CS 686 Programming SuperVGA Graphics Devices Introduction An exercise in working with graphics file formats 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 pixel s color is an individually programmable mix of red green and blue Special dual ported memory VRAM CRT CPU 16 MB of VRAM RAM 2048 MB of RAM Graphics programs What a graphics program must do is put appropriate bit patterns into the correct locations in the VRAM such 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 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 screen s 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 Certain types of applications also need to use extra VRAM for multiple displays or for special effects like computer game animations How truecolor works 24 longword alpha 16 8 red green R G 0 blue B pixel The intensity of each color component within a pixel is an 8 bit value Intel uses little endian order VRAM 0 1 2 B G R 3 4 5 6 B G R Video Screen 7 8 9 10 B G R 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 mmake vram Device node mknod dev vram c 99 0 Make it writable chmod a w dev vram 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 they re for interfacing with real mode MS DOS So special software is needed int86 cpp What is int86 cpp This is boilerplate code that we can link with all of our Linux graphics applications It invokes some kernel services provided by our dosio c device driver which set up a pre boot execution environment PXE allowing Linux to run firmware routines in virtual 8086 mode via the vm86 call Later we will discuss how it works but for now we just use it to get on with our demo In class demo pcxphoto cpp First several system setup requirements Some steps need root privileges or sudo Create device nodes in etc udev devices etc udev devices dos read only etc udev devices vram read write Restart Linux in text mode GRUB boot menu Obtain demo sources from our class website Compile and install our character mode Linux device driver modules dosio c and vram c 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 Hardware Initialization The VGA system has over 300 registers They must be individually reprogrammed Eventually we shall study those registers For now we just reuse vendor routines Such routines are built into VGA firmware But invoking them isn t trivial since they weren t designed for a Linux environment Obtaining our image data Eventually we want to compute images For now we reuse pre computed data Data was generated using an HP scanner It s stored in a standard graphic file format Lots of different graphic file formats exist Some are proprietary details are secret Other formats are public search the web BitMaP bmp file format FILE HEADER INFO SECTION Optional color palette IMAGE DATA usually uncompressed This is an image format often used by Microsoft Windows PiCture eXchange pcx format FILE HEADER 128 bytes IMAGE DATA compressed COLOR PALETTE 768 bytes 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 display s color depth Those parameters are programmable 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 Decompression Algorithm int i 0 do read fd dat 1 if dat 0xC0 reps 1 else reps dat 0x3F read fd dat 1 do image i dat while reps while i npixels 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 Where shall VRAM go We decide to use graphics mode 0x0121 It s a truecolor mode 32 bits per pixel It uses a screen resolution of 640x480 Size of VRAM needed 640 480 4 bytes So we map 2 MB of VRAM to user space We can map it to this user address range 0xA0000000 0xA0200000 Virtual Memory Layout kernel space 1GB Linux kernel stack VRAM 0xC0000000 0xA0000000 user space 3GB runtime library code and data 0x40000000 0x08048000 How to compile our demo The demo program is pcxphoto cpp It needs to include a header pcx h It needs to read a datafile issxmas pcx It needs to be linked with int86 cpp It needs to include the header int86 h Compile it with the GNU C compiler g pcxphoto cpp int86 cpp o pcxphoto Programming Project 1 Write an analagous demo program that will display an image in bmp file format The image file is named fwright bmp You will need to utilize a different graphics display mode 1024 by 768 mode 0x123 You will need a different header bmp h You will need a different image algorithm Exercise 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 red green blue these components have EQUAL intensity Color conversion Algorithm struct unsigned char r g b color int