Object-oriented Visualization

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Object-oriented Visualization

• João Duarte Cunha
• Univ. de Lisboa - Faculdade de Ciências
• Socrates Programme
• Plzen, May 2001

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Visualization

• Overcoming spatial, temporal and physical
  barriers

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Sensorial limitations

• Our senses are our interface everything we got
  from the outside world comes through them
• Our senses are quite limited
• in the type of stimuli they can perceive
• in their sensitivity

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Sensorial limitations

• Because of those limitations, the world within
  our reach is
• limited in space
• limited in time
• limited in the type of objects and phenomena that
  we can perceive!

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Mans inconformism

• Man always struggled against his sensorial
  limitations!
• Oral tradition in primitive societies was a away
  of overcoming the time barrier (and, combined
  with travelling, also the space barrier)
• Painting and other art forms have a similar role!

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Mans inconformism

• The introduction of writing was an enormous step
  forward.
• And the discovery of the press by Gutemberg in
  the 15th century amplified its impact by orders
  of magnitude.

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Mans inconformism

• And so we got the 19th century!

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Mans inconformism

• All these ways of overcoming the space and time
  barriers are human mediated
• by ONE man in the case of writing and the plastic
  arts
• by MANY men in the case of oral tradition, music,
  dance.

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Mans inconformism

• The senses involved are always vision and hearing!

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Mans inconformism

• Galileos telescope and other optical instruments
  pushed away the space barrier.
• And we know well how
• important this was for
• our understanding of
• the universe!

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Mans inconformism

• With the development of measuring instruments the
  struggle against the physical barrier proceeded
  step by step.
• But slowly!

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Mans inconformism

• In the 19th century a new era begun!
• Telegraphy (1856) introduced the instanta-neous
  transmission of the written word.
• We can say, at Earth scale at least,
• that with telegraphy
• space became independent of time!

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Mans inconformism

• The telephone (1876) and radio or wireless
  telephony (already in the 20th century) are new
  advances allowing for a limited form of direct
  (not human mediated) perception.
• Photography (1839), sound recording (1877) and
  the cinema (1895) are other important landmarks.

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Mans inconformism

• But it was undoubtedly the television (1926) the
  most significant step in mans struggle against
  the space barrier!

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"Sensing" information

• From Visualization to "Perception" !

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SEEING information

• Knowledge is based on information and action is
  determined by knowledge.
• Computers are essential to generate new
  information (simulation and data acquisition).
• But they are, more than anything else, the ideal
  instrument to store, search, retrieve and process
  information.

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SEEING information

• In their book "Readings in Information
  Visualization - Using vision to think", Card,
  Mackinlay e Shneiderman define visualization as
• The use of computer-supported,
• interactive, visual representations
• of data to amplify cognition

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SEEING information

• Look at the following table
• 0,000 0,087 0,174 0,259 0,342 0,423 0,500 0,574
• 0,643 0,707 0,766 0,819 0,866 0,906 0,940 0,966
• 0,985 0,996 1,000 0,996 0,985 0,966 0,940 0,906
• 0,866 0,819 0,766 0,707 0,643 0,574 0,500 0,423
• 0,342 0,259 0,174 0,087 0,000
• and at the graph in the following slide

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SEEING information
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SEEING information

• These two slides just point out what we already
  knew very well our ability to get information is
  highly dependent on its form.
• In general, a graph or image allow a much faster
  understanding than a list of numbers or a textual
  description.

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SEEING information

• Visualization was used long before, but the
  Scientific Visualization and Information
  Visualization areas were born in the late 80s.
• The confluence of push and pull factors was
  determinant for that.

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SEEING information

• Push factors
• 30 years of experience in Computer Graphics
• Availability of powerful graphical hardware
• Developments in human-machine interfaces

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SEEING information

• Pull factors
• Ever greater amounts of data to store and analyze
  coming from
• Sophisticated simulations made possible by the
  available computing power
• Very large and complex data bases

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SEEING information

• Simplified reference model for visualization

Data
Mapping
Rendering
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SEEING information
Mapping
Information is converted into geometry and/or
graphical attributes.
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Scientific Visualization

• Scientific Visualization or Visualization of
  Scientific Data is usually defined as concer-ned
  with visualization of data associated with a
  spatial grid, as opposed to Informa-tion
  Visualization which concentrates on more abstract
  kinds of data.

