Computer-Generated Pen-and-Ink Illustration

1
Computer-Generated Pen-and-Ink Illustration

• Georges Winkenbach and David H. Salesin
• University of Washington
• SIGGRAPH 1994

2
NPR illustrations
Pen-and-Ink some advantages over realism

• Convey information better
• Consume less storage
• Easily reproduced and transmitted
• Vary the levels of detail
• Add sense of artistic vitality

3
Pen-and-ink illustration

• Strokes
• Tones and Textures
• Outlines

4
Strokes
Strokes are produced by placing the nib of a
pen in contact with the paper and tracing a path

• Too thin a stroke can give a washed-out
  appearance
• Too coarse a stroke can detract from delicate
  details
• Pen position is often altered by turning the nib
  during a stroke to create artistic
  inconsistencies
• Even-weighted line drawings appear lifeless. The
  thickness of a line should vary along its length
• Wavy lines indicate hand motion and make images
  appear less computer rendered

5
Tone/Textures
Tone is a function of the ratio of black ink to
white paper over a certain region of the
illustration Texture is created when the
character of the strokes is varied over the same
region

• In traditional pen-and-ink illustration,
  combinations of strokes are used to create an
  overall impression of the desired tone
• Pen-and-ink drawing is unique in that
  combinations of strokes create both the tone and
  the texture, and these elements cannot be
  separated

6
Tone/Texture

• Tones should be created from lines of roughly
  equal weight and spacing
• To clarify distinctions between similarly toned
  objects, it is sometimes important to enhance
  contrast or invent shadows
• Strokes are important for conveying texture, as
  well as depicting geometry and indicating lighting

7
Tone/Texture

• indication is used to imply a texture without
  drawing every single stroke. It provides a means
  to emphasize selected portions of the image

8
Outlines
Outlines depict the borders of an image and
distinguish between the shapes within that image

• The quality of the outline stroke is important
  for conveying texture
• Thick outlines are used to suggest shadows or
  bring objects forward in the scene
• Outlines should become haloed and fade away
  where one object passes behind another
• When tones are not used, outlines are required to
  convey the shape
• Using indication when drawing outlines is just
  as important as when drawing tones

9
Outlines
10
graphics rendering pipeline

• Modifications
• Standard aspects
• Differences

11
Modifications

• dual nature of strokes
• traditionally, texture and tone are rendered
  independently. However, in pen-and-ink, the same
  strokes that produce tone also convey texture
• need to combine 2D and 3D information
• traditionally, rendering info is entirely 3D
  with final projection to 2D largely a matter of
  sampling the rendered shades
• For pen-and-ink, size of projected areas must be
  used to compute proper stroke density in order
  to accommodate dual nature of strokes
  (tone/texture)
• Also, outlining depends on type of junction
  between 2D boundaries and level of contrast
  between tones of adjacent 2D regions

12
Standard aspects of pipeline

• the model (a standard 3D polygonal model)
• assignment of texture
• lighting model
• visible surface algorithm
• shadow algorithm

13
Differences

• maintaining 2D spatial subdivision
• rendering of texture and tone- polygons are no
  longer scan converted. Texture and tone must both
  be conveyed through hatching
• clipping- stroke based clipping to texture region
  maintains hand-drawn effect
• outlining- boundary outlines surround visible
  regions, interior outlines used within
  polygons, suggest shadow direction or give
  view-dependent accents to stroke texture

14
Strokes

• Waviness function
• Pressure function

15
Strokes
16
Textures
Prioritized stroke texture

• When rendering a prioritized stroke texture,
  highest priority strokes are drawn first if
  rendered tone is too light, next highest priority
  strokes are added, etc. until you get proper tone

17
Textures
18
Textures
Resolution dependence

• Problem enlargement is performed either by pixel
  replication, which yields ugly aliasing
  artifacts, or by drawing at higher resolution
  which yields thinner strokes and therefore
  lighter illustration. Reduction is performed by
  scan-converting curves at lower resolution,
  yielding a large black mass of overlapping
  strokes
• Solution Prioritized stroke texture chooses
  proper texture and tone for given size and
  resolution

19
Textures
Indication

• It is important to suggest texture without
  drawing every last stroke

• They implemented a semi-automatic method where
  user specifies where detail should appear, and
  indication is used everywhere else

20
Textures
21
Textures
Indication

• A field w(x,y) is generated by detail segment l
  at point (x,y) in texture space according to
  w(x,y) (a b distance((x,y), l))-c where a,b
  and c are non-negative constants that can change
  the effect of the field
• The field w(x,y) is perturbed by a small random
  value so as not to create patterns that are too
  regular

22
Outline
Boundary outlines

• Surround visible polygons of the image
• Take into account both textures of surrounding
  regions and the adjacency info stored in the
  planar map
• Each stroke texture has a boundary outline
  texture

23
Outline
Interior outlines

• used within polygons to suggest shadow directions
  or give view-dependent accents to stroke texture

24
Outline
Minimizing outline

• Let E be an edge that is shaded by two faces F
  and G of a planar subdivision. E is drawn only if
  the tones of face F and G are not sufficiently
  different for the two faces to be easily
  disambiguated by their shading alone

25
Outline
26
Outline
Accenting outline

• Brick edges that cast shadows are rendered with
  thickened edges, while illuminated brick edges
  are not drawn at all

27
Outline
Viewing direction alterations

• Viewed from above, all edges between individual
  shingles are clearly visible viewed from the
  side, the shingles tend to blend together and
  vertical edges begin to disappear
• Each stroke texture is outfitted with a
  simplified Anisotropic Bi-directional Reflectance
  Distribution Function which describes its outline
  features in terms of both lighting and viewing
  directions

28
Outline
29
Contributions of this paper

• Surveyed established principles from traditional
  illustration that can be used for communicating
  visual information effectively
• Showed that a large number of these principles
  can be incorporated as part of an automated
  rendering system, and that the information
  present for driving the ordinary graphics
  pipeline is in many respects also sufficient for
  achieving important non-photorealistic effects

30
Contributions of this paper

• Introduced the concept of a prioritized stroke
  texture, a general framework for creating
  textures from strokes, and provided a methodology
  for building procedural versions of these
  textures
• Allowed a form of resolution-dependent rendering,
  in which the choice of strokes used in an
  illustration is appropriately tied to the
  resolution of the target medium