A Non-Photorealistic Model for Automatic Technical Illustration

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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