Abstract: This paper presents an approach for determining stroke thickness in computer-generated illustrations of smooth surfaces. We assume that dark strokes are drawn to approximate the dark regions of the shaded surface. This assumption leads to a simple formula for thickness of contours and suggestive contours; this formula depends on depth, radial curvature, and light direction in a manner that reproduces aspects of thickness observed in hand-made drawings. These strokes convey local shape and depth relationships, and produce appealing imagery. Our method is simple to implement, provides temporally-coherent strokes, and runs at interactive rates.