Star Field Effect in C by writing in VGA memory

For past few time I was working on the 16 Bit version of C compiler namely TurboC++. and was programing in few Graphical libraries. I was keen on writing a program in C to show a similar effect like that of Star field effect in the Windows Screen Saver but writing the routes at low level directly to VGA memory. I designed a library for that form a bit of help form net.
                                  Here I am posting a C program to Create a Star field effect in C.  Also I have well Documented the code so that you can under Stand it well.

download the executable  and source code file form exe form here

http://rapidshare.com/files/302392162/Star_Field_Effect.rar.html

 NOTE this will work only in widows XP or windows 2000 ... for VISTA or other OS  you will need dosbox. download it form here http://www.dosbox.com/download.php?main=1

the code :-

/////////////////////////////////////////////////////////////////////
//                 3D star field effect                        
//                RITESH RANJAN .... 16.03.2008  
//                                                      
//    See  guideline at http://programing-tutorial.blogspot.com/ 
//                   for compiling using TuboC++              
//    Find more Programing Hacking and Tutorial stuff at      
//     http://programing-tutorial.blogspot.com/              
/////////////////////////////////////////////////////////////////////

#include< stdio.h >
#include< stdlib.h >
#include< time.h >
#include< dos.h >
#include< conio.h >
// include my graphics library
#include< ritgraph.h >

// user defined function for writing the pixel values
// to virtual screen (double buffer)
void virpixel(int , int, byte);

// structure to define position of star and it color by c
typedef struct star
    {
       int x,y,z;
       byte c; // note byte if defined in ritgraph.h
    };

byte * virscr=NULL;    /// virtual screen pointer

int main()
{
    int x,y,z,i,sx,sy; // sx and sy screen x and y positions
    byte pal[256][3],temp[3]; // for pallet and color of star
     time_t t;
    srand((unsigned) time(&t)); // randomisation
    randomize();
    star st[70]; // take 70 stars
    settextmode();
    printf(" Another creation by Ritesh Ranjan ");
    printf("\n \n\t This star field effect is created by ");
    printf("\n\t by using double buffering for faster and smooth effect ");
    printf("\n\n I would like to hear ur comments at... :-  ritesh_ranjan007@yahoo.com");
    printf("Find more Programing Hacking and Tutorial stuff at--\n");
    printf("http://programing-tutorial.blogspot.com/");

    printf("\n\npress any key to exit");
    getch();
    setvgamode(); // set the screen mode form text mode to vga mode in ritgraph.h
////////////////////  create virtual screen //////////////////////////////

    virscr=(byte *)(calloc(64000,1));
    if(virscr==NULL)
    {
       printf(" error in calloc ");
       getch();
       exit(1);
    }

///////////////  Craete the white shade for star  ////////////////
    for(i=10;i>0;i--)
    setpal(i,55-3*i,55-3*i,55-3*i);    /// white shade

    for(i=25;i>10;i--)
       setpal(i,25,25,25);


    for(i=0;i<70;i++)
       st[i].z=0;

///////////////  create stars //////////////////////

     for(i=0;i<70;i++)
     {
     if(st[i].z<1)
    {
    st[i].x=(rand()%300)-150;
    st[i].y=(rand()%199)-100;
    st[i].z=256;
    /////////////////// colouring of stars //////////////

    if(st[i].x>0 && st[i].y>0)
       st[i].c= (st[i].x+st[i].y)/25 ;
     else if(st[i].x<0 && st[i].y>0)
       st[i].c= ((-1)*st[i].x+st[i].y)/25;
     else if(st[i].x>0 && st[i].y<0)
       st[i].c=(st[i].x+(-1)*st[i].y)/25;
     else
       st[i].c=(byte)( ( (-1)*(st[i].x+st[i].y) ) /25);



    }
   
    // transformation of 3d cordinate to 2d system for showing in screen
    // note sx and sy and real x,y coodinate of screen calculated for each star
    sx=((st[i].x*256)/st[i].z + 150);
    sy=((st[i].y*256)/st[i].z + 100);

    // if inside screen of the write the  pixel to virtual screen
    if(sx>0 && sx<320 && sy>0 && sy<200)
     virpixel(sx,sy,st[i].c);




     }

     // after calculating the whole screen copy to main VGA memory
      _fmemcpy(vga,virscr,64000);


/////////////////////////  move b/w stars ///////////////

  // start infinite loop
  while(!kbhit())
  {


   for(i=0;i<70;i++)
    {
    if(st[i].z<1)
    {
    st[i].x=(rand()%300)-150;
    st[i].y=(rand()%199)-100;
    st[i].z=256;
    if(st[i].x>0 && st[i].y>0)
       st[i].c=(st[i].x+st[i].y)/12;
     else if(st[i].x<0 && st[i].y>0)
       st[i].c=((-1)*st[i].x+st[i].y)/12;
     else if(st[i].x>0 && st[i].y<0)
       st[i].c=(st[i].x+(-1)*st[i].y)/12;
     else
       st[i].c=(-1)*(st[i].x+st[i].y)/12;

    }

    else
     st[i].z=st[i].z-2;

       sx=((st[i].x*256)/st[i].z + 150);
       sy=((st[i].y*256)/st[i].z + 100);

       if(sx>0 && sx<320 && sy>0 && sy<200)
     virpixel(sx,sy,st[i].c);
       else
      st[i].z=0;

     }
    //////////////  copy from virtual screen to real ///////

    _fmemcpy(vga,virscr,64000);
     memset(virscr,0,64000); // clear the virtual screen
     waitretrace();   // wait for the screen to draw defined in ritgraph.h
}
    free(virscr);
    settextmode();
    return 0;
}
/////////////  fuction to write in virtual memory ///////////////////////
void virpixel(int x,int y,unsigned char col)
{
   //this is x+y*2^8+y*2^6 = x+y*320
   // remeber bit wise letfshit is faster than mutiplication
   virscr[x+(y<<8)+(y<<6)]=col;
}