rbclipper

Synopsis

These are Ruby bindings to Clipper, Angus Johnson's Polygon clipping library. Because Clipper is not readily packaged, and is so beautifully self-contained, I've included the two required files in the package.

This release contains version 6.4.2 of Clipper and some improvements by Kjell Morgenstern to add tests and support polylines.

• Clipper Homepage
• rbclipper

Installation

This version is published as the rbclipper gem. The original mieko-based gem seems not to be maintained.

gem install rbclipper

To build locally:

rake install

Simple Usage:

This should be enough to get you started. Slightly fuller documentation is below.

require 'clipper'

a = [[0, 0], [0, 100], [100, 100], [100, 0]]
b = [[-5, 50], [200, 50], [100, 5]]

c = Clipper::Clipper.new

c.add_subject_polygon(a)
c.add_clip_polygon(b)
c.union :non_zero, :non_zero

=> [[[100.0, 0.0], [0.0, 0.0], [0.0, 47.85714326530613], [-4.999999, 50.0],
     [0.0, 50.0], [0.0, 100.0], [100.0, 100.0], [100.0, 50.0],
     [200.0, 50.0], [100.0, 5.0]]]

Documentation

Clipper is a two-dimensional polygon clipping library. rbclipper, the Ruby bindings can be accessed by:

require 'clipper'

Polygons

Operations that accept or return polygons are specified as an array of [x,y] coordinates, for example, to specify a triangle:

triangle = [[0,0], [0,100], [50, -100]]

Clipper supports both holes and complex polygons. Coordinates for output polygons are clockwise for shells, and and counter-clockwise for holes. See force_orientation.

Note that since 2.8, Clipper defines orientation with respect to a downward-increasing Y axis, similar to how many 2D GUI/drawing APIs position coordinate (0,0) at the top-left corner. The bindings have followed Clipper proper in this regard.

Multiple polygons are represented as simply an array of polygons.

Fill Types

• :even_odd

  A point is considered inside the polygon if the number of edge-crossings to get there from outside the shape is an even number.

• :non_zero

  A point is considered inside the polygon if the number of edge-crossings to get there is greater than zero.

• :positive

  See Clipper Homepage.

• :negative

  See Clipper Homepage.

Clipper::Clipper Methods

• Clipper#initialize

  Creates a new clipper object.

• Clipper#add_subject_polygon(polygon)

• Clipper#add_clip_polygon(polygon)

  Adds a subject or clip polygon to the engine. Boolean operations are calculated as SUBJECT operatation CLIP. Multiple polygons can Pay attention to the orientation of the coordinates given; counter-clockwise for shells and clockwise for holes.

  Multiple subject and clip polygons can be added to the engine for operations.

• Clipper#add_subject_polygons(expolygon)

• Clipper#add_clip_polygons(expolygon)

  Add an "ExPolygon" to the engine. Which is basically a list of polygons - the first is the outside (counter-clock-wise) and the rest, if any, are the holes (clock-wise). Boolean operations consider every expolygon added in this manner to be the same object.

• Clipper#add_subject_polyline(polyline)

  Add an open polygon (ie a polyline) to the engine. NB. polylines can only be present as a subject shape, and will only interact with clip shapes.

• Clipper#multiplier

• Clipper#multiplier=

  Defaults to 2^10 = 1048576. Clipper since version 4.0 uses integer math instead of floating point. To simplify using floating point coordinates, this multiplier is multiplied to each coordinate value before beeing sent to Clipper, and each result coordinate is divided by the multiplier. Use 1 if you want to use integer coordinates.

• Clipper#intersection(subject_fill=:even_odd, clip_fill=:even_odd, result_type=:polygons)

• Clipper#union(subject_fill=:even_odd, clip_fill=:even_odd, result_type=:polygons)

• Clipper#difference(subject_fill=:even_odd, clip_fill=:even_odd, result_type=:polygons)

• Clipper#xor(subject_fill=:even_odd, clip_fill=:even_odd, result_type=:polygons)

  Performs a boolean operation on the polygons that have been added to the clipper object. The result is a list of polygons or expolygons, depending on result_type being :polygons or :expolygons FIXME: :expolygons functionality is not implemented.

  If the result contains open polygons, they will also be returned. To see if a polygon is open, check if the first point is nil. FIXME: Instead of this, directly support Polytrees from Clipper

• Clipper#offset_polygons(polygons, delta, join_type, miter_limit=0)

  Expands the polygons by delta. Use negative delta to compress the polygons. join_type is any of :jtSquare, :jtButt, :jtMiter or :jtRound. Use miter_limit to make sharp joints not extend too long, by cutting off the edge.

• Clipper#area(polygon)

  Returns the area of the supplied polygon. The returned area is negative if the polygon points are oriented clockwise, positive otherwise. Obeys the multiplier and use_full_coordinate_range settings for the clipper object.

• Clipper#Orientation(polygon)

  Orientation returns a boolean value that is based on the polygon's orientation relative to the display's orientation (ie Y-axis positive upward vs Y-axis positive downward).

• On Y-axis positive upward displays, Orientation will return true if the polygon's orientation is counter-clockwise.

• On Y-axis positive downward displays, Orientation will return true if the polygon's orientation is clockwise.

Minkowski sum

# Greek capital sigma (sum sign) ...
sigma = [[300, 400], [100, 400], [200,300], [100,200], [300,200]]

# diagonal brush pattern ...
brush = [[4, -6], [6, -6], [-4, 6], [-6, 6]]

solution = Clipper::Clipper.new.minkowski_sum(brush, [sigma], false)

# ...

PointInPolygon

# drop all points inside of polygon
points.select! { |e| c.point_in_polygon(*e, poly) == 0 }