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);
    }