A Non-Photorealistic Lighting Model For Automatic Technical Illustration

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A Non-Photorealistic Lighting Model For Automatic
Technical Illustration
CS780 ????
SIGGRAPH 98 Amy Gooch Bruce Gooch Peter
Shirley Elaine Cohen
2003. 5. 30 ? ? ?
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Outline
1. Introduction 2. Related works 3.
Inspections 4. Traditional shading of matte
objects 5. Tone-based shading of matte objects 6.
Shading of metal objects 7. Implementation 8.
Future work and Conclusion
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Introduction
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Introduction (cont.)
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Characteristics of illustration
- edge lines with black curves - shaded matte
objects with intensities far from black and white
- surface normal by color temperature - no
shadowing - anisotropic shading for metal objects
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Related works
1. Saito and Takahashi. Comprehensible Rendering
of 3D Shapes. SIGGRAPH 90 2. Seligmann and
Feiner. Automated Generation of Intent-based 3D
Illustrations. SIGGRAPH 91 3. Dooley and
Cohen. Automatic Illustration of 3D Geometric
Models Surfaces. IEEE Computer Graphics and
Applications 90 4. Williams. Shading in Two
Dimensions. Graphics Interface 91
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Basic idea
make use of variations in hue and luminance to
convey shape
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Hue, luminance, saturation
? Hue apparent color irrespective of brightness
or saturation (e.g. red, green, yellow, purple,
). Hue is defined by the dominant wavelength of
the colored light. ? Luminance intensity of a
light source. ? Saturation the level of
purity in a single colors hue.
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Tones
? Tone a hue with some amount of black and
white added. Creating tones decreases the
saturation of a color. ? To create tones, artists
mix in black or white to darken or brighten a
color ? Different tones are useful if only a
limited range of intensity values are available.
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Tones, tints, shades
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Color temperature
- visual perception very sensitive to color
temperature - depth impression from color
temperature (object with a cool color appear
further away)
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Color temperature (cont.)
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Cool to warm transition
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Traditional shading of matte objects
Matte A perfect diffuse surface reflects the
light equally in all directions and such a
surface is usually called matte.
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Traditional shading of matte objects (cont.)
Only highlights and edges
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Cool - warm blending
- interpolation between kcool and kwarm if the
full range (-11) of L?N is considered - light
perpendicular to the gaze direction to get the
maximum range for L?N
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Cool-to-warm color shifts
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Cool - warm blending result
- Luminance does not vary much over the image but
tone does. - A sense of depth can be communicated
at least partially by a hue shift. - Subtle shape
information. - Unnatural colors.
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Scaled object color shades
? use cool-warm blending formula to create
different shades of the objects actual color ?
scale would range from black (kcool) to the
objects color(kwarm kd) ? resembles
traditional shading ? luminance also varies in
areas where L?Nlt0
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Tone - based shading of matte objects
? scale from fully saturated blue to fully
saturated yellow
? scale based on objects reflectance
? linear blend between both tones
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Tone creation for a pure object
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Results
b0.4, y0.4, ?0.2, ?0.6
b0.55, y0.3, ?0.25, ?0.5
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Results (cont.)
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Results (cont.)
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Shading of metal objects
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Shading of metal objects (cont.)
- illustrators represent metallic surfaces by
alternating dark and light bands - simulates real
effects on milled metal parts - milling creates
anisotropic reflection - realism is not the
primary goal of technical illustration -
simulation of milled objects by texture mapping -
texture n stripes of varying intensity mapped
along the axis of maximum curvature - intensity
of stripes between 0.0 and 0.5 randomly - stripe
closest to light direction overwritten with
white - linear interpolation of the colors
between the stripe centers
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Results
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Implementation
- using an existing shading model would be
preferable above replacing the shading model -
Phong model supported by many libraries
(OpenGL) - using the implemented Phong model
saves execution time (hardware support) ?
hack
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Implementation (cont.)
- key use of negative colors for light
sources - approximation by placing 2 light
sources as follows
- ambient light should have an intensity of
(kcoolkwarm)/2 if object color is white -
switch off highlighting
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Results
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Future work and Conclusion
? Future works - how illustration rules may
change or evolve for a scene instead of single
objects - practical issues of viewing and
interacting with 3D technical illustrations -
automation other application-specific
illustrations forms like medical illustration
? Conclusion - This model is tailored to imitate
colored technical drawings - Once the global
parameters of the model are set, the technique is
automatic - This model can be approximated by
interactive graphics techniques - Very useful
where communicating shape and function is
paramount