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OpenGL Frequently Asked Questions (FAQ) [3/3]

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Archive-Name: graphics/opengl-faq/part3
Lines: 412
Xref: comp.answers:14597 news.answers:54157

See reader questions & answers on this topic! - Help others by sharing your knowledge
The FAQ has been split into 3 parts.
This is Part 3 of 3.

Subject:  Q22:  Where can I find OpenGL source code examples?  
For instance, where is an example which combines
OpenGL with Motif, using the Motif widget?

You can get the source code examples which are found in the
OpenGL Programming Guide via anonymous, public ftp from

Mark Kilgard has created an ftp site for source code, which is
part of his articles in the X Journal magazine.  This includes
the GLUT toolkit (version 2.0) and OpenGL with Motif examples.
The directory is:

Some contributed source code of useful tools for developing
OpenGL code can be found on
Source code found here includes:
    isfast--compares the performance of OpenGL states 
    samples--more OpenGL program examples
    toogl--helps port IRIS GL code to OpenGL
    xglinfo--display information on X visuals extended for OpenGL
    xscope--examines OpenGL protocol sent to an extended X server 

Subject:  Q23:  How do I contribute OpenGL code examples to a publicly
accessible archive?

A:  To contribute to the public OpenGL archive, send mail to  Your mail should contain:

    The material to be archived, or instructions for obtaining it.

    An announcement suitable for posting to

SGI will place the material in the opengl/contrib directory on and post the announcement to this newsgroup.

To retrieve something from the archive, use anonymous ftp to  Once connected, cd to the directory OpenGL.
(Case is significant.)  Currently there are two subdirectories:

	Manual pages for OpenGL and related libraries.

	Contributions from the public.  

Note that all contributions are distributed as-is; neither SGI nor the
other companies on the OpenGL Architecture Review Board make any
legally valid claims about the robustness or usefulness of this software.

If you do not have access to anonymous ftp, consider using an
"ftp-by-mail" server.  For information on one such server, send mail
to with a message body containing only the word

Subject:  Q24:  What is the GLUT toolkit?  Where do I get it?

GLUT is a portable toolkit which performs window and event 
operations to support OpenGL rendering.

GLUT version 2.0 has:
  o  window functions, including multiple windows for OpenGL rendering
  o  callback driven event processing
  o  sophisticated input devices, including dials and buttons box, 
     tablet, Spaceball(TM)
  o  idle routines and timers
  o  a simple cascading pop-up menu facility
  o  routines to generate wire and solid objects
  o  bitmap and stroke fonts
  o  request and queries for multisample and stereo windows
  o  OpenGL extension query support

The version 2 functionality is fully backward compatible
with the version 1 functionality.

The specification, source code (including FORTRAN bindings), 
and articles for GLUT (Graphics Library Utility Toolkit) is in:

This distribution of GLUT should compile on:
  o  DEC Alpha workstation running OSF/1 with Open3D layered product
  o  IBM RS/6000 workstations running AIX with OpenGL support
  o  SGI workstation running IRIX 5.2 or higher supporting OpenGL
  o  Template Graphics Software's OpenGL for Sun workstations
  o  Mesa 1.1 for Unix workstations.

Subject:  Q25:  What is the relationship between IRIS GL and OpenGL?
Is OpenGL source code or binary code compatible with IRIS GL?

A:  IRIS GL is the predecessor to OpenGL.  After other implementors
had experience trying to port the IRIS GL to their own machines, it
was learned that the IRIS GL was too tied to a specific window system
or hardware.  Based upon consultations with several implementors,
OpenGL is much more platform independent.

IRIS GL is being maintained and bugs will be fixed, but SGI will no
longer add enhancements.  OpenGL is now the strategic interface for
3-D computer graphics.

OpenGL code is neither binary nor source code compatible with IRIS
GL code.  It was decided to bite the bullet at this time to make
OpenGL incompatible with IRIS GL and fix EVERYTHING that made IRIS GL
difficult to port or use.  For example, the gl prefix has been added
to every command:  glVertex(), glColor(), etc.

Subject:  Q26:  Why should I port my IRIS GL application to OpenGL?

SGI will be maintaining the old IRIS GL, but not enhancing it.  
OpenGL is the API of choice on all new SGI machines.

OpenGL has no subsets.  You can use the same functionality
on all machines from SGI or from other vendors.

OpenGL is better integrated with the X Window System than
the old IRIS GL.  For example, you can mix OpenGL and X
or Display PostScript drawing operations in the same window.

The OpenGL naming scheme, argument list conventions, and
rendering semantics are cleaner than those of IRIS GL.  This
should make OpenGL code easier to understand and maintain.

Subject:  Q27: How much work is it to convert an IRIS GL program to OpenGL?
What are the major differences between them?

< from Mason Woo ( and Debbie Herrington ( >

There is a fair amount of work, most of which is in substituting
for window management or input handling routines, for which the 
equivalents are not OpenGL, but the local window system, such as
the X Windows System or Windows NT.  

