Handling%20Exceptions

1
Lecture 9

• Handling Exceptions
• 3D graphics
• Help Options
• More on Events, Producers/Consumers.
• DND 3 Demo
• AnimCursor Demo
• QA. Final.

2
Dealing with Exceptions

• Exceptions as a way of life.
• Cant avoid them when doing File I/O, external
  processes, or networking.
• Catch block should restore program to a
  continuablestate.
• In most cases, the user must be notified of the
  failure. Often a retry option should be
  offered.

3

• try
• // Input filename 1
• // Input filename 2
• // exec(movecommand f1 f2)
• // capture output from process
• // refresh directory windows involved.
• catch (FileNotFoundException e)
• String mesg e.getLocalizedMessage()
• if (mesg null)
• mesg DEFAULT_FNF_MESSAGE
• JOptionPane.showMessageDialog(mesg)
• catch (IOException e)
• //...

4
Notifying the User

• Throwable.getLocalizedMessage() gets a
  locale-specific message for the error.
• Exceptions do not always provide good messages.
  If you know more precisely what the problem is,
  tell the user.
• Provide default messages.

5

• try
• // ...
• // exec(movecommand1) backup f2(undo)
• // exec(movecommand2)
• // capture outputs
• // refresh directory windows involved.
• catch (Exception e)
• JOptionPane.showMessageDialog(mesg)
• // What if movecommand1 was done?

6
Dont catch, prevent!

• When possible, catch Exceptions before they
  occur.
• Example In move command, user gives a
  non-existent source filename. This should be
  checked for immediately, and the move command
  should be aborted with a warning message (and
  option to correct filename).

7
3D Graphics, Overview

• Internal representation of a 3D environment.
• Representation of screen location within the
  context of the above.
• Rendering mechanism to display screen view based
  on both the above.
• Methods of manipulating all the above.

For each of these, there are many methods!
8
3D Internal Model

• Primary object is a Surface (2-dimensional
  manifold).
• Surfaces are usually represented as a quilt
  made up of triangular or polygonal pieces.
• Each vertex of these is represented by an
  (x,y,z) triple of real numbers.
• Each piece also has a color attribute.

9
3D Screen as Window

• As with 2D graphics, we think of the screen as
  corresponding to a particular 2-dimensional
  window within the model. For 3D graphics, we
  also must include a point representing the eyes
  of the user. This point and rectangle together
  determine the viewable area which is to be
  displayed on-screen.

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3D Internals
Eye
Screen
Surfaces
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The Visible Cone

• Each point on the screen corresponds to a ray,
  from the eye through the point, extending
  infinitely onward.
• Each rectangle on the screen corresponds to a
  rectangular cone of points, which lie on rays
  from the eye through the points in the rectangle.
• Visible cone cone for entire screen.

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The Visible Cone
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Ideal on-screen image

• Surfaces which intersect the visible cone are
  potentially visible. Principal light rays
  reflected from surfaces within the visible cone
  can pass through the window, reaching the eye.
• Closer surfaces block farther surfaces. For
  each screen point, take the first surface struck
  by its ray.

14
Actual on-screen image

• Cant check every point in screen.
• Find first surface for each integer point in
  screen rectangle.
• Color pixel by averaging the 4 colors of its
  corners.
• Alternative approach often used draw each
  polygon on screen, working back-to-front
  (z-ordering).

15
Colors Paints

• The previous approach is pretty good at
  displaying shapes of surfaces. We assumed the
  color of each polygon was given to us. More
  likely, it is not.
• Prefer to deduce the screen image from a Paint
  applied to the surface. Texture-mapping is the
  art of rendering such an image.

16
Level-coloring

• To emphasize differences in depth between
  polygons, often want the coloration to depend on
  distance from eye. Simplest scheme color
  darkens to black darkness increases with
  distance. Fog effect is reverse fartherwhiter.
• Base color and brightness can be specified for
  each surface.

17
Texture-mapping

• Slightly more complicated than displaying
  TexturePaints, because the polygon which is
  painted may be at an oblique angle to the screen.
• Conversion is affine transformation.

Polygon
Eye
Screen
18
Operations on 3D model

• Change of view
• Can be thought of as moving the eye and screen,
  or moving everything else.
• Can also think of as changing coordinate system.
  The screen rectangle defines an x and a y axis,
  and the eye defines a z-axis.
• Any 2 coordinate systems are related by an affine
  transformation, just as in 2D case. Completely
  described by a 4x4 matrix.

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Operations on 3D model II

• Translating and rotating individual surfaces or
  pieces of surfaces requires applying an affine
  transformation to all the vertices defining those
  surfaces.

20
Proper 3D support

• High-level constructs for creating surfaces and
  specifying Paints on surfaces.
• Trivial view support you specify only where the
  eye and screen are.
• VRML support.
• 3D affine transformations on Surfaces.
• Fast performance.

21
Hardware support

• Many video cards provide support for high-speed
  graphics operations. So do some microprocessors
  (Pentium, AMD).
• Generally, this means special hardware commands
  for defining, transforming, and displaying 3D
  surfaces, often with texture-mapped paints.
• Performance. Standards??.

22
Ray-tracing

• Idea simulate real life. Rays of light emanate
  from various sources, and are reflected,
  absorbed, and scattered to varying extents by
  each surface.
• Proceeds in steps. Initially, only lights are
  lit, and all surfaces are jet black. Each step
  re-evaluates the light from each surface, based
  on previous step.

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Ray-tracing

• Can produce almost photo-realistic images,
  especially when combined with texture-mapping.
• Very computationally intensive (slow).

24
Existing 3D Systems, I

• OpenGL (faq) (home page). Introduced in 1992.
  Not open-source, yes 3rd party, platform
  independent.
• DirectX Microsofts multimedia suite.
• Both of these are medium-level langs, designed
  as intermediaries between programs and
  acceleration hardware.

25
Existing 3D systems, II

• Java3D still in alpha-beta-testing at Sun.
  Follows VRMLs approach.
• VRML multi-participant interactive simulations.
  Invented 1994, latest standard VRML97. VRML 2.0
  soon.
• Nodes may be sounds, images, 3D surfaces, web
  sites, affine transformations.
• Nodes are arranged in scene graphs.

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Advanced Help

• Topic Selector
• Use a JTabbedPane or CardLayout to offer more
  than one topic selection method.
• Table of Contents
• Use a JTree to present hierarchically arranged
  help topics.
• Searchable Index
• Store topics as string, search with
  String.indexOf()

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History Tools
Preferences
Hypertext, Expandable Nodes
Card Layout
ToolTip
Topic Display
Topic Selector
28
Events, Sources, Listeners
Source
Listener(s)
listener added to the source
new event
Event
pass reference to event
process the event
new event
Event
pass reference to event
process the event
29
Events

• For each event a single Event is created, and
  all registered Listeners are given a reference to
  this Event.
• Listeners are not generally aware of the event
  source. After the event happens, they can
  discover the source by interacting with the Event
  itself. (Event.getSource).

30
Producers and Consumers

• Similar to Sources and Listeners, except that
  production is not generally a result of an
  external user action.
• Producer notifies consumers when the produced
  item is ready by invoking a special method,
  rather than using the event mechanism. Consumers
  need not all be treated the same way.