I believe the correct answer is by increasing the density of the lines.
If you take a look at the painting I've attached, you will see that the lines are quite thin in the middle where Jesus is crucified. However, as your eye moves away from Christ, you will see that the lines become thicker and thicker until they become completely black on the sides of the painting.
If you take a look at the painting I've attached, you will see that the lines are quite thin in the middle where Jesus is crucified. However, as your eye moves away from Christ, you will see that the lines become thicker and thicker until they become completely black on the sides of the painting.