Computer-Generated Pen-and-Ink Illustration of Trees

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Computer-Generated Pen-and-Ink Illustration of
Trees

• Oliver DeussenThomas StrothotteUniversity of
  Magdeburg

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Why pen, ink, and trees

• Drawings convey visual information
• Illustrations in books
• Architecture and landscaping

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Main ideas

• Using existing models
• Drawing the foliage
• Large data
• Abstract representation
• Drawing styles

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Related work

• Leaves
• Alvy Smith
• Fractals
• Small disks
• Reeves and Blau
• Particle system to represent leaves
• Sasada
• Tree sketch in architecture environments
• View facing polygons

• Strokes
• Simulate brush
• Tone and texture

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Interactive Pen and Ink Illustration

• 2D drawing
• Tone and texture
• High level cumulative effect
• Randomness to the strokes
• Stored stroke textures

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Traditional methods

• Trunk
• Silhouette
• Shading
• Leaves
• Primitive shape

• Foliage
• Well lit
• Half shadow
• Deep shadow

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Image from xfrog
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Input model

• xfrog system
• Tree skeleton
• 2 levels of branches
• Foliage
• Particles vs. polygons
• 183,000 to 8,800

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Tree skeleton

• Silhouette
• Mokosian
• Rakar
• Shading
• Floyd Steinberg method
• Difference image algorithm
• Stroke area and direction

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Foliage

• First attempt
• Merging shapes into outlines
• Abstractions
• Depth difference
• Saito and Takahashi
• Enhance outlines

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Depth differences

• Zero order difference determines outlines
• z-buffer
• Difference with neighbor
• Threshold

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Using z-buffer

• Parameters
• Depth value from the camera z0, z1
• z-buffer value d00, d165535
• Threshold 10 of the depth of the tree

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Disk size and threshold

• a) size 0.15, threshold 1000 b) size 0.7,
  threshold 2000

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Abstract primitives

• 3D impression

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Level of abstraction

• Using z-buffer value
• Primitive size

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Software framework

• Interactive vs. high quality
• Software shadows
• Vectoriation of primitive polygons

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Results

• Interactive 3 trees of 20,000 particles each and
  25,000 ground particles at 3 fps.
• 13,200 elliptic primitives - 10 seconds (SGI
  Octane)

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Results

• 200,000 leaves reduce to 16,200 particles
• 90,000 tree particles, 23,000 grass particles - 1
  minute

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Conclusion

• Pro
• Using existing models
• Primitive shape
• Level of abstraction
• Hand drawn style

• Con
• Interactive speed
• Dealing with large models

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Extensions

• Trees only?
• Depth difference extends to other models.