Provided by: xscreensaver-gl_6.08+dfsg1-1ubuntu3_amd64 
NAME
etruscanvenus - Draws a 3d immersion of a Klein bottle that smoothly deforms between the Etruscan Venus
surface, the Roman surface, the Boy surface surface, and the Ida surface.
SYNOPSIS
etruscanvenus [--display host:display.screen] [--install] [--visual visual] [--window] [--root]
[--window-id number] [--delay usecs] [--fps] [--mode display-mode] [--wireframe] [--surface]
[--transparent] [--appearance appearance] [--solid] [--distance-bands] [--direction-bands] [--colors
color-scheme] [--onesided-colors] [--twosided-colors] [--distance-colors] [--direction-colors] [--no-
change-colors] [--view-mode view-mode] [--walk] [--turn] [--no-deform] [--deformation-speed float]
[--initial-deformation float] [--etruscan-venus] [--roman] [--boy] [--ida] [--orientation-marks]
[--projection mode] [--perspective] [--orthographic] [--speed-x float] [--speed-y float] [--speed-z
float] [--walk-direction float] [--walk-speed float]
DESCRIPTION
The etruscanvenus program shows a 3d immersion of a Klein bottle that smoothly deforms between the
Etruscan Venus surface, the Roman surface, the Boy surface, and the Ida surface. You can walk on the
Klein bottle or turn it in 3d. Topologically, all surfaces are Klein bottles, even the Roman and Boy
surfaces, which are doubly covered and therefore appear to be an immersed real projective plane. The
smooth deformation between these surfaces was constructed by George K. Francis.
The Klein bottle is a non-orientable surface. To make this apparent, the two-sided color mode can be
used. Alternatively, orientation markers (curling arrows) can be drawn as a texture map on the surface
of the Klein bottle. While walking on the Klein bottle, you will notice that the orientation of the
curling arrows changes (which it must because the Klein bottle is non-orientable). Since all the
surfaces except the Ida surface have points where the surface normal is not well defined for some points,
walking is only performed on the Ida surface.
As mentioned above, the Roman and Boy surfaces are doubly covered and therefore appear to be an immersed
real projective plane. Since some of the parameter names are based on this interpretation of the
surface, the geometry of the real projective plane will be briefly disussed. The real projective plane
is a model for the projective geometry in 2d space. One point can be singled out as the origin. A line
can be singled out as the line at infinity, i.e., a line that lies at an infinite distance to the origin.
The line at infinity is topologically a circle. Points on the line at infinity are also used to model
directions in projective geometry. Direction and distance bands refer to this interpretation of the
surface. If direction bands are used, the bands extend from the origin of the projective plane in
different directions to the line at infinity and back to the origin. If distance bands are used, the
bands lie at constant distances to the origin. The same interpretation is used for distance and
direction colors. Although there is no conceptually equivalent geometric interpretation for the two
Klein bottle surfaces (the Etruscan Venus and Ida surfaces), the smooth deformation between the surfaces
results in a natural extension of these concepts to the Klein bottle surfaces.
The immersed surfaces can be projected to the screen either perspectively or orthographically. When
using the walking mode, perspective projection to the screen will be used.
There are three display modes for the Klein bottle: mesh (wireframe), solid, or transparent.
Furthermore, the appearance of the surface can be as a solid object or as a set of see-through bands.
The bands can be distance bands or direction bands, as explained above.
The colors with with the surface is drawn can be set to one-sided, two-sided, distance, or direction. In
one-sided mode, the surface is drawn with the same color on both sides of the underlying triangles. In
two-sided mode, the surface is drawn with one color on one side of the underlying triangles and the
complementary color on the other side. Since the surface actually only has one side, the color jumps
from red to green along a line on the surface. This mode enables you to see that the surface is non-
orientable. In distance mode, the surface is displayed with fully saturated colors that depend on the
distance of the points on the projective plane to the origin, as described above. If the surface is
displayed as distance bands, each band will be displayed with a different color. In direction mode, the
surface is displayed with fully saturated colors that depend on the angle of the points on the projective
plane with respect to the origin (see above for an explanation). If the surface is displayed as
direction bands, each band will be displayed with a different color. The colors used to color the
surface can either be static or can be changed dynamically.
