Notwithstanding that it’s not optimized, Listing 36.1 is reasonably fast. If you run Listing 36.2 (a sample line-drawing program that you can use to test-drive Listing 36.1), you may be as surprised as I was at how quickly the screen fills with vectors, considering that Listing 36.1 is entirely in C and has some redundant divides. Or perhaps you won’t be surprised—in which case I suggest you not miss the next chapter.

LISTING 36.2 L36-2.C

/* Sample line-drawing program. Uses the optimized
line-drawing functions coded in LListing L36.1.C.
Tested with Borland C++ in the small model. */

#include <dos.h>

#define GRAPHICS_MODE   0x13
#define TEXT_MODE       0x03
#define BIOS_VIDEO_INT  0x10
#define X_MAX           320      /* working screen width */
#define Y_MAX           200      /* working screen height */

extern void LineDraw(int XStart, int YStart, int XEnd, int YEnd, int Color);

/* Subroutine to draw a rectangle full of vectors, of the specified
 * length and color, around the specified rectangle center.  */
void VectorsUp(XCenter, YCenter, XLength, YLength, Color)
int XCenter, YCenter;   /* center of rectangle to fill */
int XLength, YLength;   /* distance from center to edge of rectangle */
int Color;              /* color to draw lines in */
{
   int WorkingX, WorkingY;

   /* lines from center to top of rectangle */
   WorkingX = XCenter - XLength;
   WorkingY = YCenter - YLength;
   for ( ; WorkingX < ( XCenter + XLength ); WorkingX++ )
   {
      LineDraw(XCenter, YCenter, WorkingX, WorkingY, Color);
   }
   /* lines from center to right of rectangle */
   WorkingX = XCenter + XLength - 1;
   WorkingY = YCenter - YLength;
   for ( ; WorkingY < ( YCenter + YLength ); WorkingY++ )
   {
      LineDraw(XCenter, YCenter, WorkingX, WorkingY, Color);
   }
   /* lines from center to bottom of rectangle */
   WorkingX = XCenter + XLength - 1;
   WorkingY = YCenter + YLength - 1;
   for ( ; WorkingX >= ( XCenter - XLength ); WorkingX-- )
   {
      LineDraw(XCenter, YCenter, WorkingX, WorkingY, Color);
   }
   /* lines from center to left of rectangle */
   WorkingX = XCenter - XLength;
   WorkingY = YCenter + YLength - 1;
   for ( ; WorkingY >= ( YCenter - YLength ); WorkingY-- )
   {
      LineDraw(XCenter, YCenter, WorkingX, WorkingY, Color);
   }
}
/* Sample program to draw four rectangles full of lines.  */
int main()
{
   union REGS regs;

   /* Set graphics mode */
   regs.x.ax = GRAPHICS_MODE;
   int86(BIOS_VIDEO_INT, &regs, &regs);

   /* Draw each of four rectangles full of vectors */
   VectorsUp(X_MAX / 4, Y_MAX / 4, X_MAX / 4, Y_MAX / 4, 1);
   VectorsUp(X_MAX * 3 / 4, Y_MAX / 4, X_MAX / 4, Y_MAX / 4, 2);
   VectorsUp(X_MAX / 4, Y_MAX * 3 / 4, X_MAX / 4, Y_MAX / 4, 3);
   VectorsUp(X_MAX * 3 / 4, Y_MAX * 3 / 4, X_MAX / 4, Y_MAX / 4, 4);

   /* Wait for a key to be pressed */
   getch();

   /* Return back to text mode */
   regs.x.ax = TEXT_MODE;
   int86(BIOS_VIDEO_INT, &regs, &regs);
}