several hatchings such as cross lines and dots are used to represent the depth within this portrait. as seen in this image the darkness of an area is shown by doing a much more of a closer/tight hatchings.
Albrecht Dürer, six pillows (verso), pen and brown ink, 1493. The Metropolitan Museum of Art.
Basic Pen Strokes - creating texture. Taken from "The Technical Pen" by Gary Simmons
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