size(400, 400);

int gap = 5;
int N = width;

// for astetics, we break black into two,
// starting with portion that is eventually covered by red
stroke(0); 
for (int k=0; k < N; k += gap) {
  float partial = 1.0 * (N-k)/N;
  line(partial*k, k + partial*(N-k), k, N);
}

stroke(255, 0, 0); // red in bottom-right quadrant
for (int k=0; k < N; k += gap) {
  line(k, N, N, N-k);
}

stroke(0, 0, 255); // blue in top-right quadrant
for (int k=0; k < N; k += gap) {
  line(k, 0, N, k);
}

stroke(0, 255, 0); // green in top-left quadrant
for (int k=0; k < N; k += gap) {
  line(N-k, 0, 0, k);
}

// for astetics, one more time with black,
// but only with portion that (re)covers green
stroke(0); 
for (int k=0; k < N; k += gap) {
  float partial = 1.0 * (N-k)/N;
  line(0, k, partial*k, k + partial*(N-k));
}