NAME

lintran -- perform a linear transformation of a list

USAGE

lintran files

PARAMETERS

files

List file or files to be transformed.

xfield = 1, yfield = 2

Fields containing the x and y coordinates.

x= 0.0, y1 = 0.0

Coordinates of current origin.

xscale = 1.0, yscale = 1.0

Scale factor to be applied to the input coordinates, relative to the current origin [x1,y1].

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 radians = yes.

x= 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.

radians = no

If this parameter is set to yes, angle is input in radians, not degrees.

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.

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:

	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

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.

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

SEE ALSO