package graphics;

//clips triangle against plane
public class ClipPlane
{
  private double a, b, c, d; //plane coordinates

  public ClipPlane(double _a, double _b, double _c, double _d)
  {
    a = _a; b = _b; c = _c; d = _d;
  }

  //plane function for a point
  private double planeFunction(double x, double y, double z)
  {
    return a*x + b*y + c*z + d;
  }

  public int clipTriangle(double[][] triangle, double[][] poly)
  {
    int numVerts = 0;
    for (int i = 0; i< 3; i++) //go thru each vertex
    {
      double[] a = triangle[i], b = triangle[(i+1)%3];
      double fA = planeFunction(a[0], a[1], a[2]);
      double fB = planeFunction(b[0], b[1], b[2]);

      if (fA <= 0) //is vertex in front of camera?
        poly[numVerts++] = (double[])a.clone();

      //are points on opposite side of plane?
      if (fA * fB < 0)
      {
        //interpolate between points a and b
        double t = -fA / (fB - fA);

        if (t > 1 || t < 0) System.err.println("Warning: In ClipPlane.clipTriangle - t seems off... t = " + t);

        //create new vertex
        double[] newVert = (double[])a.clone();
        for (int j=0; j<newVert.length; j++)
          newVert[j] = lerp(t, a[j], b[j]);

        poly[numVerts++] = newVert;
      }
    }

    return numVerts;
  }

  //returns whether to draw the line at all
  public boolean clipLine(double[] a, double[] b)
  {
    double fA = planeFunction(a[0], a[1], a[2]);
    double fB = planeFunction(b[0], b[1], b[2]);

    //both vertices are on the wrong side of the camera
    if (fA > 0 && fB > 0) return false;

    //each vertex is on opposite sides of the camera
    if (fA * fB < 0)
    {
      //interpolate between points a and b
      double t = -fA / (fB - fA);

      if (t > 1 || t < 0) System.err.println("Warning: In ClipPlane.clipTriangle - t seems off... t = " + t);

      //create new vertex
      double[] newVert = new double[a.length];
      for (int i=0; i<newVert.length; i++)
        newVert[i] = lerp(t, a[i], b[i]);

      //replace vertex on wrong side of the camera
      for (int i=0; i<newVert.length; i++)
      {
        if (fA > 0) a[i] = newVert[i];
        else b[i] = newVert[i];
      }
    }

    return true;
  }

  //linear interpolation
  public double lerp(double t, double a, double b)
  {
    return a + (b-a)*t;
  }

  public void dumpVertices(double[][] vertices)
  {
    for (int i = 0; i < vertices.length; i++)
      System.err.println("("+vertices[i][0]+", "+vertices[i][1]+", "+vertices[i][2]+")");
  }
}