Math::Trig - trigonometric functions
- 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';
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.
The tangent
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
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.
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, 2-dimensional) angles may be converted with the following functions.
- $radians = deg2rad($degrees);
- $radians = grad2rad($gradians);
- $degrees = rad2deg($radians);
- $degrees = grad2deg($gradians);
- $gradians = deg2grad($degrees);
- $gradians = rad2grad($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().
- $radians_wrapped_by_2pi = rad2rad($radians);
- $degrees_wrapped_by_360 = deg2deg($degrees);
- $gradians_wrapped_by_400 = grad2grad($gradians);
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, $phi);
All angles are in radians.
Cartesian coordinates are the usual rectangular (x, y, z)-coordinates.
Spherical coordinates, (rho, theta, pi), are three-dimensional coordinates which define a point in three-dimensional space. They are based on a sphere surface. The radius of the sphere is rho, also known as the radial coordinate. The angle in the xy-plane (around the z-axis) is theta, also known as the azimuthal coordinate. The angle from the z-axis is phi, also known as the polar coordinate. The North Pole is therefore 0, 0, rho, and the Gulf of Guinea (think of the missing big chunk of Africa) 0, pi/2, rho. In geographical terms phi is latitude (northward positive, southward negative) and theta is longitude (eastward positive, westward negative).
BEWARE: some texts define theta and phi the other way round, some texts define the phi to start from the horizontal plane, some texts use r in place of rho.
Cylindrical coordinates, (rho, theta, z), are three-dimensional coordinates which define a point in three-dimensional space. They are based on a cylinder surface. The radius of the cylinder is rho, also known as the radial coordinate. The angle in the xy-plane (around the z-axis) is theta, also known as the azimuthal coordinate. The third coordinate is the z, pointing up from the theta-plane.
Conversions to and from spherical and cylindrical coordinates are available. Please notice that the conversions are not necessarily reversible because of the equalities like pi angles being equal to -pi angles.
- ($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);
Notice that when $z
is not 0 $rho_s
is not equal to $rho_c
.
- ($x, $y, $z) = spherical_to_cartesian($rho, $theta, $phi);
- ($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
.
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.
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);
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);
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.
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, $distance);
or the midpoint if you know the end points:
- use Math::Trig 'great_circle_midpoint';
- ($thetam, $phim) =
- great_circle_midpoint($theta0, $phi0, $theta1, $phi1);
The great_circle_midpoint() is just a special case of
- use Math::Trig 'great_circle_waypoint';
- ($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.)
To calculate the distance between London (51.3N 0.5W) and Tokyo (35.7N 139.8E) in kilometers:
The direction you would have to go from London to Tokyo (in radians, straight north being zero, straight east being pi/2).
The midpoint between London and Tokyo being
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.
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%.
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:
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.
- 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.
- 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.
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.
Jarkko Hietaniemi <jhi!at!iki.fi> and Raphael Manfredi <Raphael_Manfredi!at!pobox.com>.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.