SECTG                                                              Menu: POLYG
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Creates a section on a specified plane through the geometric model.

        1. (CHAR) option: plane: (o-orthogonal, d-diagonal, f-free) <o>
     if option `o' is selected:
        2. (CHAR) axis of section (x,y or z)
        3. (CHAR) define the section by: v-value, p-point <p>
     if `p' is selected:
        4. (POINT) point on section plane
        5. (POINT) pick point on side to view
     if `v' is selected:
        4. (POINT) value on axis
        5. (POINT) value on viewing side
     if option `d' is selected:
        2. (POINT) first point on section plane
        3. (POINT) second point on section plane
        4. (POINT) pick point on side to view




     if option `f' is selected:
        2. (POINT) first point on section plane
        3. (POINT) second point on section plane
        4. (POINT) third point on section plane
        5. (POINT) pick point on side to view
     for all options:
        6. (Y/N) save the section ? <y>
     if reply `y':
        7. (CHAR) name of extracted file (1-7 char.)

Description:
This command creates a section through the geometric model along a plane:
1. Orthogonally at a specified coordinate value (option `o', the default).
2. Perpendicular to an axial plane (xy, xz, or yz  option `d').
3. By a free plane defined by three points (option `f').
For option `o':specify an axis and select one of two further options: `p' the
default  to point to the position of the section or `v'  state a coordinate
value on the axis selected. The section plane will be perpendicular to the
axis at the specified value. Then enter another coordinate value on the same
axis to indicate which side of the section plane is to be kept.


For option `d':Pick two points on one of the axial planes (xy, yz or xz). The
section will be cut through the two points perpendicular to the plane on which
the points are located. Pick another point to indicate the side of the section
that is to be retained.
For option `f':Pick three points to define a free plane and a fourth point to
indicate the side to be kept.
The model will be replaced on the screen by a 2D section (presented in heavy
line type) through the specified point. The section has no thickness.
Reply `y' to the prompt to keep the section, and give it a name. If you reply
`n', the section will be erased and the original model will be redisplayed.
When the section is displayed, the rest of the model is blanked. If you wish
to see the section in the context of the whole model, you may execute the
\nblnk command. The whole model will then appear, and the section will be
classified as being in selection group 10 (\apsel).

        Example: \sectg o z v 1 0 y sect1

Result: The model will be sectioned along a plane normal to the z axis
(parallel to the xy plane) at the point where z = 1. The section plane will be
displayed in thick lines, and will be kept in a file named $sect1.
