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Math::Trig(3pm) Perl Programmers Reference Guide Math::Trig(3pm)
NAME
Math::Trig - trigonometric functions
SYNOPSIS
use Math::Trig;
$x = tan(0.9);
$y = acos(3.7);
$z = asin(2.4);
$halfpi = pi/2;
$rad = deg2rad(120);
# Import constants pi2, pip2, pip4 (2*pi, pi/2, pi/4).
use Math::Trig ':pi';
# Import the conversions between cartesian/spherical/cylindrical.
use Math::Trig ':radial';
# Import the great circle formulas.
use Math::Trig ':great_circle';
DESCRIPTION
"Math::Trig" defines many trigonometric functions not defined by the core Perl which
defines only the "sin()" and "cos()". The constant pi is also defined as are a few
convenience functions for angle conversions, and great circle formulas for spherical
movement.
TRIGONOMETRIC FUNCTIONS
The tangent
tan
The cofunctions of the sine, cosine, and tangent (cosec/csc and cotan/cot are aliases)
csc, cosec, sec, sec, cot, cotan
The arcus (also known as the inverse) functions of the sine, cosine, and tangent
asin, acos, atan
The principal value of the arc tangent of y/x
atan2(y, x)
The arcus cofunctions of the sine, cosine, and tangent (acosec/acsc and acotan/acot are
aliases). Note that atan2(0, 0) is not well-defined.
acsc, acosec, asec, acot, acotan
The hyperbolic sine, cosine, and tangent
sinh, cosh, tanh
The cofunctions of the hyperbolic sine, cosine, and tangent (cosech/csch and cotanh/coth
are aliases)
csch, cosech, sech, coth, cotanh
The area (also known as the inverse) functions of the hyperbolic sine, cosine, and tangent
asinh, acosh, atanh
The area cofunctions of the hyperbolic sine, cosine, and tangent (acsch/acosech and
acoth/acotanh are aliases)
acsch, acosech, asech, acoth, acotanh
The trigonometric constant pi and some of handy multiples of it are also defined.
pi, pi2, pi4, pip2, pip4
ERRORS DUE TO DIVISION BY ZERO
The following functions
acoth
acsc
acsch
asec
asech
atanh
cot
coth
csc
csch
sec
sech
tan
tanh
cannot be computed for all arguments because that would mean dividing by zero or taking
logarithm of zero. These situations cause fatal runtime errors looking like this
cot(0): Division by zero.
(Because in the definition of cot(0), the divisor sin(0) is 0)
Died at ...
or
atanh(-1): Logarithm of zero.
Died at...
For the "csc", "cot", "asec", "acsc", "acot", "csch", "coth", "asech", "acsch", the
argument cannot be 0 (zero). For the "atanh", "acoth", the argument cannot be 1 (one).
For the "atanh", "acoth", the argument cannot be "-1" (minus one). For the "tan", "sec",
"tanh", "sech", the argument cannot be pi/2 + k * pi, where k is any integer.
Note that atan2(0, 0) is not well-defined.
SIMPLE (REAL) ARGUMENTS, COMPLEX RESULTS
Please note that some of the trigonometric functions can break out from the real axis into
the complex plane. For example asin(2) has no definition for plain real numbers but it has
definition for complex numbers.
In Perl terms this means that supplying the usual Perl numbers (also known as scalars,
please see perldata) as input for the trigonometric functions might produce as output
results that no more are simple real numbers: instead they are complex numbers.
The "Math::Trig" handles this by using the "Math::Complex" package which knows how to
handle complex numbers, please see Math::Complex for more information. In practice you
need not to worry about getting complex numbers as results because the "Math::Complex"
takes care of details like for example how to display complex numbers. For example:
print asin(2), "\n";
should produce something like this (take or leave few last decimals):
1.5707963267949-1.31695789692482i
That is, a complex number with the real part of approximately 1.571 and the imaginary part
of approximately "-1.317".
PLANE ANGLE CONVERSIONS
(Plane, 2-dimensional) angles may be converted with the following functions.
deg2rad
$radians = deg2rad($degrees);
grad2rad
$radians = grad2rad($gradians);
rad2deg
$degrees = rad2deg($radians);
grad2deg
$degrees = grad2deg($gradians);
deg2grad
$gradians = deg2grad($degrees);
rad2grad
$gradians = rad2grad($radians);
The full circle is 2 pi radians or 360 degrees or 400 gradians. The result is by default
wrapped to be inside the [0, {2pi,360,400}[ circle. If you don't want this, supply a true
second argument:
$zillions_of_radians = deg2rad($zillions_of_degrees, 1);
$negative_degrees = rad2deg($negative_radians, 1);
You can also do the wrapping explicitly by rad2rad(), deg2deg(), and grad2grad().
rad2rad
$radians_wrapped_by_2pi = rad2rad($radians);
deg2deg
$degrees_wrapped_by_360 = deg2deg($degrees);
grad2grad
$gradians_wrapped_by_400 = grad2grad($gradians);
RADIAL COORDINATE CONVERSIONS
Radial coordinate systems are the spherical and the cylindrical systems, explained shortly
in more detail.
