VGA Color Registers

1
VGA Color Registers

• How we can reprogram the
• Digital-to-Analog Converters
• 256 color-table registers

2
VGAs color-innovation

• The VGA introduced display-mode 19
• Allowed showing 256 simultaneous colors
• Used one byte of VRAM for every pixel
• Convenient addressing for programming
• Screen-resolution was only 320-by-200
• But SVGA offers improved resolutions
• VESA mode 0x101 640-by-480
• VESA mode 0x103 800-by-600

3
digital versus analog

• MDA and CGA used digital cable-signals
• VGA introduced analog cable-signals
• Special hardware needed DAC controller
• Implements a table of 256 color-registers
• Supports primary colors red, green, blue
• Supports 64 different intensities per color
• So each color-register implements 18-bits

4
Format of a color-register
17 12 11
6 5 0
I/O port-addresses (for DAC programming) 0x03C8
index-register 0x03C9 data-register for
writes 0x03C7 data-register for reads
5
Data-structure for colors

• typedef struct char r, g, b rgb_t
• rgb_t red 63, 0, 0
• rgb_t green 0, 63, 0
• rgb_t blue 0, 0, 63
• rgb_t white 63, 63, 63

6
Writing to a color-register

• rgb_t color 32, 48, 16 )
• int index 15
• // example reprogramming a DAC register
• outb( index, 0x03C8 ) // select register
• outb( color.r, 0x3C9 ) // set r-component
• outb( color.g, 0x3C9 ) // set g-component
• outb( color.b, 0x3C9 ) // set b-component

7
Reading a color register

• rgb_t color
• int index 14
• // example reading a DAC color-register
• outb( index, 0x3C8 ) // select register
• color.r inb( 0x3C7 ) // get r-component
• color.g inb( 0x3C7 ) // get g-component
• color.b inb( 0x03C7 )// get b-component

8
Demo studydac.cpp

• This is a testbed for color experiments
• It uses VESA display-mode 0x4101 (i.e.,
  640-by-480, in 256 simultaneous colors)
• It draws a 16-by-16 grid of color-boxes
• Each box shows a color-registers value
• Initially we see the default color-values
• Then the 256 registers are reprogrammed
• But you can try out different color schemes

9
An array of color intensities

• Each color-component is a 6-bit number
• So there are 26 64 possible intensities
• (This applies to each color-component)
• So heres how to build all the cyan-values
• rgb_t table 64
• for (int i 0 i lt 64 i)
• tablei.b i tablei.g i tablei.r
  0

10
Why do we need to do this?

• We will soon study lighting and shadows, in order
  to create photorealistic images
• It requires showing varied color-intensity
• Well want to be sure our hardware can display
  all intensities a given image needs
• Well want to arrange needed colors in an array,
  for conveniently accessing a given
  color-intensity in terms of its array-index

11
In-class exercise 1

• Modify the studydac.cpp demo-program so that it
  will simultaneously display
• 32 intensities of blue
• 32 intensities of cyan
• 32 intensities of green
• 32 intensities of magenta
• 32 intensities of red
• 32 intensities of yellow
• 64 intensities of white

12
In-class exercise 2

• Modify the studydac.cpp demo-program so it will
  simultaneously display intensity-arrays for
  several pastel colors.
• You can build these colors by mixing a pure
  color with the color white
• For example pink ½ red ½ white