//====================================================================================
// lineval2.cpp : uses PixelFlow style linear-expression evaluator to resolve pixel
//   coverage, uses combined circle and depth tests, 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) in screen space, pixel radius,
// Min and Max depth Z-value (Max is "deeper").
//------------------------------------------------------------------------------------
void BruteForceSphere(int a, int b, float r, float MinZ, float MaxZ)
{
  for (int x=0; x<WW; x++)
    for (int y=0; y<WH; y++)                   // FOR EVERY PIXEL (x,y)
    {
      int PixelIsEnabled=0;                    // SET ENABLE FLAG TO FALSE

      // CALCULATE DEPTH USING LEE AND QBUFFER
      float A_ = (MaxZ-MinZ)/(r*r);
      float A = -2*a;
      float B = -2*b;
      float C = (a*a+b*b) + MinZ/A_;

      float LEE = A*x + B*y + C;               // EVALUATE LINEAR EXPRESSION
      LEE += Qbuffer(x,y);                     // INCR BY QBUFFER
      float Z = LEE * A_;                      // MULT BY A' TO OBTAIN Z

      if (Z<=MaxZ) PixelIsEnabled=1;           // CHECK IF PIXEL IS INSIDE THE CIRCLE
                                               // DO Zbuffer DEPTH TEST HERE!
      if (PixelIsEnabled)                      // DRAW THE PIXEL IF ENABLED
      {
        float C=1-Z;
        fb.SetForeground(C,C,C);
        fb.SetPixel(x,y);
      }
    }
}

void main()
{
  int NumSphereTiles=3;

  float TileWidth = (float)WW/NumSphereTiles;
  float TileHeight = (float)WH/NumSphereTiles;
  float r = (TileWidth+TileHeight)*0.25; // CIRCLE RADIUS
  float a,b;                             // CIRCLE CENTER
  b=TileHeight*0.5;
  for (int i=0; i<NumSphereTiles; i++)
  {
    a=TileWidth*0.5;
    for (int j=0; j<NumSphereTiles; j++)
    {
      BruteForceSphere(a,b,r,0,1);
      a+=TileWidth;
    }
    b+=TileHeight;
  }

  fb.MakePPM("output.ppm");
}