Overview

The Clipper Library

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Overview

The Clipper Library performs clipping, and offsetting of both lines and polygons.

A number of features set Clipper apart from other clipping libraries:

  • it accepts all types of polygons including self-intersecting ones
  • it supports multiple polygon filling rules (EvenOdd, NonZero, Positive, Negative)
  • it's very fast relative to other libraries
  • it's numerically robust
  • it also performs line and polygon offsetting
  • it's free to use in both freeware and commercial applications

Current Version: 6.2.0

Author & copyright:
Angus Johnson. Copyright © 2010-2014
License, terms and conditions: Boost Software License

Terminology:

  • Clipping: commonly refers to the process of cutting away from a set of 2-dimensional geometric shapes those parts that are outside a rectangular 'clipping' window. This can be achieved by intersecting subject paths (lines and polygons) with a clipping rectangle. In a more general sense, the clipping window need not be rectangular but can be any type of polygon, even multiple polygons. Also, while clipping typically refers to an intersection operation, in this documentation it will refer to any one of the four boolean operations (intersection, union, difference and exclusive-or).
  • Path: is an ordered sequence of vertices defining a single geometric contour that's either a line (an open path) or a polygon (a closed path).
  • Line: or polyline is an open path containing 2 or more vertices.
  • Polygon: commonly refers to a two-dimensional region bounded by an outer non-intersecting closed contour. That region may also contain a number of 'holes'. In this documentation however, polygon will simply refer to a closed path.
  • Contour: synonymous with path.
  • Hole: is a closed region within a polygon that's not part of the polygon. A 'hole polygon' is a closed path that forms the outer boundaries of a hole.
  • Polygon Filling Rule: the filling rule, together with a list of closed paths, defines those regions (bounded by paths) that are inside (ie regions 'brush filled' in a graphical display) and those which are outside (ie 'holes').

Distribution package contents:

The ZIP package contains the Clipper library's source code, a Windows CHM help file, HTML help, and a number of compiled demo applications (with full source code). The library was initially written in Delphi Pascal (and compiles with Delphi version 7 or above) but now contains C++, C# and Python translations too. The library's source code in each language is about 5000 lines. The Delphi code contains reasonably extensive comments, but comments are fewer in the C++ and C# code. The included sample applications show how Clipper can be used with the different languages using a number of graphics display libraries including - AGG, Cairo, OpenGL, Graphics32, GDI+ and SVG.

Download Link:

SourceForge

References:

The Library is based on but significantly extends Bala Vatti's polygon clipping algorithm as described in "A generic solution to polygon clipping", Communications of the ACM, Vol 35, Issue 7 (July 1992) pp 56-63.

A section in "Computer graphics and geometric modeling: implementation and algorithms" by By Max K. Agoston (Springer, 2005) discussing Vatti Polygon Clipping was also helpful in creating the initial Clipper implementation.

The paper titled "Polygon Offsetting by Computing Winding Numbers" by Chen & McMains (Paper no. DETC2005-85513, ASME 2005. Pages 565-575) contains helpful discussion on the complexities of polygon offsetting together with some solutions.

See Also

Source, License, Clipper, ClipperOffset, ClipType, PolyFillType

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