The rotation speed for each of the three coordinate axes around which the Klein bottle rotates can be
chosen.
Furthermore, in the walking mode the walking direction in the 2d base square of the surface and the
walking speed can be chosen. The walking direction is measured as an angle in degrees in the 2d square
that forms the coordinate system of the surface. A value of 0 or 180 means that the walk is along a
circle at a randomly chosen distance from the origin (parallel to a distance band). A value of 90 or 270
means that the walk is directly along a direction band. Any other value results in a curved path along
the surface. As noted above, walking is performed only on the Ida surface.
By default, the immersion of the Klein bottle smoothly deforms between the Etruscan Venus surface, the
Roman surface, the Boy surface, and the Ida surface. It is possible to choose the speed of the
deformation. Furthermore, it is possible to switch the deformation off. It is also possible to
determine the initial deformation of the immersion. This is mostly useful if the deformation is switched
off, in which case it will determine the appearance of the surface. A value of 0 corresponds to the
Etruscan Venus surface, a value of 1000 to the Roman surface, a value of 2000 to the Boy surface, and a
value of 3000 to the Ida surface.
This program is inspired by George K. Francis's book "A Topological Picturebook", Springer, 1987, by
George K. Francis's paper "The Etruscan Venus" in P. Concus, R. Finn, and D. A. Hoffman: "Geometric
Analysis and Computer Graphics", Springer, 1991, and by a video entitled "The Etruscan Venus" by Donna J.
Cox, George K. Francis, and Raymond L. Idaszak, presented at SIGGRAPH 1989.
OPTIONS
etruscanvenus accepts the following options:
--window
Draw on a newly-created window. This is the default.
--root Draw on the root window.
--window-id number
Draw on the specified window.
--install
Install a private colormap for the window.
--visual visual
Specify which visual to use. Legal values are the name of a visual class, or the id number
(decimal or hex) of a specific visual.
--delay microseconds
How much of a delay should be introduced between steps of the animation. Default 10000, or
1/100th second.
--fps Display the current frame rate, CPU load, and polygon count.
The following four options are mutually exclusive. They determine how the Klein bottle is displayed.
--mode random
Display the Klein bottle in a random display mode (default).
--mode wireframe (Shortcut: --wireframe)
Display the Klein bottle as a wireframe mesh.
--mode surface (Shortcut: --surface)
Display the Klein bottle as a solid surface.
--mode transparent (Shortcut: --transparent)
Display the Klein bottle as a transparent surface.
The following four options are mutually exclusive. They determine the appearance of the Klein bottle.
--appearance random
Display the Klein bottle with a random appearance (default).
--appearance solid (Shortcut: --solid)
Display the Klein bottle as a solid object.
--appearance distance-bands (Shortcut: --distance-bands)
Display the Klein bottle as see-through bands that lie at increasing distances from the origin
(see above for an explanation).
--appearance direction-bands (Shortcut: --direction-bands)
Display the Klein bottle as see-through bands that lie at increasing angles with respect to the
origin (see above for an explanation).
The following five options are mutually exclusive. They determine how to color the Klein bottle.
--colors random
Display the Klein bottle with a random color scheme (default).
--colors onesided (Shortcut: --onesided-colors)
Display the Klein bottle with a single color.
--colors twosided (Shortcut: --twosided-colors)
Display the Klein bottle with two colors: one color on one "side" and the complementary color on
the "other side."
--colors distance (Shortcut: --distance-colors)
Display the Klein bottle with fully saturated colors that depend on the distance of the points on
the projective plane to the origin (see above for an explanation). If the Klein bottle is
displayed as distance bands, each band will be displayed with a different color.
--colors direction (Shortcut: --direction-colors)
Display the Klein bottle with fully saturated colors that depend on the angle of the points on
the projective plane with respect to the origin (see above for an explanation). If the Klein
bottle is displayed as direction bands, each band will be displayed with a different color.
The following options determine whether the colors with which the Klein bottle are displayed are static
or are changing dynamically.
--change-colors
Change the colors with which the Klein bottle is displayed dynamically (default).
--no-change-colors
Use static colors to display the Klein bottle.
