Notes on developing
a Unix Screensaver for BOINC

The OpenGL library contains basic rendering functions, but a collection of commonly used but more complicated functions is provided by an additional library, called the GL Utilities (GLU). The GLU library is the same for all platforms. All of the GLU functions are written in terms of OpenGL primatives.

OpenGL handles graphics rendering, but it does not deal with system-dependent aspects of dealing with the windowing system, such as creating, resizing or destroying a window, getting input from the keyboard or mouse, or writing text to the screen (ie., fonts). These tasks are handled by different libraries written for the different platforms: For Windows there is wgl, for the Mac there is agl, and for X11 there is glX.

A convenient way to write OpenGL programs which are portable across all platforms is to use a library written by Mark J. Kilgard called the OpenGL Utility Toolkit, GLUT. GLUT presents a common API for the programmer but takes care of the system-dependent tasks of opening or managing windows, keyboard and mouse input, and text. GLUT on Windows is implemented using wgl, while on a Mac it is implemented using agl, and on X11 it it implemented with glX. GLUT also takes care of other common graphics tasks, such as creating the event loop and handling windowing events. GLUT makes it much easier to write OpenGL programs quickly while focusing on the graphics, not the underlying structure to support graphics with event processing, and your programs are more readily portable.

BOINC presently uses GLUT for graphics on Linux and the Mac. BOINC presently does not use GLUT on Windows, except for some reference to GLUT fonts in the file gutil.C.

Although GLUT provides many advantages, there are also some drawbacks. One is that the way keyboard input is handled in GLUT is slightly different from how it is handled (without GLUT) on Windows, and is somewhat restrictive. On Windows a keyboard event routine is called whenever an individual key is pressed (or released), and the keycode for that key is passed as an argument. With GLUT, the key-pressed routine is only invoked when certain keys are pressed, and in this case the character code is passed as an argument. The difference is subtle but important. For example, on Windows if you press and hold the control key and then press the C key, the key-pressed routine is called first for the control key, then for the C key. Using GLUT on Linux, the key-pressed routine is only called once, sending the character code for ctrl-C, only after the C key has been pressed.

The BOINC code for Windows now has a feature which allows one to hold down the control key and then manipulate the screensaver image with the mouse or keyboard, without terminating the screensaver. (Actually, this is only possible right now if you have selected "Run Always", but it may be possible to extend this feature even to when "Run according to user preferences" is selected. This will likely require a change to the BOINC core client, not just the graphics routines used by the application.) In Linux, since GLUT is used for keyboard input, it is not possible to sense when just the control key has been pressed, so this screensaver manipulations feature is not possible.

GLUT also has problems with creating a full-screen borderless window on some X11 desktops. You get a window the size of the screen, but it has a border and it does not exactly cover the screen, but instead exends off the screen onto the next desktop segment (this is observed with fvwm2).

It turns out that in X11 there are some conventions used by screensaver programs like xscreeensver and desktop environments and window managers such as Gnome, KDE, and fvwm, and GLUT does not pay any attention to these conventions.

Thus it may be desirable to abandon GLUT for Unix screensaver graphics for BOINC, and instead replace the relevant code with the appropriate glX code, or with code based on SDL (the Simple Directmedia Layer). Potential options are discussed further below.

The graphical hacks are also compiled with a special header file, vroot.h, written by Andreas Stolcke, which redefines the X11 macros RootWindow and DefaultRootWindow to make them look for a virtual root window. If none is found, the ordinary root window is returned which retains backward compatibility with standard window managers.

The whole idea of this "vroot hack" is to leverage backward compatibility with existing graphics programs and window managers. Many graphics programs already drew to the root window if the -root flag was given on the command line, and several window managers put screensaver graphics in a virtual root window. The "vroot.h" hack just brought the two ideas together.

More modern window managers still "play nice" with virtual root windows. Thus xscreensaver works with the Gnome and KDE desktop environments, and the twm, fvwm1 and fvwm2 window managers. Some of these may also have other mechanisms for indicating that a window is to be considered a screensaver window. In any case, a Unix screensaver for BOINC should therefore utilize the same mechanism(s).

If we go this way then the easiest way to do it would be to also emulate the glX code which already exists in xscreensaver. In particular, the event loop and window initialization are in the file screenhack.C, with the GL code enabled when the macro USE_GL is defined. Some additional functions required just for GL are in the file xlock-gl.C, and some important headers are in xlockmore.h and xlockmoreI.h, though not everything in those files is actually need.