CS 686: Programming SuperVGA Graphics Devices

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CS 686 Programming SuperVGA Graphics Devices

• Introduction An exercise in working with
  graphics file formats

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Raster Display Technology
The graphics screen is a two-dimensional array of
picture elements (pixels)
These pixels are redrawn sequentially,
left-to-right, by rows from top to bottom
Each pixels color is an individually
programmable mix of red, green, and blue
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Special dual-ported memory
CRT
CPU
VRAM
RAM
32-MB of VRAM
1024-MB of RAM
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Graphics programs

• What a graphics program must do is put
  appropriate bit-patterns into the correct
  locations in the VRAM, so that the CRT will show
  an array of colored dots which in some way is
  meaningful to the human eye
• So the programmer must understand what the CRT
  will do with the contents of VRAM

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How much VRAM is needed?

• This depends on (1) the total number of pixels,
  and on (2) the number of bits-per-pixel
• The total number of pixels is determined by the
  screens width and height (measured in pixels)
• Example when our screen-resolution is set to
  1280-by-960, we are seeing 1,228,800 pixels
• The number of bits-per-pixel (color depth) is a
  programmable parameter (varies from 1 to 32)
• Some types of applications also need to use extra
  VRAM (for multiple displays, or for special
  effects like computer game animations)

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How truecolor works
0
8
16
24
alpha
red
green
blue
longword
R
G
B
pixel
The intensity of each color-component within a
pixel is an 8-bit value
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Intel uses little-endian order
0 1 2 3
4 5 6 7
8 9 10
B
G
R
B
G
R
B
G
R
VRAM
Video Screen
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Some operating system issues

• Linux is a protected-mode operating system
• I/O devices normally are not directly accessible
• On Pentiums Linux uses virtual memory
• Privileged software must map the VRAM
• A device-driver module is needed vram.c
• We can compile it using make vram.o
• Device-node mknod /dev/vram c 99 0
• Make it writable chmod aw /dev/vram

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VGA ROM-BIOS

• Our graphics hardware manufacturer has supplied
  accompanying code (firmware) that programs
  VGA device components to operate in various
  standard modes
• But these firmware routines were not written with
  Linux in mind theyre for interfacing with
  real-mode MS-DOS
• Some special software is needed (lrmi)

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Class demo pcxphoto.cpp

• First several system-setup requirements
• Some steps need root privileges (sudo)
• Obtain demo sources from class website
• Install the mode3 program (from svgalib)
• Compile character device-driver vram.c
• Create dev/vram device-node (read/write)
• Start Linux in text mode (need to reboot)

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Typical program-structure

• Usual steps within a graphics application
• Initialize video system hardware
• Display some graphical imagery
• Wait for a termination condition
• Restore original hardware state

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Hardware Initialization

• The VGA system has over 300 registers
• They must be individually reprogrammed
• Eventually we will study those registers
• For now, we just reuse vendor routines
• Such routines are built into VGA firmware
• However, invoking them isnt trivial (since they
  werent designed for Linux systems)

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Obtaining our image-data

• Eventually we want to compute images
• For now, we reuse pre-computed data
• Data was generated using an HP scanner
• Its stored in a standard graphic file-format
• Lots of different graphic file-formats exist
• Some are proprietary (details are secret)
• Other formats are public (can search web)

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Microsofts .pcx file-format
FILE HEADER (128 bytes)
IMAGE DATA (compressed)
COLOR PALETTE (768 bytes)
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Run-Length Encoding (RLE)

• A simple technique for data-compression
• Well-suited for compressing images, when adjacent
  pixels often have the same colors
• Without compression, a computer graphics
  image-file (for SuperVGA) would be BIG!
• Exact size depends on screen-resolution
• Also depends on the displays color-depth
• (Those parameters are programmable)

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How RLE-compression works

• If multiple consecutive bytes are identical
• example 0x29 0x29 0x29 0x29 0x29
• (This is called a run of five identical bytes)
• We compress five bytes into two bytes
• the example compressed 0xC5 0x29
• Byte-pairs are used to describe runs
• Initial byte encodes a repetition-count
• (The following byte is the actual data)

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Decompression Algorithm

• int i 0
• do
• read( fd, dat, 1 )
• if ( dat lt 0xC0 ) reps 1
• else reps (dat 0x3F) read( fd, dat, 1
  )
• do image i dat while ( --reps )
• while ( i lt npixels )

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Standard I/O Library

• We call standard functions from the C/C runtime
  library to perform i/o operations on a
  device-file (e.g., vram) open(), read(),
  write(), lseek(), mmap()
• The most useful operation is mmap()
• It lets us map a portion of VRAM into the
  address-space of our graphics application
• So we can draw directly onto the screen!

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Where will VRAM go?

• We decided to use graphics mode 0x013A
• Its a truecolor mode (32bpp)
• It uses a screen-resolution of 640x480
• Size of VRAM needed 6404804 bytes
• So we map 2-MB of VRAM to user-space
• We can map it to this address-range
  0xB0000000-0xB2000000

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Virtual Memory Layout
Linux kernel
kernel space (1GB)
0xC0000000
stack
VRAM
0xB0000000
user space (3GB)
runtime library
0x40000000
code and data
0x08048000
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Color-to-Grayscale

• Sometimes a color image needs to be converted
  into a grayscale format
• Example print a newspaper photograph (the
  printing press only uses black ink)
• How can we transform color photos into
  black-and-white format (shades of gray)?
• gray colors use a mix of redgreenblue, these
  components have EQUAL intensity

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Color-conversion Algorithm

• struct unsigned char r, g, b color
• int avg ( 30r 49g 11b )/100
• color.r avg color.g avg color.b avg
• long pixel 0
• pixel ( avg ltlt 16 ) // r-component
• pixel ( avg ltlt 8 ) // g-component
• pixel ( avg ltlt 0) // b-component
• vram address pixel // write to screen

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In-class exercise

• Revise the pcxphoto.cpp program so that it will
  display (1) the color-table, and then (2) the
  scanned photograph, as grayscale images (i.e.,
  different intensities of gray)