The following three options are mutually exclusive. They determine how to view the Klein bottle.
--view-mode random
View the Klein bottle in a random view mode (default). The walking mode will be randomly
selected in approximately 10% of the cases.
--view-mode turn (Shortcut: --turn)
View the Klein bottle while it turns in 3d.
--view-mode walk (Shortcut: --walk)
View the Klein bottle as if walking on its surface.
The following options determine whether the surface is being deformed.
--deform
Deform the surface smoothly between the Etruscan Venus surface, the Roman surface, the Boy
surface surface, and the Ida surface (default).
--no-deform
Don't deform the surface.
The following option determines the deformation speed.
--deformation-speed float
The deformation speed is measured in percent of some sensible maximum speed (default: 10.0).
The following options determine the initial deformation of the surface. As described above, this is
mostly useful if --no-deform is specified.
--initial-deformation float
The initial deformation is specified as a number between 0 and 4000. A value of 0 corresponds to
the Etruscan Venus surface, a value of 1000 to the Roman surface, a value of 2000 to the Boy
surface, and a value of 3000 to the Ida surface. The default value is 0.
--etruscan-venus
This is a shortcut for --initial-deformation 0.
--roman This is a shortcut for --initial-deformation 1000.
--boy This is a shortcut for --initial-deformation 2000.
--ida This is a shortcut for --initial-deformation 3000.
The following options determine whether orientation marks are shown on the Klein bottle.
--orientation-marks
Display orientation marks on the Klein bottle.
--no-orientation-marks
Don't display orientation marks on the Klein bottle (default).
The following three options are mutually exclusive. They determine how the Klain bottle is projected
from 3d to 2d (i.e., to the screen).
--projection random
Project the Klein bottle from 3d to 2d using a random projection mode (default).
--projection perspective (Shortcut: --perspective)
Project the Klein bottle from 3d to 2d using a perspective projection.
--projection orthographic (Shortcut: --orthographic)
Project the Klein bottle from 3d to 2d using an orthographic projection.
The following three options determine the rotation speed of the Klein bottle around the three possible
axes. The rotation speed is measured in degrees per frame. The speeds should be set to relatively small
values, e.g., less than 4 in magnitude. In walk mode, all speeds are ignored.
--speed-x float
Rotation speed around the x axis (default: 1.1).
--speed-y float
Rotation speed around the y axis (default: 1.3).
--speed-z float
Rotation speed around the z axis (default: 1.5).
The following two options determine the walking speed and direction.
--walk-direction float
The walking direction is measured as an angle in degrees in the 2d square that forms the
coordinate system of the surface of the Klein bottle (default: 83.0). A value of 0 or 180 means
that the walk is along a circle at a randomly chosen distance from the origin (parallel to a
distance band). A value of 90 or 270 means that the walk is directly along a direction band.
Any other value results in a curved path along the surface. As noted above, walking is performed
only on the Ida surface.
--walk-speed float
The walking speed is measured in percent of some sensible maximum speed (default: 20.0).
INTERACTION
If you run this program in standalone mode in its turn mode, you can rotate the Klein bottle by dragging
the mouse while pressing the left mouse button. This rotates the Klein bottle in 3d. To examine the
Klein bottle at your leisure, it is best to set all speeds to 0. Otherwise, the Klein bottle will rotate
while the left mouse button is not pressed. This kind of interaction is not available in the walk mode.
ENVIRONMENT
DISPLAY to get the default host and display number.
XENVIRONMENT
to get the name of a resource file that overrides the global resources stored in the
RESOURCE_MANAGER property.
XSCREENSAVER_WINDOW
The window ID to use with --root.
SEE ALSO
X(1), xscreensaver(1)
COPYRIGHT
Copyright © 2019-2020 by Carsten Steger. Permission to use, copy, modify, distribute, and sell this
software and its documentation for any purpose is hereby granted without fee, provided that the above
copyright notice appear in all copies and that both that copyright notice and this permission notice
appear in supporting documentation. No representations are made about the suitability of this software
for any purpose. It is provided "as is" without express or implied warranty.
AUTHOR
Carsten Steger <carsten@mirsanmir.org>, 05-jan-2020.