Updated: 5.40 (12-Aug-2018)

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The real projective plane 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 projective plane. While walking on the projective plane, you will notice that the orientation of the curling arrows changes (which it must because the projective plane is non-orientable).

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. The origin can be visualized in different manners. When using distance colors, the origin is the point that is displayed as fully saturated red, which is easier to see as the center of the reddish area on the projective plane. Alternatively, when using distance bands, the origin is the center of the only band that projects to a disk. When using direction bands, the origin is the point where all direction bands collapse to a point. Finally, when orientation markers are being displayed, the origin the the point where all orientation markers are compressed to a point. The line at infinity can also be visualized in different ways. When using distance colors, the line at infinity is the line that is displayed as fully saturated magenta. When two-sided colors are used, the line at infinity lies at the points where the red and green "sides" of the projective plane meet (of course, the real projective plane only has one side, so this is a design choice of the visualization). Alternatively, when orientation markers are being displayed, the line at infinity is the place where the orientation markers change their orientation.

Note that when the projective plane is displayed with bands, the orientation markers are placed in the middle of the bands. For distance bands, the bands are chosen in such a way that the band at the origin is only half as wide as the remaining bands, which results in a disk being displayed at the origin that has the same diameter as the remaining bands. This choice, however, also implies that the band at infinity is half as wide as the other bands. Since the projective plane is attached to itself (in a complicated fashion) at the line at infinity, effectively the band at infinity is again as wide as the remaining bands. However, since the orientation markers are displayed in the middle of the bands, this means that only one half of the orientation markers will be displayed twice at the line at infinity if distance bands are used. If direction bands are used or if the projective plane is displayed as a solid surface, the orientation markers are displayed fully at the respective sides of the line at infinity.

The immersed projective plane can be projected to the screen either perspectively or orthographically. When using the walking modes, perspective projection to the screen will be used.

There are three display modes for the projective plane: mesh (wireframe), solid, or transparent. Furthermore, the appearance of the projective plane can be as a solid object or as a set of see-through bands. The bands can be distance bands, i.e., bands that lie at increasing distances from the origin, or direction bands, i.e., bands that lie at increasing angles with respect to the origin.

When the projective plane is displayed with direction bands, you will be able to see that each direction band (modulo the "pinching" at the origin) is a Moebius strip, which also shows that the projective plane is non-orientable.

Finally, the colors with with the projective plane is drawn can be set to two-sided, distance, or direction. In two-sided mode, the projective plane is drawn with red on one "side" and green on the "other side". As described above, the projective plane only has one side, so the color jumps from red to green along the line at infinity. This mode enables you to see that the projective plane is non-orientable. In distance mode, the projective plane is displayed with fully saturated colors that depend on the distance of the points on the projective plane to the origin. The origin is displayed in red, the line at infinity is displayed in magenta. If the projective plane is displayed as distance bands, each band will be displayed with a different color. In direction mode, the projective plane is displayed with fully saturated colors that depend on the angle of the points on the projective plane with respect to the origin. Angles in opposite directions to the origin (e.g., 15 and 205 degrees) are displayed in the same color since they are projectively equivalent. If the projective plane is displayed as direction bands, each band will be displayed with a different color.

The rotation speed for each of the three coordinate axes around which the projective plane rotates can be chosen.

Furthermore, in the walking mode the walking direction in the 2d base square of the projective plane 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 of the projective plane. 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 from the origin to the line at infinity and back (analogous to a direction band). Any other value results in a curved path from the origin to the line at infinity and back.

By default, the immersion of the real projective plane smoothly deforms between the Roman and Boy surfaces. 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.

As a final option, it is possible to display generalized versions of the immersion discussed above by specifying the order of the surface. The default surface order of 3 results in the immersion of the real projective described above. The surface order can be chosen between 2 and 9. Odd surface orders result in generalized immersions of the real projective plane, while even numbers result in a immersion of a topological sphere (which is orientable). The most interesting even case is a surface order of 2, which results in an immersion of the halfway model of Morin's sphere eversion (if the deformation is switched off).

