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+.. _stereogram:
+
+.. index::
+ pair: menu item; Random dot stereogram
+ pair: command line option; stereogram
+ pair: filter; Random dot stereogram
+
+Random dot stereogram
+=====================
+
+Fractal images are good as a base for random dot stereograms. In case you don't
+know what these are, please point your browser to Google or another search
+engine and find some articles about such images, because learning to read such
+images takes some effort. They make it possible to generate three dimensional
+images on a normal monitor without any additional hardware, by exploiting bugs
+in the human brain (although you need two working eyes, and some people never
+learn to see them; they can simply ignore this feature).
+
+XaoS is able to generate these images in animations, so you may use all normal
+XaoS functions (except palette changing and palette rotation, which makes no
+sense applied to a stereogram). To make the animation yet more exciting, XaoS
+emulates "falling" into the set; while you zoom in, your distance from the set
+drops and drops--but you never hit it; when the set reaches the level of your
+monitor, the distance is changed again so you are far away.
+
+To make this work right, XaoS needs to know the **exact size of your monitor**.
+Because most platforms have no way to determine this, you need to use **command
+line options** to tune it. If it's not set or is wrong, the stereograms will
+probably be impossible to see (if your monitor is too big or resolution too
+low), or the images will seem to be shallow (if your monitor is too small or
+resolution too high).
+
+By default XaoS expects my 15" monitor (29.0cm x 21.5 cm). Another cause of
+problems is the virtual screen supported by some windowed environments (like
+some X servers) that makes a program think that the resolution is higher than
+it actually is, and you see only part of this extra-large screen.
+
+The worst thing you could possibly do is to run full-screen XaoS in some
+graphical windowing system (OS/2 on top of Windows or Wine on top of Linux,
+perhaps) where XaoS can't tell the real size of its window at all. In such
+cases, it's normally better (not to mention faster) to run XaoS natively,
+rather than under such an emulation layer.
+
+The following command line options are provided to specify sizes:
+
+-screenwidth, -screenheight
+
+ Lets you specify the size of your screen in centimeters. Note that you need
+ to specify the size of the visible image on the monitor, not the size with
+ edge borders, or the size of the tube. The simplistic 'my monitor is 17",
+ just turn 17" into centimeters' doesn't work; that 17" is a marketing
+ figure and has only a vague connection to reality. Get out a ruler and
+ measure it.
+
+-pixelwidth, -pixelheight
+
+ Lets you specify the exact size of a single pixel, if XaoS cannot determine
+ this for itself from your screen size.
+
+These options are used by some other parts of XaoS as well, so you should use
+them even when you don't want to see stereograms. You should probably write a
+small starting script (or alias, or shortcut; whatever your environment uses)
+that passes the correct parameters to XaoS.
+
+If the window is **smaller than 8cm in any direction**, you will probably be
+unable to see anything; make the window bigger. The correct way to see XaoS
+stereograms is:
+
+1
+
+ Start XaoS with options specifying the exact size of your screen or one
+ pixel on it
+
+2
+
+ Sit 60cm away from monitor
+
+3
+
+ If you use a windowed environment, resize XaoS's window to make it wider
+ than, say, 15 cm.
+
+4
+
+ Enable the filter (by pressing E)
+
+5
+
+ focus on a point far away from the monitor (try to use your own reflection,
+ if your monitor's not antireflective); the random blurring should
+ eventually fall into the pattern of a Mandelbrot set.
+
+6
+
+ Carefully use your mouse to zoom into interesting areas (it is easy to lose
+ concentration when you are not trained; but you can use the autopilot...)
+
+7
+
+ Enjoy animation :)
+
+If you still can't see the stereograms, it could be that the fractal, or your
+eye, is deformed. A deformed fractal can be caused by your specifying your
+monitor size wrongly. Visual problems that damage depth perception, as well as
+problems like astigmatism, can make it impossible to see stereograms at all.
+
+:ref:`More information about filters <filter>`
+
+**Available as**: menu item, command line option