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    {
    MICHAEL NICOLAI

    The basic formula (and quickest) For drawing a circle is: x^2 + y^2 = r^2.
    The r stands For radius (half the diameter). You know this formula, i am
    sure. A guy called Phytagoras set i up very long ago to calculate the
    hypotenuse of a given triangle. (there has to be a 90ø angel between a and b)

                       |\
                       | \
                     a |  \ c      c^2 = a^2 + b^2
                       |   \
                       |____\

                         b

    Remember?

    Now look at this:        ...|     a quater of the circle
                           ..   |
                          . ____|y
                         . |\   |
                        .  | \  |
                        .  | r\ |
                        .  |   \|
                   --------------------------
                        r  x    |0
                                |
                                |

    r is given and take 0 - r as a starting point For x. Then all you have to do
    is to calculate y and plot the point.

        y = sqrt((r * r) - (x * x))      sqrt : square root

    After each calculation x is increased Until it has reached 0. Then one
    quarter of the circle is drawn. The other three quarters are symmetrical.

    I have written a short Program For you to draw a circle in 320x200x256
    Graphics mode. When you key in some values please remember that NO error
    checking will be done. x has to be between 0 and 319, and y between 0 and
    199. The radius must not be greater than x and y.

    Example: x : 160; y : 100; r : 90

    When you start this Program you will not get correct circles because in
    Graphics mode ONE pixel is not square!!! You have to calculate an aspect
    ratio to get nice looking circles.
    }

    Program circle;

    Uses
      Crt, Dos;

    Var
      regs    : Registers;
      x0, y0  : Word;
      x, y, R : Real;
      temp    : Real;
      c       : Char;

    Procedure putpixel(x, y : Word; color : Byte);
    begin
      mem[$A000: (y * 320 + x)] := color;
    end;

    begin
      ClrScr;
      Writeln('Enter coordinates of middle-point :');
      Writeln;
      Write('x : '); readln(x0);
      Write('y : '); readln(y0);
      Writeln;
      Write('Enter radius :'); readln(R);

      { Switch to 320x200x256 }

      regs.ax := $0013;
      intr($10, regs);

      x := (-1) * R;  { go from 0 - R to 0 }
      temp := R * R;
      Repeat
        y := sqrt(temp - (x * x));
        putpixel((x0 + trunc(x)), (y0 - trunc(y)), 15); { 4.th quadrant }
        putpixel((x0 - trunc(x)), (y0 - trunc(y)), 15); { 1.st quadrant }
        putpixel((x0 + trunc(x)), (y0 + trunc(y)), 15); { 3.rd quadrant }
        putpixel((x0 - trunc(x)), (y0 + trunc(y)), 15); { 2.nd quadrant }
        x := x + 0.1; { change this if you want coarse or fine circle. }
      Until (x >= 0.0);
      c := ReadKey;  { wait For keypress. }

      { Switch back to Textmode. }

      regs.ax := $0003;
      intr($10, regs);
    end.

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