You can import radial coordinate conversion functions by using the ":radial" tag:
use Math::Trig ':radial';
($rho, $theta, $z) = cartesian_to_cylindrical($x, $y, $z);
($rho, $theta, $phi) = cartesian_to_spherical($x, $y, $z);
($x, $y, $z) = cylindrical_to_cartesian($rho, $theta, $z);
($rho_s, $theta, $phi) = cylindrical_to_spherical($rho_c, $theta, $z);
($x, $y, $z) = spherical_to_cartesian($rho, $theta, $phi);
($rho_c, $theta, $z) = spherical_to_cylindrical($rho_s, $theta, $phi);
All angles are in radians.
COORDINATE SYSTEMS
Cartesian coordinates are the usual rectangular (x, y, z)-coordinates.
Spherical coordinates, (rho, theta, pi), are three-dimensional coordinates which define a
point in three-dimensional space. They are based on a sphere surface. The radius of the
sphere is rho, also known as the radial coordinate. The angle in the xy-plane (around the
z-axis) is theta, also known as the azimuthal coordinate. The angle from the z-axis is
phi, also known as the polar coordinate. The North Pole is therefore 0, 0, rho, and the
Gulf of Guinea (think of the missing big chunk of Africa) 0, pi/2, rho. In geographical
terms phi is latitude (northward positive, southward negative) and theta is longitude
(eastward positive, westward negative).
BEWARE: some texts define theta and phi the other way round, some texts define the phi to
start from the horizontal plane, some texts use r in place of rho.
Cylindrical coordinates, (rho, theta, z), are three-dimensional coordinates which define a
point in three-dimensional space. They are based on a cylinder surface. The radius of
the cylinder is rho, also known as the radial coordinate. The angle in the xy-plane
(around the z-axis) is theta, also known as the azimuthal coordinate. The third
coordinate is the z, pointing up from the theta-plane.
3-D ANGLE CONVERSIONS
Conversions to and from spherical and cylindrical coordinates are available. Please
notice that the conversions are not necessarily reversible because of the equalities like
pi angles being equal to -pi angles.
cartesian_to_cylindrical
($rho, $theta, $z) = cartesian_to_cylindrical($x, $y, $z);
cartesian_to_spherical
($rho, $theta, $phi) = cartesian_to_spherical($x, $y, $z);
cylindrical_to_cartesian
($x, $y, $z) = cylindrical_to_cartesian($rho, $theta, $z);
cylindrical_to_spherical
($rho_s, $theta, $phi) = cylindrical_to_spherical($rho_c, $theta, $z);
Notice that when $z is not 0 $rho_s is not equal to $rho_c.
spherical_to_cartesian
($x, $y, $z) = spherical_to_cartesian($rho, $theta, $phi);
spherical_to_cylindrical
($rho_c, $theta, $z) = spherical_to_cylindrical($rho_s, $theta, $phi);
Notice that when $z is not 0 $rho_c is not equal to $rho_s.
GREAT CIRCLE DISTANCES AND DIRECTIONS
A great circle is section of a circle that contains the circle diameter: the shortest
distance between two (non-antipodal) points on the spherical surface goes along the great
circle connecting those two points.
great_circle_distance
You can compute spherical distances, called great circle distances, by importing the
great_circle_distance() function:
use Math::Trig 'great_circle_distance';
$distance = great_circle_distance($theta0, $phi0, $theta1, $phi1, [, $rho]);
The great circle distance is the shortest distance between two points on a sphere. The
distance is in $rho units. The $rho is optional, it defaults to 1 (the unit sphere),
therefore the distance defaults to radians.
If you think geographically the theta are longitudes: zero at the Greenwhich meridian,
eastward positive, westward negative -- and the phi are latitudes: zero at the North Pole,
northward positive, southward negative. NOTE: this formula thinks in mathematics, not
geographically: the phi zero is at the North Pole, not at the Equator on the west coast of
Africa (Bay of Guinea). You need to subtract your geographical coordinates from pi/2
(also known as 90 degrees).
$distance = great_circle_distance($lon0, pi/2 - $lat0,
$lon1, pi/2 - $lat1, $rho);
great_circle_direction
The direction you must follow the great circle (also known as bearing) can be computed by
the great_circle_direction() function:
use Math::Trig 'great_circle_direction';
$direction = great_circle_direction($theta0, $phi0, $theta1, $phi1);
great_circle_bearing
Alias 'great_circle_bearing' for 'great_circle_direction' is also available.
use Math::Trig 'great_circle_bearing';
$direction = great_circle_bearing($theta0, $phi0, $theta1, $phi1);
The result of great_circle_direction is in radians, zero indicating straight north, pi or
-pi straight south, pi/2 straight west, and -pi/2 straight east.
