Does anyone out there know the formula for a circle.

I know you can work out the corresponding x and y value for any point given
the centre and radius. I used to know two methods about 15 years ago but
have since forgotten them.

Any help gratefully received.

Sean Radford

In article <01bcd8ee$2303d100$9763989e@bladesys>,

PROCEDURE circle(f: GraphicFrames.Frame; col: INTEGER; x0, y0, r: LONGINT);
        VAR
                x, y, u: LONGINT;
        BEGIN
                u := 1 - r;
                x := r;
                y := 0;
                WHILE y <= x DO
                        Display.DotC(f, col, SHORT(x0+x), SHORT(y0+y), 0);
                        Display.DotC(f, col, SHORT(x0+y), SHORT(y0+x), 0);
                        Display.DotC(f, col, SHORT(x0-y), SHORT(y0+x), 0);
                        Display.DotC(f, col, SHORT(x0-x), SHORT(y0+y), 0);
                        Display.DotC(f, col, SHORT(x0-x), SHORT(y0-y), 0);
                        Display.DotC(f, col, SHORT(x0-y), SHORT(y0-x), 0);
                        Display.DotC(f, col, SHORT(x0+y), SHORT(y0-x), 0);
                        Display.DotC(f, col, SHORT(x0+x), SHORT(y0-y), 0);
                        IF u < 0 THEN
                                INC(u, 2*y+3)
                        ELSE
                                INC(u, 2*(y-x)+5);
                                DEC(x)
                        END ;
                        INC(y)
                END
        END circle;

--
Lazlo's Chinese Relativity Axiom: No matter how great your
triumphs or how tragic your defeats, over a billion Chinese
couldn't care less.

for d := 0 to 360 do
  x := cos(d) * radius;
  y :=  sin(d) * radius;
  plot(x,y);
end;

Of course if you want a solid circle then save the previous x and y's
and join
them with a line.

In article <01bcd8ee$2303d100$9763989e@bladesys>,

                        Paul Bolchover

Sean,
  The equation for a circle is:

Sqr(X1 - X0) + Sqr(Y1 - Y0) := Sqr(R)

Where X0 and Y0 are the coords for the center and R is the radius.

To solve for X1 given a value for Y1 you would use:

X1 := X0 + Sqrt(Sqr(R) - Sqr(Y1-Y0));
X1 := X0 - Sqrt(Sqr(R) - Sqr(Y1-Y0));

The reason for 2 equations is that for each Y coord there will be two
values of X.

Also you might want to solve Sqr(R) - Sqr(Y1-Y0) first as a negative value
here would indicate that the Y1 value is outside the circle and will raise
an error.

Hope this helps!
--

Rodney E Geraghty
GERA-Tech
Ottawa, Canada
gera...@ibm.net

Sean Radford <s...@bladesys.demon.co.uk> wrote in article
<01bcd8ee$2303d100$9763989e@bladesys>...

On Tue, 14 Oct 1997 22:13:38 GMT, "Sean Radford" <s...@bladesys.demon.co.uk>
wrote:

Pete Barrett

don't most 1anguages which offer a plot function also offer a circle function?

Eric
--
   _    "just a Texan a long way from home"
 _| ~-. Eric Barber, Nortel, London Road, Harlow, England CM17 9NA
 \, *_} AT&T/BT/etc. +44 1279 403527/429531 x 3527, ESN (Nortel) 6 742 3527
   \(DISCLAIMER: Working at, not speaking for, Nortel | Eric.Bar...@nortel.co.uk

But the question is, why?  You can SelectObject(hDC, hBitmap);
and then call Arc() or another curve function for to alter the bitmap.

--

-Thanh Lim
Fanatic Games Programmer

   x :=  a + cos(d) * radius;
   y :=  b + sin(d) * radius;

if for some reason you don't want the center at the origin...

--
David Ullrich

sig.txt not found

In article <01bcd9d0$1166b560$1962420c@efgdell>,
Earl F. Glynn <EarlGl...@worldnet.att.net> wrote:

Totoro <tot...@netcom.com> wrote in article
<totoroEI3pM3....@netcom.com>...

Why not just use Canvas.Ellipse and pass it a "square":

For example,  Canvas.Ellipse(0,0, 100,100);

efg
_____________________________________
Earl F. Glynn          EarlGl...@WorldNet.att.net
MedTech Research Corporation
Lenexa, KS  66219  USA                    

        (Sorry, spillover from another thread. Seriously,
seems like it must be that it's a non-Delphi, presumably
non-MS-Windows and also non-who-knows-what-else question.)

--
David Ullrich

sig.txt not found

PROCEDURE DrawCircle* (x, y, r: INTEGER);
VAR u, v, r2: INTEGER;
BEGIN
  r2 := r*r;    (* r < Sqrt(MAX(INTEGER)) *)
  FOR u := -r TO r DO
    v := SHORT(ENTIER(0.5 + Math.sqrt(r2-u*u)));
    Dot(x+u,y+v);
    Dot(x+u,y-v);
  END;
END DrawCircle;

where Dot is any pixel drawer (with color previously set).

--
Douglas G. Danforth, Ph.D.      danfo...@csli.stanford.edu
Senior Research Engineer        (650) 723-2487
Center for the Study of Language and Information
Stanford University