int numSteps = 20;
int stepSize = 20;

int N = (1+numSteps) * stepSize;
size(N, N);
background(255);
fill(200);  // light gray

beginShape();
vertex((0.5+numSteps)*stepSize, (0.5+numSteps)*stepSize);
vertex((0.5+numSteps)*stepSize, 0.5*stepSize);
for (int j=0; j < numSteps; j++) {
  float x = (numSteps - j - 0.5)*stepSize;
  float y = (1.5+j)*stepSize;
  vertex(x,y-stepSize);
  vertex(x,y);
}
endShape(CLOSE);