3D Models Where does the data come from

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3D ModelsWhere does the data come from?
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Sources of 3D data

• Directly specifying the Three-Dimensional data

y
(-1, -1, -1) ( 1, -1, -1) ( 1, 1, -1) (-1, 1,
-1) (-1, -1, 1) ( 1, -1, 1) ( 1, 1, 1) (-1,
1, 1)

Fine for this
x
z
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... But not for this!
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Sources of 3D data

• Directly specifying the Three-Dimensional data
• Transforming data found in other forms into
  surfaces or volumes e.g. taking molecule data
  and transforming it into spheres and cylinders

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Sources of 3D data

• Directly specifying the Three-Dimensional data
• Transforming data found in other forms
• Writing programs that create such data this is
  called procedural modelling.

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Procedural modelling of buildings (Parish and
Muller, SIGGRAPH 2001)
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Procedural Modelling of Natural Sceneries
(Guerraz et al., CASA 2003)
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Sierpinskis Triangle Self-similarity common in
nature
Fractal Landscape
www.flash.net/djconnel/Vue/
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L-systems for plant modelling Prusinkiewicz
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Sources of 3D data

• Directly specifying the Three-Dimensional data
• Transforming data found in other forms
• Writing programs that create such data this is
  called procedural modelling.
• Using a modelling program, e.g. Blender, Maya, 3D
  Studio, Auto CAD, Softimage

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Sources of 3D data

• Directly specifying the Three-Dimensional data
• Transforming data found in other forms
• Writing programs that create such data this is
  called procedural modelling.
• Using a modelling program, e.g. Blender, Maya, 3D
  Studio Max, Auto CAD, Softimage
• Photogrammetric techniques reconstruction from
  photos of a real object.

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Photogrammetry

• Find corresponding points in a series of images
• Triangulate to find their location in space

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From photos, extract models using image
information
later you can texture and even relight
Oh et al. SIGGRAPH 2001
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Sources of 3D data

• Directly specifying the Three-Dimensional data
• Transforming data found in other forms
• Writing programs that create such data this is
  called procedural modelling.
• Using a modelling program, e.g. Blender, Maya, 3D
  Studio Max, Auto CAD, Softimage
• Photogrammetric techniques reconstruction from
  photos of a real object.
• Use 3D scanner or digitizers, which gather depth
  information

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The Digital Michelangelo project
Stanford University
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Gives an awful lot of data!!!!
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Sources of 3D data

• Directly specifying the Three-Dimensional data
• Transforming data found in other forms
• Writing programs that create such data this is
  called procedural modelling.
• Using a modelling program, e.g. Blender, Maya, 3D
  Studio Max, Auto CAD, Softimage
• Photogrammetric techniques reconstruction from
  photos of a real object.
• Use 3D scanner or digitizers, which gather depth
  information
• Other Sources
• Implicit surfaces,
• Medical scans, e.g. CT, MRI

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Implicit Surfaces blobby objects that can merge
and deform Usually defined by an equation of the
form f(x,y,z)0
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Volumetric data can be derived from CT or MRI
scans
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Representing 3D objects
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What representation?

• Graphics scenes contain all kinds of objects
  terrain, trees, rocks, buildings, characters,
  rain, cloth etc..
• No one method that will describe objects that
  will be suited to their very different
  characteristics
• So, How do we ...
• Represent 3D objects in a computer?
• Construct these objects quickly?
• Manipulate them with the computer?
• The algorithms depend on the representation.

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Representation schemes

• Two broad categories (but not everything fits
  neatly into one or the other)
• Boundary Representations (B-reps)
• and
• Space-partitioning Representations

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Boundary Representations

• B-reps describe a 3D object as a set of surfaces
  that separate the object interior from the
  environment.
• Examples
• Polygon Meshes
• Subdivision Surfaces
• Implicit Surfaces
• Parametric/spline patches
• Constructive Solid Geometry

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Polygonal Model
Tessellation or polygonisation
A polygon mesh is a B-rep or boundary
representation model
Geometry defined by the sub-space bounded by sets
of planes and surfaces.
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Space-partitioning

• Space-partitioning representations are used to
  describe interior properties of solid objects.
• They typically partition a region containing an
  object into a set of small, non-overlapping,
  contiguous solids, such as cubes or tetrahedra.
• Examples
• Voxels
• Quadtrees
• Octrees
• BSP-trees (Binary Space Partition)
• Kd-trees (k-dimensional)
• CSG ?

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Volumetric model.
CSG constructive solid geometry
3D Object is a collection of volume elements or
voxels which flag spatial occupancy in a discrete
region.
3D Objects defined as boolean operations on solid
primitives.
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Materials

• Visual appearance of objects also depends on the
  properties of the materials they are made of
• How we represent materials depends on the
  rendering technique used
• Realistic techniques require physical parameters
  (albedo, reflectance)
• Often, intermediate information is used
  (colour)
• Eventually
• Volumetric, parameters are f(x,y,z)
• Surface based, parameters are f(u,v) (Texture
  maps)