great_circle_destination
You can inversely compute the destination if you know the starting point, direction, and
distance:
use Math::Trig 'great_circle_destination';
# $diro is the original direction,
# for example from great_circle_bearing().
# $distance is the angular distance in radians,
# for example from great_circle_distance().
# $thetad and $phid are the destination coordinates,
# $dird is the final direction at the destination.
($thetad, $phid, $dird) =
great_circle_destination($theta, $phi, $diro, $distance);
or the midpoint if you know the end points:
great_circle_midpoint
use Math::Trig 'great_circle_midpoint';
($thetam, $phim) =
great_circle_midpoint($theta0, $phi0, $theta1, $phi1);
The great_circle_midpoint() is just a special case of
great_circle_waypoint
use Math::Trig 'great_circle_waypoint';
($thetai, $phii) =
great_circle_waypoint($theta0, $phi0, $theta1, $phi1, $way);
Where the $way is a value from zero ($theta0, $phi0) to one ($theta1, $phi1). Note that
antipodal points (where their distance is pi radians) do not have waypoints between them
(they would have an an "equator" between them), and therefore "undef" is returned for
antipodal points. If the points are the same and the distance therefore zero and all
waypoints therefore identical, the first point (either point) is returned.
The thetas, phis, direction, and distance in the above are all in radians.
You can import all the great circle formulas by
use Math::Trig ':great_circle';
Notice that the resulting directions might be somewhat surprising if you are looking at a
flat worldmap: in such map projections the great circles quite often do not look like the
shortest routes -- but for example the shortest possible routes from Europe or North
America to Asia do often cross the polar regions. (The common Mercator projection does
not show great circles as straight lines: straight lines in the Mercator projection are
lines of constant bearing.)
EXAMPLES
To calculate the distance between London (51.3N 0.5W) and Tokyo (35.7N 139.8E) in
kilometers:
use Math::Trig qw(great_circle_distance deg2rad);
# Notice the 90 - latitude: phi zero is at the North Pole.
sub NESW { deg2rad($_[0]), deg2rad(90 - $_[1]) }
my @L = NESW( -0.5, 51.3);
my @T = NESW(139.8, 35.7);
my $km = great_circle_distance(@L, @T, 6378); # About 9600 km.
The direction you would have to go from London to Tokyo (in radians, straight north being
zero, straight east being pi/2).
use Math::Trig qw(great_circle_direction);
my $rad = great_circle_direction(@L, @T); # About 0.547 or 0.174 pi.
The midpoint between London and Tokyo being
use Math::Trig qw(great_circle_midpoint);
my @M = great_circle_midpoint(@L, @T);
or about 69 N 89 E, in the frozen wastes of Siberia.
NOTE: you cannot get from A to B like this:
Dist = great_circle_distance(A, B)
Dir = great_circle_direction(A, B)
C = great_circle_destination(A, Dist, Dir)
and expect C to be B, because the bearing constantly changes when going from A to B
(except in some special case like the meridians or the circles of latitudes) and in
great_circle_destination() one gives a constant bearing to follow.
CAVEAT FOR GREAT CIRCLE FORMULAS
The answers may be off by few percentages because of the irregular (slightly aspherical)
form of the Earth. The errors are at worst about 0.55%, but generally below 0.3%.
Real-valued asin and acos
For small inputs asin() and acos() may return complex numbers even when real numbers would
be enough and correct, this happens because of floating-point inaccuracies. You can see
these inaccuracies for example by trying theses:
print cos(1e-6)**2+sin(1e-6)**2 - 1,"\n";
printf "%.20f", cos(1e-6)**2+sin(1e-6)**2,"\n";
which will print something like this
-1.11022302462516e-16
0.99999999999999988898
even though the expected results are of course exactly zero and one. The formulas used to
compute asin() and acos() are quite sensitive to this, and therefore they might
accidentally slip into the complex plane even when they should not. To counter this there
are two interfaces that are guaranteed to return a real-valued output.
asin_real
use Math::Trig qw(asin_real);
$real_angle = asin_real($input_sin);
Return a real-valued arcus sine if the input is between [-1, 1], inclusive the
endpoints. For inputs greater than one, pi/2 is returned. For inputs less than minus
one, -pi/2 is returned.
acos_real
use Math::Trig qw(acos_real);
$real_angle = acos_real($input_cos);
Return a real-valued arcus cosine if the input is between [-1, 1], inclusive the
endpoints. For inputs greater than one, zero is returned. For inputs less than minus
one, pi is returned.
BUGS
Saying "use Math::Trig;" exports many mathematical routines in the caller environment and
even overrides some ("sin", "cos"). This is construed as a feature by the Authors,
actually... ;-)
The code is not optimized for speed, especially because we use "Math::Complex" and thus go
quite near complex numbers while doing the computations even when the arguments are not.
This, however, cannot be completely avoided if we want things like asin(2) to give an
answer instead of giving a fatal runtime error.
Do not attempt navigation using these formulas.
Math::Complex
AUTHORS
Jarkko Hietaniemi <jhi!at!iki.fi>, Raphael Manfredi <Raphael_Manfredi!at!pobox.com>,
Zefram <zefram AT fysh.org>
LICENSE
This library is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
perl v5.16.3 2013-03-04 Math::Trig(3pm)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
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