CLIP(1) Geometry Center (23 November 1994) CLIP(1)
NAME
clip - Clip an OOGL object against planes or other surfaces
SYNOPSIS
clip [-v axisx,y,z,...]
[-g value-or-point] [-l value-or-point]
[-s nstrips[,fraction]] [-e]
[-sph centerx,y,z,...] [-cyl centerx,y,z,...]
[ooglfile]
DESCRIPTION
Clip, adapted from Daeron Meyer's ginsu module, allows
clipping an OOGL object against planes, spheres, or
cylinders from the UNIX command line. Its input can come
from a file or standard input; output is written to standard
output.
Options specify a function of space position; the output is
the portion of the object where the function is greater or
less than some given value, or the portion lying between two
values. Alternatively, an object can be sliced into
equally-spaced strips. Objects may be of any dimension (but
see the BUGS section).
Options are:
-g value-or-point
-l value-or-point
Select the portion of the object where the function is
greater than (-g) or less than (-l) the given value.
If both are specified, the result is the portion of the
object satisfying both conditions.
If, rather than a single number, the argument to -l or
-g is a point (a series of x,y,z,... values separated
by commas, with no embedded blanks), then the clipping
surface is one chosen to pass through that point.
-v axisx,y,z,...
Specifies a direction in space. For planar clipping
(the default), it's the plane normal direction; the
clipping function is the inner product between the
direction vector and the point on the object. For
cylindrical clipping, -v specifies the direction of the
cylinder's axis; the clipping function is the distance
from the axis.
-sph centerx,y,z,...
Clip against spheres centered on x,y,z,.... The
clipping function is the distance from the given
center. Coordinates must be separated by commas
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without intervening spaces.
-cyl centerx,y,z,...
Clip against cylinders with an axis passing through
centerx,y,z,..., with axis direction given by the -v
option. The clipping function is the distance from the
axis.
-s nslices[,fraction]
Clip an object into a series of nslices ribbons
spanning its entire extent -- the range of function-
values over the object. Part of each ribbon is
omitted; the fraction, default .5, sets the width of
the visible part of a ribbon compared to the ribbon
period. There are a total of (nslices+fraction-1)
ribbon periods across the object, so e.g. -s 2,.5
slices the object into equal thirds, omitting the
middle third. The output OOGL object is a LIST of
OFFs, one per ribbon.
-e Don't emit a clipped OOGL object, just print two
numbers, listing the minimum and maximum function
values for the object. If -g or -l clipping options
are specified, the object is clipped before determining
the function range. If none of the object remains,
clip prints "0 0".
EXAMPLES
To extract the portion of an object lying below the x+y+z=1
plane:
clip-l 1 -v 1,1,1 file.oogl > portion.oogl
To extract the portion of an object lying in the positive
octant and below the x+y+z=1 plane, we can pipe multiple
instances of clip together to find the intersection of
several half-spaces:
clip-g 0 -v 1,0,0 file.oogl | \
clip-g 0 -v 0,1,0 | \
clip-g 0 -v 0,0,1 | \
clip-l 1 -v 1,1,1 > portion.oogl
To find the region lying between two surfaces (either above
one and below the other, or below the first and above the
second), say the planes 2x + y -.5z = 1 and y + 2z = 0:
echo "{ LIST"
clip-v 2,1,-.5 -g 1 file.oogl | clip-v 0,1,2 -l 0
clip-v 2,1,-.5 -l 1 file.oogl | clip-v 0,1,2 -g 0
echo "}"
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CLIP(1) Geometry Center (23 November 1994) CLIP(1)
We use pipelines to compute intersections, and a LIST to
form their union.
SEE ALSO
ginsu(1)
BUGS
Uses anytooff(1) to convert input data to OFF format
internally; this can lose information. The only arbitrary-
dimensional form accepted at present is nOFF, not nSKEL or
nMESH. However the four-dimensional 4OFF, 4QUAD, 4MESH,
4VECT, etc. formats should work.
Clip really only clips edges. If a curved clipping surface
cuts an edge twice, or removes only an interior portion of
some polygon, clip misses it entirely. Clipping against a
curved surface yields a straight edge (a chord of the ideal
curved edge segment). This latter failing might be fixed
someday.
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