//====================================================================================
// lineval.cpp : uses PixelFlow style linear-expression evaluator to resolve pixel
//   coverage, uses paraboloid approximation for depth, uses Qbuffer.
//====================================================================================
#include "fbppm.hpp"
#include <math.h>

// THE "MOCK" FRAMEBUFFER
int WW=256, WH=256;
FrameBufferPPM fb(WW,WH);

//------------------------------------------------------------------------------------
// Qbuffer : table of precomputed values for each (x,y) pixel location.
//------------------------------------------------------------------------------------
int Qbuffer(int x, int y)
{
  return( x*x + y*y );  // FOR NOW, JUST EVALUATE
}

//------------------------------------------------------------------------------------
// scan-converts sphere given center (a,b,c) in screen space and pixel radius.
//------------------------------------------------------------------------------------
void LinEvalSphere(int a, int b, int c, int r)
{
  // FOR EVERY PIXEL (x,y)
  for (int x=0; x<WW; x++)    
    for (int y=0; y<WH; y++)
    {
      int PixelIsEnabled=0;

      // DETERMINE IF PIXEL IS ENABLED BY EVALUATING LINEAR EXPRESSION
      int A = 2*a;
      int B = 2*b;
      int C = r*r - a*a - b*b;
      int LinEval = A*x + B*y + C;
      if ( LinEval >= Qbuffer(x,y) ) PixelIsEnabled=1;

      // CALCULATE DEPTH USING PARABOLOID APPROXIMATION
      if (PixelIsEnabled)
      {
        float z = (LinEval + c*r - Qbuffer(x,y)) / r;

        float Color = (z-c)/r;
        fb.SetForeground(Color,Color,Color);
        fb.SetPixel(x,y);
      }
    }
}

void main()
{
  int NumSphereTiles=1;

  float TileWidth = (float)WW/NumSphereTiles;
  float TileHeight = (float)WH/NumSphereTiles;
  float Radius = (TileWidth+TileHeight)*0.25;
  float Cx, Cy;
  Cy=TileHeight*0.5;
  for (int i=0; i<NumSphereTiles; i++)
  {
    Cx=TileWidth*0.5;
    for (int j=0; j<NumSphereTiles; j++)
    {
      LinEvalSphere(Cx, Cy, 0, Radius);
      Cx+=TileWidth;
    }
    Cy+=TileHeight;
  }
    
  fb.MakePPM("output.ppm");
}