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The Visualization Toolkit - vtk

• VTK is an object-oriented toolkit for 3D
  graphics(http//www.kitware.com/vtk.html)
• Although tailored for the scientific
  visuali-zation field it can also be sucessfully
  used to illustrate the basics of 3D graphics and
  rendering.

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The Visualization Toolkit - vtk

• The toolkit consists of a set of C classes
  providing the functionality to build 3D graphics
  applications, namely visualization applications,
  without the need for any low level programming.
• Vtk is compatible with several graphical systems,
  namely OpenGL and X-Windows.

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The Visualization Toolkit - vtk

• Vtk may be used from C, Java or Tcl/Tk
  programmes.
• In this talk Tcl/Tk will be used.

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The Visualization Toolkit - vtk

• Vtk being object-oriented, we need only to create
  objects of the appropriate types and call the
  available methods.

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The Visualization Toolkit - vtk

• Vtk object types can be grouped according to
  their functionality.
• The most important object groups are Sources,
  Mappers, Graphics and Filters.
• We will now look in some detail at each group.

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The Visualization Toolkit - vtk

• Sources are information provider objects.
• Some sources are readers, that is, they get the
  information from files in appropriate formats.
  Examples
• vtkPLOT3DReader
• vtkBYUReader
• vtkPolyDataReader (one of vtk own formats)

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The Visualization Toolkit - vtk

• Others generate data algorithmically. The typical
  example are the sources that create basic
  geometric shapes like
• vtkSphereSource
• vtkCylinderSource
• vtkConeSource

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The Visualization Toolkit - vtk

• Mappers, as suggested by the name itself, get the
  information provided by source objects, directly
  or through appropriate filters, and generate
  graphic primitives.
• There are two types of mappers
• vtkDataSetMapper
• vtkPolyDataMapper

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The Visualization Toolkit - vtk

• A special type of mappers are the writers, which
  write out information in files with differente
  formats. Examples
• vtkBYUWriter
• vtkTIFFWriter
• vtkPolyDataWriter

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The Visualization Toolkit - vtk

• Objects in the graphics group are respon-sible
  for the rendering portion of the visua-lization
  pipeline.

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The Visualization Toolkit - vtk

• Objects in this group include
• vtkRenderer
• vtkRenderWindow
• vtkActor
• vtkProperty
• vtkTransform
• vtkCamera
• vtkLight

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The Visualization Toolkit - vtk

• vtkRenderer is a virtual class.
• But vtkOpenGLRenderer, which inherits from
  vtkRenderer, is a concrete class instan-tiated
  when the graphics systems is OpenGL.

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The Visualization Toolkit - vtk

• Objects of type vtkRenderWindow repre-sent the
  application window.
• The window will be a Windows window or an X
  window, according to the graphics system being
  used.

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The Visualization Toolkit - vtk

• Each instance of vtkActor represents a scene
  object, combining the geometry (provided by the
  mapper) with the optical properties (colour,
  texture, etc) and con-crete values for the
  location, orientation and size.

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The Visualization Toolkit - vtk

• Instances of vtkProperty are associated with
  actors to control its appearance.
• Similarly, instances of vtkTransform are
  associated with actors to determine their
  location, orientation and size.

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The Visualization Toolkit - vtk

• Instances of vtkCamera and vtkLight are
  associated with Renderer objects to specify how
  the scene is seen and illuminated.
• If the user does not explicitely create objects
  of these two types, default ones are provided.

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The Visualization Toolkit - vtk

• Filters are transformation objects and there
  are many types of filters available.
• For example, the marching cubes and mar-ching
  squares algorithms are implemented as filters.
  The same is true for decimation, sampling,
  geometry extraction, threshol-ding, particle and
  many other algorithms.

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The visualization pipeline in vtk

• The simplest pipeline includes a source, a mapper
  and an actor.
• To be seen, actors must be associated with a
  renderer and the renderer associated to a window.

