int numSteps = 10;
int stepSize = 40;

int sz = numSteps * stepSize;
size(sz, sz);
background(255);

fill(200);  // light gray

for (int level = 0; level < numSteps; level += 1) {
  float topX = (numSteps-level-1)*stepSize/2;
  float leftY = level * stepSize;
  rect(topX, leftY, stepSize * (1+level), stepSize);
}

save("pyramid.jpg");