Math::Trig(3perl)

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, pi4, pip2, pip4 (2*pi, 4*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, phi), 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 rho, 0, 0, and the Gulf of Guinea (think of the missing big chunk of Africa) rho, 0, pi/2. 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
Returns the great circle distance between two points on a sphere.

    $distance = great_circle_distance($theta0, $phi0, $theta1, $phi1, [, $rho]);

Where ($theta0, $phi0) and ($theta1, $phi1) are the spherical coordinates of the two points, respectively. The distance is in $rho units. The $rho is optional. It defaults to 1 (the unit sphere).

If you are using geographic coordinates, latitude and longitude, you need to adjust for the fact that latitude is zero at the equator increasing towards the north and decreasing towards the south. Assuming ($lat0, $lon0) and ($lat1, $lon1) are the geographic coordinates in radians of the two points, the distance can be computed with

  $distance = great_circle_distance($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 $way indicates the position of the waypoint along the great circle arc through the starting point ($theta0, $phi0) and the end point ($theta1, $phi1) relative to the distance from the starting point to the end point. So $way = 0 gives the starting point, $way = 1 gives the end point, $way < 0 gives a point "behind" the starting point, and $way > 1 gives a point beyond the end point. $way defaults to 0.5 if not given.

Note that antipodal points (where their distance is pi radians) do not have unique waypoints between them, and therefore "undef" is returned in such cases. If the points are the same, so the distance between them is zero, all waypoints are identical to the starting/end point.

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 rad2deg);

    my @M = great_circle_midpoint(@L, @T);
    sub SWNE { rad2deg( $_[0] ), 90 - rad2deg( $_[1] ) }
    my @lonlat = SWNE(@M);

or about 69 N 89 E, on the Putorana Plateau 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.

SEE ALSO
Math::Complex

AUTHORS
Jarkko Hietaniemi <jhi!at!iki.fi>, Raphael Manfredi <Raphael_Manfredi!at!pobox.com>, Zefram <zefram@fysh.org>

LICENSE
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.