Quadratic Curve Pixel Shader

QuadCurvePix.c

   float4 QuadraticPS(float2 p : TEXCOORD0,  
  float4 color : COLOR0) : COLOR  
{  
  // Gradients  
   float2 px = ddx(p);  
  float2 py = ddy(p);  
  // Chain rule  
   float fx = (2*p.x)*px.x - px.y;  
  float fy = (2*p.x)*py.x - py.y;  
  // Signed distance  
   float sd = (p.x*p.x - p.y)/sqrt(fx*fx + fy*fy);  
  // Linear alpha  
   float alpha = 0.5 - sd;  
  if (alpha > 1)       // Inside  
    color.a = 1;  
  else if (alpha < 0)  // Outside  
   clip(-1);  
  else                   
   // Near boundary  
    color.a = alpha;  
  return color;  
} 

Charles Loop & Jim Blinn Quadratic Curve Pixel Shader:
http://http.developer.nvidia.com/GPUGems3/gpugems3_ch25.html