A Non-Photorealistic Model for Automatic Technical

IllustrationAmy Gooch Bruce Gooch

Peter Shirley Elaine CohenSIGGRAPH 1998

Outline

Introduction Illustration Techniques Automatic Shading Model Conclusion and Future work Demo

Introduction Method to automate some technical

illustration conventions. Technical illustrations: in textbooks,

reference books, manuals i.e. a car owner’s manual.

Method: a shading algorithm based on edges, highlighting and cool-to-warm tones.

Technical Illustrations

Communication of geometry and form is more important than aesthetics or realism.

Edge lines are usually emphasized. Important three-dimensional properties

are preserved while extraneous detail is diminished.

Shadows are usually not included Only one light is usually used

Illustration Techniques

Observed illustration characteristics:› edge lines, the set containing surface

boundaries, silhouettes, and discontinuities, are drawn with black curves.

› objects are shaded with intensities far from black or white with warmth or coolness of color indicative of surface normal;

› a single light source provides white highlights.

› shadowing is not shown.

Illustration Techniques

Subjects can infer at least as much geometric information from edge lines in drawn images verses shaded or textured images.

Hue changes are used to indicate surface orientation rather than reflectance.

Automatic Illustration Method

Automate the mentioned illustration characteristics.› Edge lines are drawn in black (mention in other paper)› Highlights are drawn using traditional term

from the Phong shading model.› Shade the surfaces of objects

Automatic Illustration Method

Traditional diffuse shading method calculates luminance as follows:

Tone-based shading Shading metal Objects

Traditional Shading kd = 1, ka = 0 The image hides shape and material

information in the dark regions.

Traditional Shading Additional information can be provided

by highlights (direction of light) and edge lines (divisions).

Image produced by adjusting kd and ka

Traditional Shading Combining the shaded and illustrated model. Poor image and loss of detail, not automated.

Tone-based Shading

Tones : color scales created by adding grey to a certain color.

Tones are important to illustration, especially when restricted to a limited luminance range.

Temperature : used to give depth cue. Warm colors advance, cool colors recede.› Warm – red, orange, yellow› Cool – blue, violate, and green› Temperate – red-violets, red-greens

Creating a Tone Tone for a pure red object: sum blue-to-

yellow and dark-to-red to tone.

Tone-based Shading

Tone-based Shading

Generalize the classic shading model to experiment with tones using the cosine term:

Use blue and yellow as two temperature extremes:

Tone-based Shading Combining luminance shift (traditional shading),

tone and temperature based shading.

b = 0.4, y = 0.4,    = 0.2, and   = 0.6

Tone-based Shading The different values of b and y determine the strength of the overall

temperature shift, where as alpha and beta determine the prominence of the object color, and the strength of the luminance shift.

b = 0.55, y = 0.3,    = 0.25, and   = 0.5

Shading of Metal Objects

Technical illustrators use a different technique to communicate whether or not an object is metal.

Illustrators represent a metallic surface by alternating dark and light bands.

Method: map a set of twenty stripes of varying intensity along the parametric axis of maximum curvature.

Shading of Metal Objects Phong vs metal-shading

Shading of Metal Objects metal-shading with edge and cool-to-warm shift

Colored Objects

Conclusion & Future Work

An automated technical illustration method is presented using edge lines, highlighting, color-shifts and metal-shading.

Improvements in illustration rules Automate other illustration forms Interactive illustration

Implementation Results

Implementation Results