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

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

Introduction Method to automate some technical illustration conventions. Method to automate some technical illustration conventions. Technical illustrations: in textbooks, reference books, manuals i.e. a car owner’s manual. 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. 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. Communication of geometry and form is more important than aesthetics or realism. Edge lines are usually emphasized. Edge lines are usually emphasized. Important three-dimensional properties are preserved while extraneous detail is diminished. Important three-dimensional properties are preserved while extraneous detail is diminished. Shadows are usually not included Shadows are usually not included Only one light is usually used Only one light is usually used

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

Motivation Same comparison between technical illustrations and photographs Same comparison between technical illustrations and photographs Use color to differentiate parts Use color to differentiate parts

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

Related work [Dooley and Cohen 1990] – Automatic illustration of 3D geometric objects. [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 use user-defined hierarchy of components (i.e. line width, line boundary conditions) to generate illustrations d=91422&type=pdf&coll=GUIDE&dl =GUIDE&CFID= &CFTOK EN=

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

Illustration Techniques Observed illustration characteristics: 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. 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. Hue changes are used to indicate surface orientation rather than reflectance.

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

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

Traditional shading kd = 1, ka = 0 kd = 1, ka = 0 The image hides shape and material information in the dark regions. 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). 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. Combining the shaded and illustrated model. Poor image and loss of detail, not automated. Poor image and loss of detail, not automated.

Tone-based shading Tones : color scales created by adding grey to a certain color. Tones : color scales created by adding grey to a certain color. Tones are important to illustration, especially when restricted to a limited luminance range. 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. Temperature : used to give depth cue. Warm colors advance, cool colors recede. Warm – red, orange, yellow Warm – red, orange, yellow Cool – blue, violate, and green Cool – blue, violate, and green Temperate – red-violets, red-greens Temperate – red-violets, red-greens

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

Tone-based shading

Generalize the classic shading model to experiment with tones using the cosine term: Generalize the classic shading model to experiment with tones using the cosine term: Use blue and yellow as two temperature extremes: Use blue and yellow as two temperature extremes:

Tone-based shading Combining luminance shift (traditional shading), tone and temperature 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 and determine the prominence of the object color, and the strength of the luminance shift. 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

Shading of Metal Objects Technical illustrators use a different technique to communicate whether or not an object is metal. 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. 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. 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 Phong vs metal-shading

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

Colored Objects

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

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