TPACircle

function TPACircle(center: TPoint; radius: Extended): TPointArray;
var
  l, i, j, r: Integer;
begin
  r := Floor(radius);
  SetLength(Result, Trunc((((r * 2) + 1) * ((r * 2) + 1)) * 0.8));
  l := 0;
  Result[0] := center;
  for i := 1 to r do
  begin
    Result[(l + 1)] := Point((center.X - i), center.Y);
    Result[(l + 2)] := Point((center.X + i), center.Y);
    Result[(l + 3)] := Point(center.X, (center.Y - i));
    Result[(l + 4)] := Point(center.X, (center.Y + i));
    l := (l + 4);
    for j := 1 to r do
      if (Sqr((j / radius)) + Sqr((i / radius)) <= 1) then
      begin
        Result[(l + 1)] := Point((center.X - j), (center.Y - i));
        Result[(l + 2)] := Point((center.X - j), (center.Y + i));
        Result[(l + 3)] := Point((center.X + j), (center.Y - i));
        Result[(l + 4)] := Point((center.X + j), (center.Y + i));
        l := (l + 4);
      end else
        Break;
  end;
  SetLength(Result, (l + 1));
end;

function TPACircleFilled(const C:TPoint; Rad:Integer): TPointArray;
var
  x, y, i: Integer;
  sqrad: single;
  B: TBox;
begin
  sqrad := Trunc(Sqr(Rad+0.5));
  B := IntToBox(C.x-Rad, C.y-Rad, C.x+Rad, C.y+Rad);
  SetLength(Result, (B.x2-B.x1+1)*(B.y2-B.y1+1));
  i := 0;
  for y:=B.y1 to B.y2 do
    for x:=B.x1 to B.x2 do
      if Sqr(x-c.x) + Sqr(y-c.y) < SqRad then
      begin
        Result[i] := Point(x,y);
        i := i+1;
      end;
  SetLength(Result, i);
end;

var
  t, i, b: Integer;
  a: TPointArray;

begin
  ClearDebug;
  b := CreateBitmap(1000, 500);
  DisplayDebugImgWindow(1000, 500);
  DrawTPABitmap(b, TPACircle(Point(250, 250), 50.5), 16777215);
  DrawTPABitmap(b, TPACircleFilled(Point(750, 250), 50), 255);
  DrawBitmapDebugImg(b);
  for i := 0 to 9 do
  begin
    t := GetSystemTime;
    a := TPACircle(Point(100, 100), 2000.5);
    WriteLn(GetSystemTime - t, ' ms. for TPACircle. [', Length(a), ']');
    SetLength(a, 0);
    t := GetSystemTime;
    a := TPACircleFilled(Point(100, 100), 2000);
    WriteLn(GetSystemTime - t, ' ms. for TPACircleFilled. [', Length(a), ']');
    SetLength(a, 0);
  end;
end.