/ 
Math-PlanePath-29
River stage one • 3 direct dependents • 4 total dependents
/ Math::PlanePath::ArchimedeanChords

NAME

Math::PlanePath::ArchimedeanChords -- radial spiral chords

SYNOPSIS

use Math::PlanePath::ArchimedeanChords;
my $path = Math::PlanePath::ArchimedeanChords->new;
my ($x, $y) = $path->n_to_xy (123);

DESCRIPTION

This path puts points at unit chord steps along an Archimedean spiral. The spiral goes outwards by 1 unit each revolution and the points are spaced 1 apart. The result is roughly

                     31              
               32          30         ...                3
         33                   29
                  14
   34       15          13          28    50             2
                              12
         16        3
35                       2             27    49          1
                4                11
      17
36           5        0     1          26    48     <- Y=0
                                 10
      18
37              6                      25    47         -1
                               9
         19        7     8          24    46
   38                                                   -2
            20                23
      39          21    22             45
                                                        -3
            40                   44
               41    42    43


                      ^
   -3    -2    -1    X=0    1     2     3     4

X,Y positions returned are fractional. Each revolution is about 2*pi longer than the previous, so the effect is a kind of 6.28 increment looping.

Because the spacing is by unit chords, adjacent unit circles centred on each N position touch but don't overlap. The spiral spacing of 1 unit per revolution means they don't overlap radially either.

The unit chords here are a little like the TheodorusSpiral. But the TheodorusSpiral goes by unit steps at a fixed right-angle and approximates an Archimedean spiral (of 3.14 radial spacing). Whereas this ArchimedeanChords is an actual Archimedean spiral (of radial spacing 1), with unit steps angling along that.

FUNCTIONS

$path = Math::PlanePath::ArchimedeanChords->new ()

Create and return a new path object.

($x,$y) = $path->n_to_xy ($n)

Return the X,Y coordinates of point number $n on the path.

$n can be any value $n >= 0 and fractions give positions on the chord between the integer points (ie. straight line between the points). $n==0 is the origin 0,0.

For $n < 0 the return is an empty list, it being considered there are no negative points in the spiral.

$n = $path->xy_to_n ($x,$y)

Return an integer point number for coordinates $x,$y. Each integer N is considered the centre of a circle of diameter 1 and an $x,$y within that circle returns N.

The unit spacing of the spiral means those circles don't overlap, but they also don't cover the plane and if $x,$y is not within one then the return is undef.

The current implementation is a bit slow.

$n = $path->n_start ()

Return 0, the first $n on the path.

$n = $path->figure ()

Return "circle".

FORMULAS

N to X,Y

The current code keeps a position as a polar angle t and calculates an increment u needed to move along by a unit chord. If dist(u) is the straight-line distance between t and t+u, then squared is the hypotenuse

dist^2(u) =   ((t+u)/2pi*cos(t+u) - t/2pi*cos(t))^2     # X
            + ((t+u)/2pi*sin(t+u) - t/2pi*sin(t))^2     # Y

which simplifies to

dist^2(u) = [ (t+u)^2 + t^2 - 2*t*(t+u)*cos(u) ] / (4*pi^2)

Switch from cos to sin using the half angle cos(u) = 1 - 2*sin^2(u/2)) in case if u is small then the cos(u) near 1.0 might lose floating point accuracy, and also as a slight simplification,

dist^2(u) = [ u^2 + 4*t*(t+u)*sin^2(u/2) ] / (4*pi^2)

Then want the u which has dist(u)=1 for a unit chord. The u*sin(u) part probably doesn't have a good closed form inverse, so the current code is a Newton/Raphson iteration on f(u) = dist^2(u)-1, seeking f(u)=0

f(u) = u^2 + 4*t*(t+u)*sin^2(u/2) - 4*pi^2

Derivative f'(u) for the slope from the cos form is

f'(u) = 2*(t+u) - 2*t*[ cos(u) - (t+u)*sin(u) ]

And again switching from cos to sin in case u is small,

f'(u) = 2*[ u + t*[2*sin^2(u/2) + (t+u)*sin(u)] ]

X,Y to N

A given x,y point is at a fraction of a revolution

frac = atan2(y,x) / 2pi     # -.5 <= frac <= .5
frac += (frac < 0)          # 0 <= frac < 1

And the nearest spiral arm measured radially from x,y is then

r = int(hypot(x,y) + .5 - frac) + frac

Perl's atan2 is the same as the C library and gives -pi <= angle <= pi, hence allowing for frac<0. It may also be "unspecified" for x=0,y=0, and give +/-pi for x=negzero, which has to be a special case so 0,0 gives r=0. The int rounds towards zero, so frac>.5 ends up as r=0.

So the N point just before or after that spiral position may cover the x,y, but how many N chords it takes to get around to there is 's not so easily calculated.

The current code looks in saved n_to_xy positions for an N below the target, and searches up from there until past the target and thus not covering x,y. With n_to_xy points saved 500 apart this means searching somewhere between 1 and 500 points.

One possibility for calculating a lower bound for N, instead of the saved positions, and both for xy_to_n and rect_to_n_range, would be to add up chords in circles. A circle of radius k fits pi/arcsin(1/2k) many unit chords, so

         k=floor(r)     pi
total = sum         ------------
         k=0        arcsin(1/2k)

and this is less than the chords along the spiral. Is there a good polynomial over-estimate of arcsin, to become an under-estimate total, without giving away so much?

N Range

For the rect_to_n_range upper bound, the current code takes the arc length along with spiral with the usual formula

arc = 1/4pi * (theta*sqrt(1+theta^2) + asinh(theta))

Written in terms of the r radius (theta = 2pi*r) as calculated from the biggest of the rectangle x,y corners,

arc = pi*r^2*sqrt(1+1/r^2) + asinh(2pi*r)/4pi

The arc length is longer than chords, so N=ceil(arc) is an upper bound for the N range.

An upper bound can also be calculated simply from the circumferences of circles 1 to r, since a spiral loop from radius k to k+1 is shorter than a circle of radius k.

         k=ceil(r)
total = sum         2pi*k
         k=1

      = pi*r*(r+1)

This is bigger than the arc length, thus a poorer upper bound, but an easier calculation. (Incidentally, for smallish r have arc length <= pi*(r^2+1) which is a tighter bound and an easy calculation, but it only holds up to somewhere around r=10^7.)

SEE ALSO

Math::PlanePath, Math::PlanePath::TheodorusSpiral, Math::PlanePath::SacksSpiral

HOME PAGE

http://user42.tuxfamily.org/math-planepath/index.html

LICENSE

Math-PlanePath is Copyright 2010, 2011 Kevin Ryde

Math-PlanePath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

Math-PlanePath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Math-PlanePath. If not, see <http://www.gnu.org/licenses/>.