To help ease the way, port to "mixed model" right away, mixing the
X Window System calls to open and manage windows, cursors, and color
maps and read events of the window system, mouse and keyboard.  
You can do that now with IRIS GL, if you are running IRIX 4.0.  

In the X Window System, display mode choices (such as single or 
double buffering, color index or RGBA mode) must be declared before 
the window is initially opened.  You may also substitute for other
IRIS GL routines, such as using a OSF/Motif menu system, in place of
the IRIS GL pop-up menus.  You should use glXUseXFont(), whenever 
you were using the font manager with IRIS GL.

OpenGL uses standard, predictable naming conventions, which required
that all names have been changed from IRIS GL.  The OpenGL
naming scheme uses unique prefixes, suffixes, and capitalization to
help prevent potential conflicts among application, system, and
library routine names.

And all routine names have changed,
at least, minimally; for example:  ortho() is now glOrtho(). 

Tables for states such as lighting or line and polygon stipples will 
be gone.  Instead of using a def/set or def/bind sequence to load a
table, you turn on the state with glEnable() and also declare the
current values for that state.

Colors are best stored as floating point values, scaled from 0.0 to
1.0 (0% to 100%).  Alpha is fully integrated in the RGBA mode and
at least source alpha will be available on all OpenGL implementations.
OpenGL will not arbitrarily limit the number of bits per color to 8.
Clearing the contents of buffers no longer uses the current color, but
a special "clearing" color for each buffer (color, depth, stencil, and

The transformation matrix has changed.  In OpenGL, there is no
single matrix mode.  Matrices are now column-major and are post-multiplied,
although that does NOT change the calling order of these routines from
IRIS GL to OpenGL.  OpenGL's glRotate*() now allows for a rotation
around an arbitrary axis, not just the x, y, and z axes.  lookat() 
of IRIS GL is now gluLookAt(), which takes an up vector value, not
merely a twist.  There is no polarview() in OpenGL, but a series
of glRotate*()s and glTranslate*()s can do the same thing.

There are no separate depth cueing routines in OpenGL.  Use linear fog.

Feedback and selection (picking) return values, which are different
from those found on any IRIS GL implementation.  For selection and
picking, depth values will be returned for each hit.  In OpenGL, feedback 
and selection will now be standardized on all hardware platforms.

Subject:  Q28:  When using Xlib, how do I create a borderless window?

A:  <from (David Blythe)>

Essentially you can create the window with override-redirect 
(see man xcreatewindow) which is the sledgehammer approach or 
you can change the _MOTIF_WM_HINTS property to tell the window 
manager to leave your windows undecorated.

<from (Alex Madarasz)>

Also of note is that the window manager decorations of any client can be
turned off by putting something like the following in the .Xdefaults
file in your home directory - assuming you aren't overriding them in
your app:

4Dwm*ClientAppOrClassName*clientDecoration: none

( see the 4Dwm / mwm man pages for a full description of this resource )

( you must restart the window manager or logout/login for 4Dwm resources
to take effect )

"none" will remove all of the window manager decorations - border, title
bar etc.

Subject:  Q29:  How do I switch between single buffer and double buffer mode?

A:  <from (Mark Kilgard) 
	and (David Blythe)>

When using OpenGL with X, switching between a double buffered and 
single buffered window can be accomplished by creating a "container" 
X window and creating two subwindows, one with a double buffer visual, 
the other with a single buffer visual.  Make sure the subwindows are each
the full size of their parent window.

You can then use XRaiseWindow or XLowerWindow to change the stacking
order of the two subwindows to switch between double buffering and
single buffering.

You will need to create a separate context for each of the
two windows since they have different visual types.  You will need
to make the appropriate window/context pair current when you switch

IRIS GL made it easy to switch between double buffering and single
buffering.  But essentially, IRIS GL implemented the above process

Subject:  Q30:  On my machine, it appears that glXChooseVisual
is only able to match double-buffered visuals.  I want to have more
bits of color resolution, so how do I render in single buffer mode?

A:  <from (Mark Kilgard)>

On mid- to low-end machines with double buffer hardware, you'll
probably find you get twice as much color resolution using a single
buffer visual.  But if there is no hardware double buffering support,
the double buffered and single buffered visuals are generally the
same depth (the back buffer is "carved" out of software).

Search again for a double buffered visual.  If you find one,
use it instead.  Call glDrawBuffer(GL_FRONT) though to make sure
you are drawing to the front buffer (the default for a double
buffered visual is to draw into the back buffer).

Subject:  Q31:  I've got a 24-bit machine, but my OpenGL windows
are not using the full color resolution.  What's wrong?  My
program looks fine on one machine, but the depth buffer doesn't work
on another.  What's wrong?

