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 programmer’s perspective-that is, through the API. There are two different approaches.

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.

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

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

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

Example Color Depth=6 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.

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 LUT[39] to the 15-bit quantity 1000111001 00100 (since 17 is 10001 in binary, 25 is 11001, and 4 is 00100).

Example Set the pixel at x =479 and y=532 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 LUT[39] 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

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.

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.

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

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."

Compare the costs