The key for real-time non-photo-realistic computer graphics
often lies in finding its silhouette (see Figure ).
Mathematically, a silhouette edge is defined as edge connecting
front-facing and back-facing polygons. An approach that uses a fast
probabilistic identification of silhouette edges in object space in
order to render line-art in real time was presented by Markosian et
al. [#!CONF_CG_INTERACTIVE-2!#].
Winkenbach and Salesin [#!winkenback90parametric!#] propose
a pen-and-ink system specificly for parametric surfaces (see
Figure ).
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The physical simulation of graphite pencil drawing
is subject to the work by Sousa and Buchanan [#!EVL-1999-365!#].
They propose a rendering framework that encapsulates three levels
of drawing: First, there is the simulation of the drawing materials
(low level): The distribution of lead particles over a piece
of paper is computed, with multiple layers being rendered above each
other. Furthermore, different pencil hardnesses, pencil tip shapes
as well as the structure of the drawing paper is taken into account
(refer to Figure ). At medium level,
the simulation deals with the placement of the strokes and drawing
of outlines. Composition of the scene and rendering as a whole is
dealt with at high level.
Line direction is a critical factor in hatching. Girshik et al. [#!SYMP_NPAR-2000-7!#] have provided evidence that line direction affects surface perception (e.g. curvature of a surface). Salisbury et al. [#!CONF_CG_INTERACTIVE-3!#] have therefore proposed a system for creating pen-and-ink style renderings with orientatable strokes. By aligning the direction field with the surface orientation, a more expressive appearance of pen-and-ink illustrations can be achieved (see Figure
Praun, Hoppe et al. [#!EVL-2001-153!#] present an approach which manages to produce hatched illustrations in real-time. Producing an animation out of a set of individually hatched images poses the following problems:
A triangle is drawn by blending several TAM images over
each other using multi-texturing (see Figure ).
Each texture image is weighted using the lighting computed at the
vertices. Thus, tones can be varied over each triangle. The use of
Real-Time hatched illustrations for 3D gaming has
been researched by Freudenberg et al. Their approach uses a per-object
specification of rendering style in order to optimize performance.
Previous work on the subject [#!SYMP_INTACT_3D-2001-1!#] focused
on intercepting OpenGL calls in order to replace them with sketched-style
rendering calls. However, frame-to-frame coherence could not be reached
with this method, resulting in a fuzzy and disturbing look. The approach
leaves the specification of discontinuities on the 3D mesh to the
artist, and introduces various simplifications in determining the
visibility of the silhouette edges. Surface detail is added via mip-mapping
(Hatch Maps and Ink Maps), with smaller images containing
fewer lines in order to achieve constant tonality. Conventional shading
is used for colorization of the objects.