1. VTK: The Visualization Toolkit
Part II: Visualization Model
March 30, 2001

2. VTK Visualization Models
Graphics and Visualization Model
Graphics objects: rendering
Visualization objects: generating geometry
Data Flow System: Pipeline execution
Data source Visualizatoin Graphics

3. What is visualization anyway?
Converting data from its original form into graphics
Deal with the issues of transformation and representation
Transformation: data to graphics
Representation: internal data structures
Example: stock price display 

4. Representations
Computational representation: An array of stock prices.
Price[i] = 9085, 9080, 9075, 9060, 9068, ….
Time[i] = 10(am), 10:30, 11, 11:30, 12, …
Graphical Representation: x-y plot
X(time)
Y (price)
Computational Graphics
Transformation
(visualization)

5. A 3D example Quadric function:
f(x,y,z) = a0 x + a1 y + a2 z + a3 xy + a4 yz + a5 xz
+ a6 x + a7 y + a8 z
Visualization process:
Sample the function f(x,y,z) -> a 3D array on
a regular grid: F[x][y][z] = f ; (-1 <= x,y,z <= 1)
Generate a 3D surface corresponding to f(x,y,z) = c
Display the surface Sample the function

6. Visualization Process
f(x,y,z)
sample
Transformation
(visualization)
Computational
representation
Graphical
representation

7. Object-oriented viewpoint
VTK Visualization Model
Representation: Object model
internal data structures
Transformation: Functional model
the steps to create visualization (how data flow through the system)

8. VTK Functional Model Sample F(x,y,z) Extract Surface Display
Point Array
Polygon
Illustrate the steps
to create visualization,
also called
Visualization Pipeline
(visualization networks)
: process
: data
: data flow
direction

9. VTK Object Model Describe which objects operate on the functional
model (visualization pipeline)
Data Object: Represent information and methods to
create, access, and delete the information
example: the 3D point array F(x,y,z)
Process Object: operates on (transform) the data objects
example: generating surfaces from F(x,y,z)

10. Process Objects Source: Interface to external data sources
or generate data procedurally
Filter: Transform the data input
Mapper: The sink of the functional model,
interface with the graphics model

11. Process Objects (cont’d)
Sample F(x,y,z)
Extract Surface
Display
Point Array
Polygon
Source
Filter
Mapper (and Graphics)

12. Data Objects Data Object: dataset (representation of information)
Structures: how the information is organized
- Topology
- Geometry
Attributes: store the information we want
to visualize. e.g. function values

13. Data Objects: structures
Topology:
Invariant under geometric transformation
(rotation, translation, scaling etc)
- Topological structures: Cells
Geometry:
The instantiation of the topology
Geometric structures: cells with
positions in 3D space

14. VTK Cell Types And more (I am too tired drawing now …) (a) vertex
(b) Polyvertex
(c) line
(d) polyline
(e) triangle
(e) Quadrilateral
(e) Polygon
(f) Tetrahedron
(f) Hexahedron
And more (I am too tired drawing now …)

15. VTK Data Attributes The information stored at each corner of the cell
Scalars: temperature, pressure, etc
Vector: velocitys
Normals: surface directions
Texture coordinates: graphics specific
Tensors: matrices

16. Where are we now ... Remember: Data Object: dataset
(representation of information)
Structures: how the information is organized
- Topology
- Geometry
Attributes: store the information we want
to visualize. e.g. function values

17. VTK Datasets A Collection of structures and attributes Structured Grid
Rectilinear Grid
Unstructured Points
Structured Grid
Polygonal Grid
Unstructured Grid

18. Examples
vol.cxx
Ugrid.cxx

19. Pipeline Execution Direction of ‘update’ Source Filter Mapper Actor
Render
Visualization model Graphics model
Source Filter Mapper Actor
Direction of data flow