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A Non-Photorealistic Model for Automatic Technical Illustration

This paper presents an automated method for creating technical illustrations using edge lines, highlighting, color shifts, and metal shading. Future work includes improving illustration rules and automating other forms of illustration.

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A Non-Photorealistic Model for Automatic Technical Illustration

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  1. A Non-Photorealistic Model for Automatic Technical Illustration Amy Gooch Bruce Gooch Peter Shirley Elaine Cohen SIGGRAPH 1998 Presented by Anteneh

  2. Outline • Introduction • Motivation • Related/previous work • Illustration Techniques • Automatic Shading Model • Conclusion and Future work

  3. 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.

  4. 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

  5. Motivation • Technical illustrations tend to show more information about the shape and function of an object compared computer generated images.

  6. Motivation • Same comparison between technical illustrations and photographs • Use color to differentiate parts http://www.khulsey.com/stockauto.html

  7. Related Work • Very little previous work in graphics related to technical illustration (1998). • [Saito and Takahashi 90] – techniques to show geometric properties of objects.

  8. Related work • [Dooley and Cohen 1990] – Automatic illustration of 3D geometric objects. • use user-defined hierarchy of components (i.e. line width, line boundary conditions) to generate illustrations http://portal.acm.org/ft_gateway.cfm?id=91422&type=pdf&coll=GUIDE&dl=GUIDE&CFID=59921272&CFTOKEN=37584317

  9. Related Work • [Williams 91] – global illumination approximation using warm-to-cool tones and drawing conventions for specular objects.

  10. 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.

  11. 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.

  12. Automatic Illustration Method • Automate the mentioned illustration characteristics. • Edge lines are drawn in black • Highlights are drawn using traditional term from the Phong shading model. • Shade the surfaces of objects

  13. Automatic Illustration Method • Traditional diffuse shading method calculates luminance as follows: • Tone-based shading • Shading metal Objects

  14. Traditional shading • kd = 1, ka = 0 • The image hides shape and material information in the dark regions.

  15. Traditional shading • Additional information can be provided by highlights (direction of light) and edge lines (divisions). Image produced by adjusting kd and ka

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

  17. 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

  18. Creating a tone • Tone for a pure red object: sum blue-to-yellow and dark-to-red to tone.

  19. Tone-based shading

  20. Tone-based shading • Generalize the classic shading model to experiment with tones using the cosine term: • Use blue and yellow as two temperature extremes:

  21. Tone-based shading • Combining luminance shift (traditional shading), tone and temperature based shading. b = 0.4, y = 0.4 ,   = 0.2, and = 0.6

  22. Tone-based shading • The different values of b and y determine the strength of the overall temperature shift, where as and 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

  23. 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.

  24. Shading of Metal Objects • Phong vs metal-shading

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

  26. Colored Objects

  27. Conclusion: • An automated technical illustration method is presented using edge lines, highlighting, color-shifts and metal-shading. Future Work: • Improvements in illustration rules • Automate other illustration forms • Interactive illustration

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