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The visualization pipeline in vtk

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Case study

• We will now see and briefly discuss the
  application of vtk to meteorological data.
• The data was provided by the Lisbon
  Meteorological Institute (IM) for use in a course
  of Computer Graphics for physics students.

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Case study

• The data were collected with intervals of six
  hours between April 5 at 12h and April 6 at 12h.
• Physical quantities measured are Tempera-ture,
  Humidity, Altitude and Wind velocity along the
  parallel and along the meridian.

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Case study

• The grid is topologically and geometrically
  regular (cartesian grid), covering a region from
  -45º W to 45º E and from 80º N to 20º N, at 0.5º
  intervals.
• This means a total of 108000 data points for each
  physical quantity.

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Case study

• The data were measured at the so called
  mid-atmosphere, defined as the 500 hPa
  isobaric.
• We will start by having a look at the data using
  WebWinds, a visualization program developped by
  the Jet Propulsion Lab (http//www.jpl.nasa.gov/di
  rectory)

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Case study

• WebWinds is also object-oriented, but the objects
  are provided with a complete inter-face and a
  visual programming approach was adopted.

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Case study

• Now the vtk application.
• First the essential parts of the Tcl code.

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Case study

• Create RenderWindow and Renderer
• vtkRenderer ren1
• vtkRenderWindow renWin
• renWin AddRenderer ren1

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Case study

• Get the Temperature data
• vtkStructuredPointsReader reader
• reader SetFileName "dados_meteo_T_vtk.vtk"
• reader Update

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Case study

• Extract planes for each time slice
• Plane 0
• vtkStructuredPointsGeometryFilter compPlane
• compPlane SetInput reader GetOutput
• compPlane SetExtent 0 200 0 200 0 0
• vtkPolyDataMapper planeMapper
• planeMapper SetInput compPlane GetOutput

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Case study

• planeMapper ScalarVisibilityOn
• eval planeMapper SetScalarRange reader
  GetOutput GetPointData GetScalars GetRange
• vtkActor planeActor
• planeActor SetMapper planeMapper
• planeMapper SetColorModeToLuminance
• planeActor GetProperty SetRepresentationToSurfac
  e

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Case study

• Plane 1
• .....
• Plane 2
• .....
• Plane 3
• .....
• Plane 4
• .....

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Case study

• Cut data using a plane
• vtkPlane plane
• Plane contains the volume center
• eval plane SetOrigin reader GetOutput
  GetCenter
• and is parallel to the latitude and time axis
• plane SetNormal 1 0 0
• vtkCutter planeCut

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Case study

• planeCut SetInput reader GetOutput
• planeCut SetCutFunction plane
• vtkDataSetMapper cutMapper
• cutMapper SetInput planeCut GetOutput
• cutMapper SetColorModeToLuminance
• eval cutMapper SetScalarRange reader
  GetOutput GetPointData GetScalars GetRange
• vtkActor cutActor
• cutActor SetMapper cutMapper

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Case study

• Draw the outline
• vtkOutlineFilter outline
• outline SetInput reader GetOutput
• vtkPolyDataMapper outlineMapper
• outlineMapper SetInput outline GetOutput
• vtkActor outlineActor
• outlineActor SetMapper outlineMapper
• eval outlineActor GetProperty SetColor 0 0 0

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Case study

• Add actors to the renderer
• ren1 AddActor outlineActor
• ren1 AddActor planeActor
• ren1 AddActor planeActor1
• ren1 AddActor planeActor2
• ren1 AddActor planeActor3
• ren1 AddActor planeActor4
• ren1 AddActor cutActor

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Case study

• Adjust camera
• set cam1 ren1 GetActiveCamera
• cam1 SetClippingRange 3.95297 50
• cam1 SetFocalPoint 0 50 3
• cam1 SetPosition 0 28 -200
• cam1 ComputeViewPlaneNormal
• cam1 SetViewUp 0 -0.99 0.06

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Case study

• Get the rendering done!
• renWin Render

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Case study

• Now we are ready to play with the application
  itself

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Case Study

• Thank you for your attention!