c# - Set the transparency of a circle on a bitmap -

i'm trying set transparency of circle on bitmap in c# .net winforms. tried doing using graphics.drawellipse this:

private void setcirclealpha(int alpha, ref bitmap b, columnvector2 pos, int diameter)     {         graphics g = graphics.fromimage(b);         solidbrush sb = new solidbrush(color.fromargb(0, 0, 0, 0));          g.fillellipse(sb, pos.x, pos.y, diameter, diameter);     } 

but not want draws transparent circle on image instead of setting transparency of circle.

i have resorted using extremely slow code:

private void setcirclealpha(int alpha, ref bitmap b, columnvector2 pos, int diameter)     {         //calculate square root of radius         float radius = diameter / 2;         float sqradius = radius * radius;          //calculate centre of circle         columnvector2 centre = pos + new columnvector2(radius, radius);          (int x = (int)pos.x; x < pos.x + diameter; x++)         {             (int y = (int)pos.y; y < pos.y + diameter; y++)             {                 //calculate distance between centre of circle , point being tested                 columnvector2 vec = new columnvector2(x, y) - centre;                  //if distance between point , centre of circle less radius of circle point in circle.                  //calculate distance using pythagoras (a^2 + b^2 = c^2)                 //square both distance , radius eliminate need square root                 float sqdist = (vec.x * vec.x) + (vec.y * vec.y);                  if (sqdist < sqradius)                 {                     b.setpixel(x, y, color.fromargb(alpha, b.getpixel(x, y)));                 }             }         }     } 

my question is: is there better/faster way this?

please note i'm not asking faster circle generation algorithms, rather asking alternative graphics options.

using hans passant's comment got working:

private void setcirclealpha(int alpha, ref bitmap b, columnvector2 pos, int diameter)     {         graphics g = graphics.fromimage(b);         g.compositingmode = system.drawing.drawing2d.compositingmode.sourcecopy;         solidbrush sb = new solidbrush(color.fromargb(alpha, 0, 0, 0));          g.fillellipse(sb, pos.x, pos.y, diameter, diameter);     }