We present a new technique to automatically generate building ex- teriors. Our technique relies on the definition of building templates that will be applied on building descriptions. The building descrip- tions require the building 3D footprints and their heights and roof heights. This information can be stored within a Geographic Infor- mation System (GIS). Most of the time, graphic designers use the texture repetition method in order to create building frontages. Due to this lack of flexibility, this method often disappoints the users who expect more realism. In this paper, we focus on the descrip- tion of our frontage generation method. Building frontages are gen- erated using a 2.5D wall grammar based on a set of rules that can be simple or detailed enough to fulfil the users wishes. Our method is as easy to use as the texture repetition but provides a higher level of realism and diversity in the resulting buildings

1. Wall Grammar for Building Generation
Mathieu Larive and Véronique Gaildrat
Visual Objects, from Reality To EXpression
http://www.irit.fr/-Equipe-VORTEX
Oktal Synthetic Environment
http://www.oktal-se.fr
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2. Contents
• Introduction
• Frontage Templates
• Groundwork and Roof Templates
• Results and Discussions
• Conclusion and Future Work
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3. Problem
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4. Goals
• Quickly create a plausible (geo-typic) city around an already
modelized zone (geo-specific)
• Reach a level of detail satisfactory enough for a navigation
at ground level:
– modeling of the buildings’inside
– geometric modeling of the urban furniture
→ usage of automatic techniques inside AGETIM
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5. Generation steps
• Hierarchical division of
city generation process in
seven steps [Larive2005]
• Each step can be seen as a
logical LoD
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6. Contents
• Introduction
• Frontage Templates
• Groundwork and Roof Templates
• Results and Discussions
• Conclusion and Future Work
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7. Frontage Templates
• A frontage template contains a primary wall and possibly a
background material
• A frontage template can be seen as a style sheet that
describes:
– wall rendering
– possible dimensions and kinds of windows and doors
(described as textures or 3D objects)
– how to place these elements on the building
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8. Frontage Templates
• Formal grammar representation [Wonka2003]
• 2,5 dimensions paramatric wall grammar
Repetition rules Position rules
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9. Frontage Templates
• Wall Panel
– the unique terminal
symbol of our grammar
– various parameters:
• texture
– Background
– Decoration
• 3D object
• possible dimensions
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10. Frontage Templates
• Bordered Wall • Extruded Wall
– four margins – depth (positive or not)
– resize policy – depth faces generation
– BW → W boolean
– EW → W
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11. Frontage Templates
• Wall Grid
– Contains an unique
child wall
– Repetition
• Vertical
• Horizontal
• Both
– Controlled by two
cardinality intervals
– WG → Wnm
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12. Frontage Templates
• Wall List
– Several child walls
– Orientation (exclusive)
• Vertical
• Horizontal
– WL → W1W2...Wn
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13. Frontage Templates
• The combination of those different walls in a tree-like
hierarchy allows the user to build simple or complex
frontage templates
• Use of repetition schemes on every part of our
frontages (wall grid)
• Usage of previously generated 3D objects (such as
balconies or cornices)
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14. Frontage Templates
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15. Contents
• Introduction
• Frontage Templates
• Groundwork and Roof Templates
• Results and Discussions
• Conclusion and Future Work
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16. Groundwork and Roof Templates
• Groundwork templates
– Z-min
– Z-max
– Extruded
Z-min Z-max Extruded
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17. Groundwork and Roofs Template
• Roof templates
– Based on the Straight
Skeleton method
[Felkel1998]
– One, two or four slopes
– Overhang (4 types)
– Currently, 10 various
roof types
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18. Contents
• Introduction
• Frontage Templates
• Groundwork and Roof Templates
• Results and Discussions
• Conclusion and Future Work
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19. Results
• Various buildings on one building footprint
94 faces 350 faces 5600 faces
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20. Results
• Large scale urban area
17 362 buildings, 920 182 faces generated in less than 8 mn 20/22

21. Conclusion
• Able to generate buildings on any kind of footprint
– convex, non-convex, non-plane, even with holes
• Generated buildings are valid
– geometrically (no hole, no overlapping face, no empty
frontage -blind frontages-)
– no window or door on a frontage edge
• Once a building template is finished and robust, it can
be immediately reused (ready-to-use building template
library)
• Control of the geometric complexity, according to the
user hints
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22. Future Work
• Finalize the building
template editor
• Usage of the Urban Land
Use Model
– Road network generation
– Creation of lots and
building footprints
• Integration of all the
various generation steps
inside the same process
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