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.help lintran Apr84 lists
.ih
NAME
lintran -- perform a linear transformation of a list
.ih
USAGE
lintran files
.ih
PARAMETERS
.ls files
List file or files to be transformed.
.le
.ls xfield = 1, yfield = 2
Fields containing the x and y coordinates.
.le
.ls x1 = 0.0, y1 = 0.0
Coordinates of current origin.
.le
.ls xscale = 1.0, yscale = 1.0
Scale factor to be applied to the input coordinates, relative
to the current origin [x1,y1].
.le
.ls angle = 0.0
Rotation angle about the origin [x1,y1]. This angle, measured
counter clockwise from the positive x axis, is entered in
degrees, unless \fBradians\fR = yes.
.le
.ls x2 = 0.0, y2 = 0.0
Coordinates of the new origin; input coordinates are shifted by an
amount (x2-x1) and (y2-y1). Positive shifts indicate increasing distance
from the current origin. Applied after scaling and rotating the input
coordinates.
.le
.ls radians = no
If this parameter is set to yes, \fBangle\fR is input
in radians, not degrees.
.le
.ls min_sigdigits = 1
The number of significant digits to be output in the transformed fields
is the maximum of this parameter and the number of input significant digits.
.le
.ih
DESCRIPTION
Specified fields from the input list can be scaled, rotated and shifted.
Two fields of each input line are designated
as the x and y coordinates to be transformed (default: fields 1, 2).
All other fields are be preserved across the transformation.
For clarification, the equations used in the transformation are shown below:
.ks
.nf
1. Subtract off the current origin:
xt = x - x1
yt = y - y1
2. Scale and rotate the coordinates:
xs = xt * xscale
ys = yt * yscale
xt = xs * cos(angle) - ys * sin(angle)
yt = xs * sin(angle) + ys * cos(angle)
3. Shift to the new origin:
xt = xt + x2
yt = yt + y2
.fi
.ke
Comment lines and blank lines are passed on to the output unmodified
(a comment line is any line beginning with the character '#').
If either x or y is indefinite
and no rotation is being performed, the corresponding
output coordinate will be indefinite. If either input coordinate is indefinite
and a rotation is being performed, both output coordinates will be indefinite.
.ih
EXAMPLES
1. Shift the coordinate list frame1 so it represents positions
in a second exposure of a star field, not registered with the first. Take
the coordinates of a star in frame1 to be the current origin
(e.g., [35.7, 389.2]); the new origin is then the coordinates of the same
star in the second exposure ([36.9, 400.0]). The shifted list is saved in
file "frame2":
cl> lintran frame1 x1=35.7 y1=389.2 x2=36.9 y2=400.0 > frame2
2. Apply a shift of +3.4 units in x, -1.3 units in y to the input list
read from the standard input, writing the output list on the standard
output.
cl> list_stream | lintran x2=3.4 y2=-1.3
3. Rotate a coordinate list of a 800x800 frame by 90 degrees. The
rotated coordinate list would represent positions in the field if it had
been rotated, for example, from East to the right to East to the top.
Note that the rotation takes place about the central pixel [400.50,400.50]
and that the current and new origins are the same:
cl> lintran picture x1=400.5 y1=400.5 x2=400.5 y2=400.5 angle=90
.ih
SEE ALSO
.endhelp
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