Introduction

In art, the most basic element is the point. It can have multiple appearances, such as a sharp-cornered spot, a blurred spot, or a stain [#!DIE_KUNST_DER_ZEICHNUNG!#] (see Figure ). The line, as next-higher element of drawing, out-lines (hence the name!) the shape of an object (this is often referred to as contour). It follows its boundaries, either as a closed line or in the form of multiple line segments (see Figure ). Each object has distinguishing marks which are used by our vision system to recombine a pair of lines into mental objects. A form surrounded by lines is seen as an area (compare with Figure ). The simple heuristic the brain uses for that is to take the simplest form for a given set of lines. This means that if details are left away, the form gets clearer and better recognizable (this is quite the opposite of what most people think).

Artists use various drawing tools to influence the appearance of their artwork: Pens, pencils, chalks, brushes or even fingers give different shapes to the produced strokes. Furthermore, the different types of the pigments used (e.g. ink, graphite, watercolor) determine their color and opacity.

• Pens and pencils are widely used for sketches and technical illustrations. Due to the thin nib, sharp lines can be produced. The main pigments are sepia, traditional ink and Indian ink (pen) as well as graphite (pencil). Contrary to pens, various tones can be produces with pencils depending on their hardness.
• Chalks and charcoals are used in portrait paintings and sketches. As for pencils, different tones are available (according to the pigment mixture).
• Brushes come in multiple shapes and are made of various materials (e.g. horse hair, marten hair). The pigments are available separately, using water or oil as binder. The application of brushes is referred to as painting (as opposed to drawing, which uses non-fluid pigments).

Figure (top): Types of points
Figure (bottom): set of lines, which we see as 'face'

The placement of the strokes is called drawing technique (e.g. Hatching, Sumi, Pointilism). Apart from this technical side of drawing, selection, composition and abstraction of a scene adds meaning to a piece of artwork, and is therefore considered to be the most important subject for an artist.

The simulation of art in computer graphics, often referred to as non-photorealistic rendering, has originally concentrated on the creation of Pen-and-Ink Style Drawings for CAD and illustrations. With growing interest in the field, researchers have also tried to mimic the style in which famous artists produce their artwork (``Painterly Rendering'') not only for still images, but also for animations. This has lead to a strong influence of the movie industry on the field, mainly in the form of Cartoon Style Renderings. In this report, an overview of these techniques is given.

Overview

The paper is organized as follows: Section (2) deals with the generation of Pen-and-Ink Style Illustrations. In section (3), approaches producing Painterly Renderings are presented. Cartoon Syle Renderings are dealt with in section (5). In order to sum up, a categorization of non-photorealistic techniques is given in section (6).