Module: RGeo::Cartesian::Analysis

Defined in:
lib/rgeo/cartesian/analysis.rb

Overview

This provides includes some spatial analysis algorithms supporting Cartesian data.

Class Method Summary collapse

Class Method Details

.ccw?(ring) ⇒ Boolean Also known as: counter_clockwise?

Check orientation of a ring, returns ‘true` if it is counter-clockwise and false otherwise.

If the factory used is GEOS based, use the GEOS implementation to check that. Otherwise, this methods falls back to ‘ring_direction`.

Note

This method does not ensure a correct result for an invalid geometry. You should make sure your ring is valid beforehand using ‘ring?` if you are using a LineString, or directly `valid?` for a `linear_ring?`. This will be subject to changes in v3.

Returns:

  • (Boolean) 


28
29
30
31
32
33
34
 
# File 'lib/rgeo/cartesian/analysis.rb', line 28

def ccw?(ring)
  if RGeo::Geos.capi_geos?(ring) && RGeo::Geos::Analysis.ccw_supported?
    RGeo::Geos::Analysis.ccw?(ring)
  else
    RGeo::Cartesian::Analysis.ring_direction(ring) == 1
  end
end

.ring_direction(ring) ⇒ Object

Given a LineString, which must be a ring, determine whether the ring proceeds clockwise or counterclockwise. Returns 1 for counterclockwise, or -1 for clockwise.

Returns 0 if the ring is empty. The return value is undefined if the object is not a ring, or is not in a Cartesian coordinate system.



45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
 
# File 'lib/rgeo/cartesian/analysis.rb', line 45

def ring_direction(ring)
  size = ring.num_points - 1
  return 0 if size == 0

  # Extract unit-length segments from the ring.
  segs = []
  size.times do |i|
    p0 = ring.point_n(i)
    p1 = ring.point_n(i + 1)
    x = p1.x - p0.x
    y = p1.y - p0.y
    r = Math.sqrt(x * x + y * y)
    if r > 0.0
      segs << x / r << y / r
    else
      size -= 1
    end
  end
  segs << segs[0] << segs[1]

  # Extract angles from the segments by subtracting the segments.
  # Note angles are represented as cos/sin pairs so we don't
  # have to calculate any trig functions.
  angs = []
  size.times do |i|
    x0, y0, x1, y1 = segs[i * 2, 4]
    angs << x0 * x1 + y0 * y1 << x0 * y1 - x1 * y0
  end

  # Now add the angles and count revolutions.
  # Again, our running sum is represented as a cos/sin pair.
  revolutions = 0
  direction = nil
  sin = 0.0
  cos = 1.0
  angs.each_slice(2) do |(x, y)|
    ready = y > 0.0 &&
      (sin > 0.0 || sin == 0 && direction == -1) ||
      y < 0.0 &&
      (sin < 0.0 || sin == 0 && direction == 1)
    unless y == 0
      s = sin * x + cos * y
      c = cos * x - sin * y
      r = Math.sqrt(s * s + c * c)
      sin = s / r
      cos = c / r
    end
    next unless ready
    if y > 0.0 && sin <= 0.0
      revolutions += 1
      direction = 1
    elsif y < 0.0 && sin >= 0.0
      revolutions -= 1
      direction = -1
    end
  end
  revolutions
end