A:  <from (Mark Kilgard) and (Mason Woo)> 

An unfortunate (but documented) semantic of glXChooseVisual is
that if you don't request GLX_RED_SIZE, GLX_GREEN_SIZE, or
GLX_BLUE_SIZE, glXChooseVisual assumes zero for these parameters
which means pick the visual with the _smallest_ amount of red,
green, and blue that matches the other visual attributes.  Make
sure you ask for at least 1 bit of GLX_RED_SIZE, GLX_GREEN_SIZE,
and GLX_BLUE_SIZE.  If these configuration parameters are
non-zero, it matches the visual with the _largest_ amount of red,
green, and blue with at least 1 bit of each (probably what you

Similarly, if you don't request GLX_DEPTH_SIZE, you may get a
visual with zero bits of depth buffer.  Some systems may have few
visuals available, and those visuals all have at least 1 bit of
depth buffer.  On other systems, there may be dozens of visuals
available, some with zero bits for the depth buffer.  In short,
if hidden surface removal appears to fail, check to see if you
have explicitly specified any bits of depth buffer you have requested.
Also check to see what visual you have received.

Subject:  Q32:  What information is available about OpenGL extensions?

A:  Examples of extensions include vertex arrays (calling several vertexes
or related data, such as normals or colors, with a single function call),
blending of constant colors, polygon offset (multiple coplanar polygons 
can be rendered without interaction), 

Procedure names and tokens for OpenGL extensions are either suffixed
with EXT or a vendor-specific acronym:  such as SGI for Silicon Graphics
machines, or INGR for Intergraph.  Also note that Silicon Graphics 
extensions to OpenGL are suffixed to indicate whether they will be 
available on all machines (SGI), on just a subset of machines (SGIS), 
or are very experimental and may become unavailable or completely
changed (SGIX).  

Vendors are encouraged to add extension information to their 
documentation.  For Silicon Graphics, extension information is 
summarized on the glIntro man page.  

Subject:  Q33:  How do I make shadows in OpenGL?

A:  There are no individual routines to control shadows nor
an OpenGL state for shadows.  However, code can be written
to render shadows.

<from (Mason Woo)> 

To project a shadow onto a flat plane (such as in the insect
demo), draw the stippled object, flattened using matrix transformations.
The easiest way to flatten an object is to use the scale function.
For example, use glScalef(1., 0., 1.) to create from an infinite light
shining straight down the y axis.  A transformation matrix 
can be used to cast a shadow from an infinite or local light source
in an arbitrary direction.  See the article:

	Thant Tessman, "Casting Shadows on Flat Surfaces,"
	IRIS Universe, Winter, 1989.

<from (Dave Shreiner)>

Check out the fast shadow and projective texture multi-pass
algorithms for producing realistic shadows using texture mapping.
See the SIGGRAPH paper:

	Mark Segal, Carl Korobkin, et al.
	"Fast Shadows and Lighting Effects using Texture Mapping"
	1992 SIGGRAPH Proceedings

Subject:  Q34:  How can I use 16 bit X fonts?

A:  <from James A. ("Jim") Miller>

Here is some code that will load any font into the server and use glXUseXFont
to build the display lists for you (this does work with 16 bit fonts, it has
been tested on IBM, DEC and SGI machines at an OpenGL interop testing).
Once your display lists are created for each character, you can use the same
basic logic to figure out which characters are valid (using first, last,
firstrow and lastrow in the sample code to get :
firstchar = 256 * firstrow + first
lastchar = 256 * lastrow + last ) and use glCallList with those characters 
to print them out.

static int LoadFont(char *fontName)
    Font id;
    int first, last, firstbitmap, i;
    GLuint base;
    Display *display=0;
    int firstrow, lastrow;
    int maxchars;

    tkGetSystem(TK_X_DISPLAY, &display);

    fontInfo = XLoadQueryFont(display, fontName);
    if (fontInfo == NULL) {
        return 0;
    id = fontInfo->fid;
     * First and Last char in a row of chars
    first = (int)fontInfo->min_char_or_byte2;
    last = (int)fontInfo->max_char_or_byte2;
     * First and Last row of chars, important for multibyte charset's
    firstrow = (int)fontInfo->min_byte1;
    lastrow = (int)fontInfo->max_byte1;
     * How many chars in the charset
    maxchars = 256 * lastrow + last;
    base = glGenLists(maxchars+1);
    if (base == 0) {
        return 0;
     * Get offset to first char in the charset
    firstbitmap = 256 * firstrow + first;
     * for each row of chars, call glXUseXFont to build the bitmaps.
    for(i=firstrow; i<=lastrow; i++)
        glXUseXFont(id, firstbitmap, last-first+1, base+firstbitmap);
        firstbitmap += 256;
    return base;

Subject:  Q35:  What's in the new GLU 1.2 tesselator?

A:  <from Mark Kilgard>

Our friends at Digital have the answers:

User Contributions:

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