Manpages - Math_Trig.3perl

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.

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.

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.

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 Ω- 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().

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 Ω- 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.)

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:

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.