Introduction to the Mouse

1
Introduction to the Mouse

• Glenn G. ChappellCHAPPELLG_at_member.ams.org
• U. of Alaska Fairbanks
• CS 381 Lecture Notes
• Wednesday, September 24, 2003

2
ReviewDisplay Lists

• A display list (or call list) is a way for OpenGL
  to save commands for later playback.
• Display lists are compiled and then executed.
• Display list storage format is
  implementation-dependent and handled internally
  by OpenGL.
• We compile a display list in four steps
• Generate a name (actually an integer) for the
  list. (glGenLists).
• Tell OpenGL that we are creating a display list
  (glNewList).
• Make the OpenGL function calls that we want to
  store in the display list.
• Only OpenGL commands are stored in the list!
• Tell OpenGL that we are finished creating the
  list (glEndList).
• We execute a display list with a single function
  call.
• We pass its name to glCallList.

3
ReviewText 1/2

• In CG, text comes in two varieties
• Bitmap
• Outline/stroke
• Both are available, in a limited form, in GLUT.

4
ReviewText 2/2

• GLUT bitmap text is made using the OpenGL bitmap
  primitive glBitmap.
• Bitmaps (and other raster images) are drawn at
  the raster position.
• The left-hand point of the baseline is drawn at
  this position.
• The baseline is often the bottom of the image.
• We can move it up, to allow for descenders in
  text.
• Set the raster position using glRasterPos.
• Parameters of glRasterPos are handled like those
  of glVertex.
• We draw a character at the current raster
  position using glutBitmapCharacter.
• The raster position is advanced.

5
The MouseOverview

• We now begin a discussion of event-driven
  programming using the mouse (or other 2-D
  pointing device). We will cover
• Basic mouse handling via mouse motion
  callbacks.
• Today
• Mouse-based pop-up menus.
• Friday
• Picking.
• Starting on Monday.
• But first
• Mouse-handling is heavily dependent on pixel
  coordinates, so we need to know how to deal with
  these.
• Therefore, we discuss GLUTs reshape callback.

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Reshape CallbackWhat It Does

• The GLUT reshape callback function is called
• When the window is first created, before any
  other callback.
• Later, whenever the window size/shape changes.
• Not when the window is merely moved.
• GLUT has a built-in reshape function.
• But it might not do what you want.
• Registering a callback replaces the built-in
  function with your own.
• Reshape has 2 parameters
• New width of window, in pixels.
• New height of window, in pixels.
• If you need information on the window size
  somewhere else, save it in the reshape function.
• Maybe you can get it by other means, but why
  bother?

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Reshape CallbackSetting the Viewport

• The first thing you usually do in a reshape
  function is
• void reshape(int w, int h)
• glViewport(0, 0, w, h)
• Function glViewport sets the viewport.
  Parameters
• Left side (pixels from left side of window).
• Bottom (pixels from bottom of window).
• Width (in pixels).
• Height (in pixels).
• So the above call sets the viewport to the entire
  windows.

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Reshape CallbackSetting the Projection

• The next thing you do in a reshape function is
  usually to set the projection.
• For now, gluOrtho2D is generally what you want to
  use.
• Later well want perspective.
• Recall You can think of this as setting up a
  coordinate system in the viewport.
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• gluOrtho2D(left, right, bottom, top)
• glMatrixMode(GL_MODELVIEW)

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Reshape CallbackWorking in Pixels

• Suppose you want the coordinate system to be in
  pixels?
• This might be convenient when dealing with the
  mouse.
• void reshape(int w, int h)
• glViewport(0, 0, w, h)
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• gluOrtho2D(0, w, 0, h)
• glMatrixMode(GL_MODELVIEW)

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Mouse-Related CallbacksMouse Motion Functions

• GLUT has two main mouse-handling callbacks.
• The mouse function.
• This is called when any of the mouse buttons
  (GLUT assumes 3) is pressed or released.
• unless the mouse event is intended for some
  other program.
• The motion function.
• This is called (possibly repeatedly) whenever the
  mouse moves while any of the buttons is pressed.
• An exception
• GLUT includes mouse-controlled pop-up menus.
• A menu can be attached to a mouse button, so
  that the menu pops up when the button is pressed.
• If this is done, then the above callbacks are not
  called when this button is down.
• We will discuss menus on Friday.
• Now the details

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Mouse-Related CallbacksMouse Function

• The mouse function is called when a mouse button
  is pressed or released.
• Registered withglutMouseFunc(mouse) // can use
  different name
• Declared asvoid mouse(int button, int state, int
  x, int y)
• button will be one of
• GLUT_LEFT_BUTTON
• GLUT_MIDDLE_BUTTON (GLUT started under
  Unix/X-Windows)
• GLUT_RIGHT_BUTTON
• state will be one of
• GLUT_DOWN (button press)
• GLUT_UP (button release)
• x and y are the mouse position, in pixels.
• x is from left side of window, y is from top (not
  bottom).

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Mouse-Related Callbacks Motion Function

• The motion function is called if the mouse is
  moved while a mouse button is down.
• If the mouse moves again, motion function is
  called again.
• Motion functions are useful for dragging and
  drawing.
• Registered withglutMotionFunc(motion) // can
  use different name
• Declared asvoid motion(int x, int y)
• x and y are the mouse position, in pixels.
• x is from left side of window, y is from top (not
  bottom).
• Guaranteed sequence of events when dragging
• First the mouse function is called (button down).
• Then the motion function may be called (zero or
  more times).
• Lastly, the mouse function is called (button up).

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Mouse-Related CallbacksConverting Coordinates
1/2

• GLUT gives mouse coordinates in pixels from the
  upper-left corner of the window.
• OpenGL deals with coordinates set by the
  projection.
• May not be in pixels.
• Based at the lower-left corner of the viewport.

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Mouse-Related CallbacksConverting Coordinates
2/2

• EXAMPLES
• Suppose we didgluOrtho2D(0.0, 1.0, 0.0,
  1.0)How do we convert between GLUT mouse
  coordinates and OpenGL coordinates?
• Given mouse _at_ x, y window size w, h.
• In OpenGL coordinates x/w, 1-y/h.
• Note The above will not work right in a C
  program, since it will do integer division.
  Usedouble(x)/w, 1-double(y)/h.
• Same question for the coordinates in pixels, as
  in the last reshape slide.
• In OpenGL coordinates x, h-y.

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Mouse-Related CallbacksOther Callbacks

• The keyboard and special functions are given the
  mouse position.
• Use it if you want.
• You might treat the keyboard as a large
  collection of mouse buttons.
• Unconventional
• Also, you dont get mouse-up events.
• There is a passive motion callback.
• It is called when the mouse moves, and no button
  is down.
• Otherwise, it works just like the motion
  function, except, of course, for the guaranteed
  sequence of events.

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Mouse-Related CallbacksExample

• Write a program the uses the mouse and motion
  callbacks.
• We will do this on Friday.
• For now, see simplemouse.cpp, on the web page.