This program is inspired by François Apéry's book "Models of the Real Projective Plane", Vieweg, 1987.

**-window**- Draw on a newly-created window. This is the default.
**-root**- Draw on the root 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 projective plane is displayed.

**-mode random**- Display the projective plane in a random display mode (default).
**-mode wireframe**(Shortcut:**-wireframe**)- Display the projective plane as a wireframe mesh.
**-mode surface**(Shortcut:**-surface**)- Display the projective plane as a solid surface.
**-mode transparent**(Shortcut:**-transparent**)- Display the projective plane as a transparent surface.

The following four options are mutually exclusive. They determine the appearance of the projective plane.

**-appearance random**- Display the projective plane with a random appearance (default).
**-appearance solid**(Shortcut:**-solid**)- Display the projective plane as a solid object.
**-appearance distance-bands**(Shortcut:**-distance-bands**)- Display the projective plane as see-through bands that lie at increasing distances from the origin.

**-appearance direction-bands**(Shortcut:**-direction-bands**)- Display the projective plane as see-through bands that lie at increasing angles with respect to the origin.

The following four options are mutually exclusive. They determine how to color the projective plane.

**-colors random**- Display the projective plane with a random color scheme (default).
**-colors twosided**(Shortcut:**-twosided-colors**)- Display the projective plane with two colors: red on one "side" and green on the "other side." Note that the line at infinity lies at the points where the red and green "sides" of the projective plane meet, i.e., where the orientation of the projective plane reverses.
**-colors distance**(Shortcut:**-distance-colors**)- Display the projective plane with fully saturated colors that depend on the distance of the points on the projective plane to the origin. The origin is displayed in red, the line at infinity is displayed in magenta. If the projective plane is displayed as distance bands, each band will be displayed with a different color.
**-colors direction**(Shortcut:**-direction-colors**)- Display the projective plane with fully saturated colors that depend on the angle of the points on the projective plane with respect to the origin. Angles in opposite directions to the origin (e.g., 15 and 205 degrees) are displayed in the same color since they are projectively equivalent. If the projective plane is displayed as direction bands, each band will be displayed with a different color.

The following three options are mutually exclusive. They determine how to view the projective plane.

**-view-mode random**- View the projective plane in a random view mode (default).
**-view-mode turn**(Shortcut:**-turn**)- View the projective plane while it turns in 3d.
**-view-mode walk**(Shortcut:**-walk**)- View the projective plane as if walking on its surface.

The following options determine whether the surface is being deformed.

**-deform**- Deform the surface smoothly between the Roman and Boy surfaces (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 1000. A value of 0 corresponds to the Roman surface, while a value of 1000 corresponds to the Boy surface. The default value is 1000.
**-roman**-
This is a shortcut for
**-initial-deformation 0**. **-boy**-
This is a shortcut for
**-initial-deformation 1000**.

The following option determines the order of the surface to be displayed.

**-surface-order***number*- The surface order can be set to values between 2 and 9 (default: 3). As described above, odd surface orders result in generalized immersions of the real projective plane, while even numbers result in a immersion of a topological sphere.

The following options determine whether orientation marks are shown on the projective plane.

**-orientation-marks**- Display orientation marks on the projective plane.
**-no-orientation-marks**- Don't display orientation marks on the projective plane (default).

The following three options are mutually exclusive. They determine how the projective plane is projected from 3d to 2d (i.e., to the screen).

**-projection random**- Project the projective plane from 3d to 2d using a random projection mode (default).
**-projection perspective**(Shortcut:**-perspective**)- Project the projective plane from 3d to 2d using a perspective projection.
**-projection orthographic**(Shortcut:**-orthographic**)- Project the projective plane from 3d to 2d using an orthographic projection.

The following three options determine the rotation speed of the projective plane 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 projective plane (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 from the origin to the line at infinity and back (analogous to a direction band). Any other value results in a curved path from the origin to the line at infinity and back.
**-walk-speed***float*- The walking speed is measured in percent of some sensible maximum speed (default: 20.0).

**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.