WELCOME TO THE FLY THROUGH DEMO


   This demonstrates draping a texture map (image) on
   a surface as well as interactive and automated
   exploration of the resulting visualization. In this
   example, the texture map is a satellite image
   acquired in the vicinity of Los Angeles,
   California. The surface is a digital elevation
   model (DEM or DTM) of the associated
   topography. You can directly manipulate the
   visualization with your mouse, navigate through a
   simulated fly-through session, and record your
   flightpath for later playback. Not limited to the
   Earth Sciences, texture draped surface
   visualizations are useful for exploring many types
   of multidimensional data.  In addition, your own
   data may be displayed instead of the default scene.
   See the example at the end of this file for details.

   IDL's advanced graphics system is built to provide
   maximum performance and flexibility for complex
   visualizations such as this. For enhanced
   interactive performance, this system supports
   OpenGL-based hardware rendering.



   MENU OPTIONS
   ------------
   File Menu:
      Select Quit to exit the Flyby Demo and return 
      to the demo system.

   About Menu:
      Select "About Flythrough" for information 
      about the Flythrough demo (this file).


   FEATURES
   --------

   <<START>> button
      Initiates the flythrough. Movement is simulated
      by translating the surface object towards the eye.
      The user can direct the movement by positioning
      the cursor over the view.  Up/down movement with the
      cursor controls pitch, left/right movement simulates
      banked turns.  Note that the middle mouse button (or
      an emulated middle mouse button, for those with two
      buttons) can be used to start and stop the flythrough.
      See the "Running IDL" section of "Using IDL" for details
      on middle mouse button emulation.

      The translations and rotations which make up each step
      of the interactive flight sequence are stored as
      transformation matrices.  The transformations for each
      flight are appended to form a saved path which can be
      replayed.  See the "Replay Path" and "Clear Path" buttons
      below.

   <<Stop>> button
      Stops the flythrough by stopping the translations of the
      model towards the eye.  The flythrough can be resumed by
      selecting Start or by clicking the middle mouse button
      (or emulated middle button).  This is useful if you want
      to change several of the controls during a flythrough.

   <<Reset>> button
      Resets the orientation to the initial location by setting
      the model transformations to the original transformations.

   <<Replay Path>> button
      Steps through the model transformation of the last flight
      or flights since startup or the last use of the "Clear Path"
      button. Note that this is not an animation of static images.
      The transformations matrices of a flight are stored during
      the flight and used to manipulate the surface object again
      to replay the flight.  If the last path ended with a crash
      the path can be replayed, but the replay will cycle
      continuously rather than stopping at the crash.

   <<Clear Path>> button
      Clears the saved path.  The transformations of the next
      flight will become the beginning of a new path which can
      be replayed. 

   <<Airspeed>> slider
      Regulates the speed by altering the amount of translation
      toward the eye position.

   <<Pitch Sensitivity>> slider
      Regulates the amount of pitch change for the same amount of
      cursor movement in the Y direction.

   <<Roll Sensitivity>> slider
      Regulates the amount of roll change for the same amount of
      cursor movement in the X direction.

   <<Ignore Crashes>> check box
      When checked this setting prevents the flight from stopping
      when the flight passes through the surface.  The default is
      to stop the flight when passing through the surface.

   Joystick display
      This small display is a visual aide which represents a
      top-down view of a joystick which is controlling the flight.
      The position of the joystick is controlled by the position
      of the cursor within the main display.

   <<Surface Controls>> button
      Displays a floating control panel with optional settings for
      the display of the surface.  The controls are described below.

   <<Texture Map>> check box
      If checked the display will show an image draped on the digital
      elevation model.  If not checked the display will show the
      shaded surface and surface paths.

   <<Bilinear Interp.>> check box
      If checked, the texture map will use Bilinear sampling.
      If not checked, the texture map will use nearest neighbor
      sampling.

   <<Grid at Z=0>> check box
      If selected, display a referece grid at Z=0.

   <<Center Box>> check box
      If selected, display a reference box at the center of the screen.
      Note that if the cursor is near this box the flight will continue
      forward.  Moving the cursor away from this box during flight will
      change the pitch or initiate banked turns.

   <<Vertical Exag.>> slider
      Alters the vertical exaggeration of the Z dimension of the surface.

   <<Done>> button
      Closes the Surface Controls control panel.



   Run this demo from the command line to supply your own data (surface
   and image).  For example:

   ; note that surface and image dimensions need not agree
   ; the image is scaled to fit the surface.
   tmpSurface=DIST(50)
   tmpImage=BYTSCL(SIN(DIST(40)))
   d_flythru, SURFACE=tmpSurface, IMAGE=tmpImage