final float MIN_X= -10;
final float MIN_Y= -10;
final float MAX_X= 10;
final float MAX_Y= 10;

final float STEP_X = 0.2;
final float STEP_Y = 0.2;

int POINTS_X = (int)((MAX_X-MIN_X)/STEP_X);
int POINTS_Y = (int)((MAX_Y-MIN_Y)/STEP_Y);

float[][] f = new float[POINTS_X][POINTS_Y];
float minz=1, maxz=0;

void setupPlot() {
  float x,y;
  for (int i=0;i<POINTS_X;i++) {
    x = MIN_X + i*STEP_X;
    for (int j=0;j<POINTS_Y;j++) {
      y = MIN_Y + j*STEP_Y;
      f[i][j] = f(x,y);
    }
  }
}

void drawPlot() {
  float x=0,y=0,z=0;
  float mn=1,mx=0;
  float a = 0;

  for (int i=0;i<POINTS_X;i++) {
    x = MIN_X + i*STEP_X;
    for (int j=0;j<POINTS_Y;j++) {
      y = MIN_Y + j*STEP_Y;
      z = f[i][j];

      float b = (1 / screenZ(x,y,z)) - 1;
      mn = min(mn,b); mx = max(mx,b);
      a = 2*((b - minz)/(maxz)) + 0.2;
      stroke(1,1,1,a);

      point(x,y,z);
    }
  }
  maxz=mx; minz=mn;
}

void drawAxis() {
  stroke(1,0,0);
  line(0,0,0,1,0,0);
  stroke(0,1,0);
  line(0,0,0,0,1,0);
  stroke(0,0,1);
  line(0,0,0,0,0,1);
}