2. Ink Line Rendering for Film Production
Daniel Teece
Walt Disney Feature Animation

3. Introduction Coming up… Background Four routes to an ink line image
Production examples

4. Ink Lines and NPR The bigger picture “Natural Media Emulation”
Several commercial ‘toon renderers’
Research papers:
Winkenbach
Elber
Gooch & Gooch
Amongst others
Books (Gooch & Gooch, Apodaca & Gritz)

5. Applications Why do we need to render lines?
Outlines convey information minimally
Simplicity of a line, variety of styles
For Disney - merging with traditionally animated artwork
Various uses, various approaches

6. Surface Shaders The shortest route
Surface normal tested against view vector, and dot product compared to threshold value
No rendering code needed (just a shader)
Easy integration with rendering pipeline
Able to take advantage of standard features in scanline renderer
Difficult to control line width

7. Surface Shaders Shader example:
float angle = abs(normalize(N) . normalize(I));
float border = 1 - step (0.3, angle);
Ci = mix (Ci, color 0, border);
(from [Apodaca, Gritz 00])

8. Image Buffers and Edge Detection
An established and proven approach
Sobel edge detection on reference images
High resolution (often 4K or higher)
Various render passes:
Z Depth (one float per pixel)
Surface Normal (one 3D vector per pixel) or N.I
Object / Surface IDs (one or more ints per pixel)

9. Image Buffers and Edge Detection
Strengths and Weaknesses
Proven approach, production tested
A lot comes “for free” - e.g. intersection lines, visibility
Slow due to image size / multiple passes
Finding threshold values can be difficult - limited fine-grain control

10. Technical illustration using edge detection
Image Buffers and Edge Detection
Technical illustration using edge detection

11. Image Buffers and Edge Detection
Examples of production use
Hercules - the Hydra
Mulan - the Hun charge
The Iron Giant - the Giant
Osmosis Jones - Drix

12. Inverted Surface Outline Rendering
A novel alternative
Standard back face culling creates outlines from surfaces
Each original surface has two surfaces, one flipped and slightly larger than original
The offset between the surfaces is the line

13. Inverted Surface Outline Rendering
A simple example

14. Inverted Surface Outline Rendering
© Disney
© Disney
© Disney
Images courtesy of Hiroki Itokazu, George Taylor and Lance Williams

15. Inverted Surface Outline Rendering
Rendering can be completely off-the-shelf
Focus shifts to modeling
No line parameterization
Surface intersections problematic

16. Geometry-based Ink Line Rendering
Edge detection in object space
Lines typically generated from polygonal data
Rendering is more easily separated from line extraction
NURBS edge extraction has also been explored [Gooch98] [Elber,Cohen90]
Memory use shifts from image buffers to geometrical data structures

17. Geometry-based Ink Line Rendering
Silhouette detection refinements
Interpolation (edges across faces)
Probabilistic Searching [Markosian et al 97]
Edge buffer [Buchanan, Sousa 00]
Other approaches:
Dual Surface [Hertzmann, Zorin 00]
Gauss Maps [Gooch et al 99]

18. Geometry-based Ink Line Rendering
Strengths and Weaknesses
Fast, but some computations are costly
Vectorized lines can be used elsewhere
Have to compute visibility
Tessellation quality is critical
Numerical precision issues
Software development effort is larger

19. Case Study : Inka A hybrid ink line renderer
Primarily a geometry-based approach
Lines derived from tessellated geometry (not from NURBS as in Gooch / Elber)
High level of localized control over lines
Part of digital production pipeline
Used for approvals / roughs and final art

20. Inka - Requirements Inka needed to support: Large models
Faster render times
High quality lines
Controllability for LookDev TDs
© Disney

21. Line Segment Generation Visibility Determination
Inka - Process
Attributes
Attribute Parsing
Geometry
Surface Tessellation
Camera
Scan Conversion
Line Segment Generation
Line Refinement
Visibility Determination
Output Image
Line Drawing
Vector Lines

22. Inka - Attributes A text file providing ink line directives
# Set line width and color
width *object1* 1.5
color *object2*
# Turn off certain lines
noLine *object1* vmin,vmax
noLine *object2* trim
# Visibility directives
outlineZMin *object1*
selfZBias *object2* 0.08

23. Inka - Tessellation NURBS / SubDs to triangles
Can have significant impact on results
View dependent / view independent
Trims are supported
Inventor format - pipeline standard

24. Inka - Line Segment Generation
Line types:
Silhouettes
Boundaries
Trims
Surface Curves
Space Curves
Creases
Intersections are computed separately

25. Inka - Visibility High resolution Z-Buffer
Appel’s quantitative visibility [Appel67] was initial approach
Z-Buffer is higher resolution than image (typically 2x - 4x)
Line edges compared with scan-converted surface depth values
Numerical precision can be an issue

26. zBias moves point towards viewer
Inka - Visibility Issues
zBias
zBias moves point towards viewer
ambiguous
intersections

27. Inka - Line Drawing Merged lines are scan converted edge by edge
Tapering and other effects may be applied
Alpha accumulation is controlled by attributes and pixel compositing modes

28. Inka - Line Quality
Line shaders allow user to control variation of color, width etc.
Line tapering lends hand-drawn appearance, using screen-space algorithm
Further extended by vector output - lines replaced by other drawing primitives (e.g. Sable stroke renderer)

29. Inka - Line Quality
© Disney
Tapering lines

30. Vector output in interactive viewer
Inka - Vector Viewer
Vector output in interactive viewer

31. Inka - Extra Features Simple paint layers
Memory management (geometry groups)
Animated attributes
SWF export
Line attenuation based on depth
Width and opacity can vary with distance

32. Inka - Summary Geometry-based Highly controllable
Part of production pipeline
Film appearances:
The Emperor’s New Groove (2000)
Atlantis - The Lost Empire (2001)
Lilo and Stitch (2002)
Treasure Planet (2002)

33. Inka - Summary Inka software developers : Yun-Chen Sung Mike King
Patrick Dalton
Rasmus Tamstorf
Joe Lohmar
Ramon Montoya Vozmediano
Daniel Teece

34. Conclusions Different approaches, each with merits and pitfalls
Image quality over speed
Controllability over interactivity
Integration is important

35. Acknowledgements
Hiroki Itokazu
Tad Gielow
Jack Brooks