Display Lists

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Display Lists Text

• Glenn G. ChappellCHAPPELLG_at_member.ams.org
• U. of Alaska Fairbanks
• CS 381 Lecture Notes
• Monday, September 22, 2003

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ReviewClient-Server Model

• Many operations in a computing environment,
  especially in a networked environment, are
  provided via a client-server model.
• The server sits and waits for requests.
• A client can request the server to provide
  services.
• This model works well in a CG context.
• Client Application program.
• Server Graphics hardware (or low-level
  software).
• The model affects optimization.
• Assumption client-server communication may be
  slow.

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Display ListsMotivation

• Many factors can slow down OpenGL rendering.
• Scenes modeled in real time may require large
  computations.
• Function-call overhead.
• Slow client-server communication.
• Slow rendering hardware.
• In order to mitigate some of these effects
  (marked with ), OpenGL provides display lists.
• In OpenGL-speak, these are often called call
  lists.

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Display ListsWhat Are They?

• A display list (or call list) is a way for an
  OpenGL implementation to store a sequence of
  OpenGL commands for later playback.
• Application programmers need no knowledge of how
  display lists are stored. You simply tell OpenGL
  Store these commands as a display list.
• A display list can be compiled once and executed
  many times.
• Display lists always reduce function-call
  overhead.
• Since display lists may be stored on the graphics
  hardware, they may reduce client-server
  communication delays.

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Display ListsUsage Overview

• We compile a display list in four steps
• Allocate 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.
• Tell OpenGL that we are finished creating the
  list (glEndList).
• We can later execute the commands in the display
  list.
• We use a display list by calling it (glCallList).
• This tells OpenGL to execute all the commands
  stored in the list, just as if we made all those
  calls again, but generally a bit faster.

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Display ListsUsage Details Naming

• An OpenGL display-list name is just a number.
• But that number must be assigned by OpenGL.
• Get a display-list name with glGenLists.
• 1 parameter of list names to generate.
• Return value 1st list name generated, or 0 if
  error.
• Put the name into a global int.
• Check for errors!
• int my_list // global
• // The following goes in function init, maybe?
• my_list glGenLists(1) // Give me 1 list name
• if (my_list 0)
• cerr ltlt "Could not make display-list" ltlt endl
• exit(1) // Error

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Display ListsUsage Details Compilation

• We compile a display list using glNewList. 2
  parameters
• The list name.
• Either GL_COMPILE or GL_COMPILE_AND_EXECUTE.
• The latter is useful when making a display list
  in the display function.
• Then give the OpenGL commands to be stored in the
  list.
• Last, glEndList (no parameters).
• glNewList(my_list, GL_COMPILE)
• // Lots of OpenGL commands here
• // Indentation helps, as with glBegin/glEnd
• glEndList()
• You can do glBeginglEnd inside the above, but do
  not do the above inside glBeginglEnd.

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Display ListsUsage Details Execution

• We execute a display list using glCallList.
• 1 parameter The list name.
• // The following goes in the display function!
• glCallList(my_list)
• There is also glCallLists, for executing multiple
  display lists.

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EXAMPLE 1

• Modify intro2d.cpp to use a display list.
• Modified the program so that the color was set
  and the square was drawn using a display list.
• The display list was created in function init.
• The display function was reduced to 3 lines
• Clear the viewport.
• Execute the display list.
• Flush.

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TextIntroduction

• In CG, text comes in two varieties
• Raster/bitmap fonts.
• Text described as an image.
• Outline/stroke fonts.
• Text described as a sequence of curves.
• Both are available, in a limited form, in GLUT.
• We will discuss bitmap text here.

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TextRaster Position Bitmaps 1/2

• When we make bitmap text, we are telling OpenGL
  to draw a raster image.
• A raster image is drawn with the left-hand point
  on its baseline at the current raster position.
• The OpenGL raster position is specified with
  glRasterPos.
• Coordinates passed via glRasterPos pass through
  the same transformations as those passed via
  glVertex.
• However, glRasterPos is not called between
  glBegin glEnd.

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TextRaster Position Bitmaps 2/2

• Bitmap images are raster images with 1 bit per
  pixel.
• They are intended specifically for rendering
  bitmap text.
• OpenGL draws bitmap images with the primitive
  glBitmap.
• This command includes an amount to move the
  raster position by.
• We will use GLUT commands, not glBitmap, but it
  is important to remember that glBitmap is what
  GLUT calls.

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TextUsing GLUT Bitmap Fonts

• We draw a character at the current raster
  position using glutBitmapCharacter. 2 parameters
• The GLUT font to use.
• I use GLUT_BITMAP_9_BY_15. See docs for others.
• The character to render.
• The raster position is advanced by an appropriate
  amount.

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EXAMPLE 2

• See the program bmptext.cpp, which draws bitmap
  text using GLUT.
• Source code is on the web page.