From http://www.jwz.org/xscreensaver/xscreensaver-5.35.tar.gz

[xscreensaver] / jwxyz / README

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+JWXYZ Is Not Xlib.
+
+But it's a bunch of function definitions that bear some resemblance to
+Xlib and that kinda sorta implement Xlib in terms of the native graphics
+substrate (Cocoa, OpenGL, GLES, Java).
+
+When porting XScreenSaver to other platforms, my goal is to keep a single
+code base that compiles for multiple platforms.  That is, I don't want to
+end up with two different files that implement "Attraction" using different
+APIs or different languages,
+
+Since the vast majority of xscreensaver was originally written in C for
+the vintage-1985 X11 API and the vintage-1992 OpenGL API, this presents
+something of a challenge.
+
+  1:  To do the MacOS port, I implemented X11 in terms of Cocoa.
+      That's what jwxyz.m is.
+
+  2:  To do the iOS port, I used that X11/Cocoa layer from #1, but also
+      had to implement OpenGL 1.1 in terms of OpenGLES 1.0.  That's what
+      jwzgles.c is.  I have some things to say about that.  You can
+      read it on my blog: http://jwz.org/b/yhM9
+
+  3:  To do the Android port, we used the OpenGL/OpenGLES layer from #2,
+      but implemented X11 in terms of OpenGL.  That's what jwxyz-gl.c,
+      jwxyz-common.c and jwxyz-android.c are.
+
+Perhaps some day we can re-target MacOS and iOS at the OpenGL port of X11
+instead of the Cocoa port of X11, and replace jwxyz.m with jwxyz-gl.c and
+jwxyz-cocoa.m.  That day has not yet arrived.