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4 Writing new XScreenSaver modules
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8 Any program that can be made to render on an X window created by another
9 process can be used as a screen saver. Just get the window ID out of
10 $XSCREENSAVER_WINDOW, draw on that, and you're done.
12 In theory, you can write a screen saver in any language you like. In
13 practice, however, languages other than C or C++ tend not to allow you to
14 draw to windows that they did not create themselves. Unfortunately, this
15 means that if you want to write a screen saver, you must write it in C.
17 Given that you're going to be writing in C, you might as well take
18 advantage of the various utility functions that I have written to make
19 that easier. Writing a new screen saver in C using the frameworks
20 included with xscreensaver simplifies things enormously.
22 Generally, the best way to learn how to do something is to find a similar
23 program, and play around with it until you understand it. Another
24 approach is to not worry about understanding it, but to just hack it out.
25 Either way, your best bet is probably to start with one of the existing
26 xscreensaver demos, included in the "hacks/" directory, rename the file,
27 and edit it until it does what you want.
29 The "Greynetic" and "Deluxe" hacks are probably good ones to start with,
30 since they are so very simple. For OpenGL programs, "DangerBall" is a
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35 Requirements for inclusion with the XScreenSaver collection
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38 If you come up with anything, send it to me! If it's good, I'd love to
39 include it in the xscreensaver distribution. However, there are a few
40 requirements for me to distribute it:
42 - Write in portable ANSI C. No C++. No nonstandard libraries.
44 - Write a .man page describing all command-line options.
46 - Write an .xml file describing the graphical configuration dialog box.
48 - Include a BSD-like copyright/license notice at the top of each source
49 file (preferably, just use the one from "screenhack.h", and include
50 your name and the current year). The GNU GPL is not compatible with
51 the rest of XScreenSaver.
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56 ==========================================================================
58 - Start with #include "screenhack.h"
60 - Define 2 global variables:
62 yoursavername_defaults -- Default values for the resources you use.
63 yoursavername_options -- The command-line options you accept.
67 yoursavername_init -- Return an object holding your global state.
68 yoursavername_draw -- Draw a single frame, quickly.
69 yoursavername_free -- Free everything you've allocated.
70 yoursavername_reshape -- Called when the window is resized.
71 yoursavername_event -- Called when a keyboard or mouse event happens.
73 The "event" function will only be called when running in a window
74 (not as a screen saver). The "reshape" event will be called when the
75 window size changes, or (as a screen saver) when the display size
76 changes as a result of a RANDR event (e.g., plugging in a new monitor).
78 It's ok for both the "event" and "resize" functions to do nothing.
80 - All other functions should be static.
82 - The last line of the file should be
83 XSCREENSAVER_MODULE ("YourSaverName", yoursavername)
85 - Finally, edit the Makefile to add a rule for your program.
86 Just cut-and-paste one of the existing rules.
88 Your "draw" must not run for more than a fraction of a second without
89 returning. This means "don't call usleep()". Everything has to be a
92 You may not store global state in global variables, or in function-local
93 static variables. All of your runtime state must be encapsulted in the
94 "state" object created by your "init" function. If you use global or
95 static variables, your screen saver will not work properly on MacOS.
97 Do not call XSync() or XFlush(). If you think you need to do that, it
98 probably means that you are you are relying on the speed of the graphics
99 card for timing, which probably means that your "draw" function is
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107 Some of the display modes that come with xscreensaver were ported from
108 XLock, and so, for historical reasons, they follow a slightly different
109 programming convention. For newly-written Xlib programs, you'd be
110 better off following the pattern used in hacks like "Deluxe" than in
111 hacks like "Flag". The XLockMore ones are the ones that begin with
112 "#ifdef STANDALONE" and #include "xlockmore.h".
114 But, all OpenGL screen savers have to follow the XLockMore API.
116 The XLockMore API is similar to the XScreenSaver API, in that you define
117 (roughly) the same set of functions, but the naming conventions are
118 slightly different. Instead of "hackname_init", it's "init_hackname",
119 and it should be preceeded with the pseudo-declaration ENTRYPOINT.
121 One annoying mis-feature of the XLockMore API is that it *requires* you
122 to make use of global variables for two things: first, for each of your
123 command line options; and second, for an array that holds your global
124 state objects. These are the only exceptions to the "never use global
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132 - Your screen saver should look reasonable at 20-30 frames per second.
133 That is, do not assume that your "draw" function will be called more
134 than 20 times a second. Even if you return a smaller requested delay
135 than that, you might not get it. Likewise, if your "draw" function
136 takes longer than 1/20th of a second to run, your screen saver may be
137 consuming too much CPU.
139 - Don't make assumptions about the depth of the display, or whether it
140 is colormapped. You must allocate all your colors explicitly: do not
141 assume you can construct an RGB value and use that as a pixel value
142 directly. Use the utility routines provided by "utils/colors.h" to
143 simplify color allocation.
145 - It is better to eliminate flicker by double-buffering to a Pixmap
146 than by erasing and re-drawing objects. Do not use drawing tricks
149 - If you use double-buffering, have a resource to turn it off. (MacOS
150 has double-buffering built in, so you'd be triple-buffering.)
152 - Understand the differences between Pixmaps and XImages, and keep in
153 mind which operations are happening in client memory and which are in
154 server memory, and which cause latency due to server round-trips.
155 Sometimes using the Shared Memory extension can be helpful, but
156 probably not as often as you might think.
158 - On modern machines, OpenGL will always run faster than Xlib. It's
159 also more portable. Consider writing in OpenGL whenever possible.
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166 Though XScreenSaver started its life as an X11 program, it also now runs
167 on MacOS X. If you do your development on an X11 system, and follow the
168 usual XScreenSaver APIs, you shouldn't need to do anything special for
169 it to also work on MacOS.
171 The preprocessor macro HAVE_COCOA will be defined when being compiled in
172 a MacOS (Cocoa/Quartz) environment, and will be undefined when compiling
175 To compile on MacOS, use the XCode project included in the source
176 distribution. You shouldn't need to have X11 installed, and shouldn't
177 need to run "configure" first. MacOS 10.4.0 and XCode 3.1 or newer are
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