRT image must be refreshed rapidly
  (typically, 60 times a second) to prevent
  disturbing flicker.
• There are monochrome video displays, which
  display a single color in different intensities.
  A single DAC converts pixel values in the frame
  buffer to voltage levels, which drive a single
  electron-beam gun. The CRT has only one type of
  phosphor, so it can produce various intensities
  of only one color.
• Note that 6 bits in the frame buffer for
  monochrome video displays 26 64 levels of gray.

7
Indexed Color

• The 6 bits stored in each pixel. These bits are
  used as an index into a table of 64 values, say,
  LUT 0 ... LUT 63. For instance, if a pixel
  value is 39, the values stored in LUT 39 are
  used to drive the DACs. As shown in the figure,
  LUT 39 contains the 15-bit value 01010
  11001 10010
• Five of these bits (01010) are routed to drive
  the "red DAC," 5 others drive the "green DAC,"
  and the last 5 drive the "blue DAC."
• Each of the LUT entries can be set under
  program control. For example, the instruction
  setPalette(39, 17, 25, 4) would set the value
  in LUT39 to the 15-bit quantity 1000111001
  00100 (since 17 is 10001 in binary, 25 is 11001,
  and 4 is 00100).

8
Example

• Set the pixel at x 479 and y532 with value
  39
• drawDot(479, 532, 39)
• Each time the frame buffer is "scanned out" to
  the display, this pixel is read as the value 39,
  which causes the value stored in LUT39 to be
  sent to the DACs.
• How many possible color could be used in the
  system of Figure 1.40?
• In the system of Figure 1.40, each entry of the
  LUT consists of 15 bits, so each color can be set
  to one of 215 32K 32,768 possible colors.
• The set of 215 possible colors that the system is
  capable of displaying is called its palette, so
  we say that this display "has a palette of 32K
  colors."
• Therefore, this system can display a maximum of
  26 64 different colors at one time selected
  from 215 32K 32,768 possible colors

9
LookUp Table

• The contents of the LUT are not changed in the
  middle of a scan-out of the image, so one whole
  scan-out uses a fixed set of 26 64 palette
  colors.
• Usually, the contents of the LUT remain fixed
  for many scan outs, although a program can change
  the contents of a small LUT between two
  successive scan-outs.

10
General color depth

• In more general terms, suppose that a raster
• display system has a color depth of b bits and
• that each LUT entry is w bits wide.
• Then the system can display 2w colors, any 2b at
  one time.

11
Examples

• A system with b 8 bit planes and an LUT width w
  12 can display 4096 colors, any 256 of them at
  a time.
• A system with b 8 bit planes and an LUT width w
  24 can display 224 16,777, 216 colors, any
  256 at a time.
• If b 12 and w 18, the system can display 256K
  262,144 colors, 4,096 at a time

12
Compare the costs

• To compare the costs of two systems
• one with a LUT (pseudo color system) and
• one without a LUT (true color system)
• Figure 1. 41 shows an example of two
  l,024-by-l,280-pixel displays (so that each of
  them supports about 1.3 million pixels).
• Both systems allow colors to be defined with a
  precision of 24 bits, often called "true color."

13
Compare the costs