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Can't find where parameter is specified in this shader

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Hi! I'm trying to figure out where it is specified if the curve is convex or concave in this sample shader provided in this article: Rendering Vector Art on the GPU. Does it need to be rendered in two batches one for convex triangles and one for concave triangles?

       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  
       // Near boundary  
        color.a = alpha;  
      return color;  

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