Draw a filled circle

[Mirage]

OMG, Sqrt/ArcCos within the inner cycle!
http://cboard.cprogramming.com/game-...algorithm.html

[Cybermonkey]

OMG, Sqrt/ArcCos within the inner cycle!
http://cboard.cprogramming.com/game-...algorithm.html

But your link does not refer to a filled circle algorithm. You see in my first post I had a solution without sqrt. But it was awfully slow.
I came to this, which is fast and does not overlap anymore (this can be seen when drawing alphablended):

procedure fillcircle (cx,cy,r:integer); 
var x,y,r2,dx:integer;
begin
    if r = 0 then begin
        r:=1;
    end;
   r2 := r * r;
   for x := r downto 0 do begin
      y := round(sqrt(r2 - x*x));
      dx := cx - x;
      line(dx-1, cy-y, dx-1, cy+y);
      dx := cx + x;
      line(dx, cy-y, dx, cy+y);
      end;
end;

[Carver413]

well I suppose that works but what if you want a 6 sided circle or an ellipse or a half circle. might be better to use a more conventual routine with a fill routine.

[User137]

Sometimes, but fill routine makes it much slower. It is checking for pixels on every draw to find where the borders are, and could also be allocating dynamic arrays for point data, depending on algorithm. You can modify algorithms to make filled half circle etc. And if you need to draw the circle on top of a complex drawing, fill no longer works. And it's in addition that you would use sin/cos/sqrt for outline anyway

Here's algorithm for above given function translated, filled and non-filled circle. (Drawing to TForm.canvas directly was bad idea, i know... but it showed it at least.)

procedure DrawPixel(x, y: longint; color: TColor);
begin
  form1.Canvas.Pixels[x, y]:=color;
end;

procedure DrawLine(x1, x2, y: longint; color: TColor);
var x: integer;
begin
  for x:=x1 to x2 do
    form1.Canvas.Pixels[x, y]:=color;
end;

procedure retro_circle(xc, yc, r: longint; color: TColor);
var x, y, d: longint;
begin
  x:=0; y:=r;
  d:=1 - r;
  while x < y do begin
    if d < 0 then
      d:=d + 2*x + 3
    else begin
      d:=d + 2*x - 2*y + 5;
      dec(y);
    end;
    DrawPixel(xc + x, yc - y, color); // Top
    DrawPixel(xc - x, yc - y, color);
    DrawPixel(xc + y, yc - x, color); // Upper middle
    DrawPixel(xc - y, yc - x, color);
    DrawPixel(xc + y, yc + x, color); // Lower middle
    DrawPixel(xc - y, yc + x, color);
    DrawPixel(xc + x, yc + y, color); // Bottom
    DrawPixel(xc - x, yc + y, color);
    inc(x);
  end;
end;

procedure retro_fill_circle(xc, yc, r: longint; color: TColor);
var x, y, d: longint;
begin
  x:=0; y:=r;
  d:=1 - r;
  while x < y do begin
    if d < 0 then
      d:=d + 2*x + 3
    else begin
      d:=d + 2*x - 2*y + 5;
      dec(y);
    end;
    DrawLine(xc - x, xc + x, yc - y, color);
    DrawLine(xc - y, xc + y, yc - x, color);
    DrawLine(xc - y, xc + y, yc + x, color);
    DrawLine(xc - x, xc + x, yc + y, color);
    inc(x);
  end;
end;

Also as far as i see, the fill function may draw some pixels overlapped. Not perfectly optimal this way.

[Mirage]

But your link does not refer to a filled circle algorithm.

If you can draw a circle bound, you know all its points. And then you can draw all the needed horizontal lines very fast.

[User137]

If you can draw a circle bound, you know all its points. And then you can draw all the needed horizontal lines very fast.

That's exactly what i did. But i now decided to go back to it a little and optimize it. No more overlapped pixels. Also screenshot to let you see the areas it makes with different colors.

... adding previous code...

procedure DrawVLine(x, y1, y2: longint; color: TColor);
var y: integer;
begin
  for y:=y1 to y2 do
    form1.Canvas.Pixels[x, y]:=color;
end;

// Optimized fill circle
procedure retro_fill_circle(xc, yc, r: longint; color: TColor);
var x, y, d, y1, y2: longint;
begin
  x:=0; y:=r;
  d:=1 - r;
  y1:=r*7 div 10; // I found this 7/10 ratio just by testing and testing a little...
  // It is the Y coordinate where the drawing sections separate
  y2:=yc+y1;
  y1:=yc-y1;
  while x < y do begin
    if d < 0 then
      d:=d + 2*x + 3
    else begin
      d:=d + 2*x - 2*y + 5;
      dec(y);
    end;
    DrawVLine(xc - x, yc - y, y1, color); // Top
    DrawVLine(xc + x, yc - y, y1, color);
    DrawLine(xc - y, xc + y, yc - x, color); // Upper middle
    DrawLine(xc - y, xc + y, yc + x, color); // Lower middle
    DrawVLine(xc - x, y2, yc + y, color); // Bottom
    DrawVLine(xc + x, y2, yc + y, color);
    inc(x);
  end;
end;

edit: Actually the real ratio might close into sin(45 degree) ~ 0.707. There is 2 pixel-lines overlapped still when drawing with 250 radius. We aren't talking of any significant performance difference though. But if drawing blended pixels you want absolute precision.