Dechunking

Contour lines and filled contours that are split into multiple chunks can have their chunks combined using the functions dechunk_lines() and dechunk_filled(). Line and fill types that are not split into chunks (LineType.Separate, LineType.SeparateCode, FillType.OuterCode and FillType.OuterOffset) and those that are but only have a single chunk are returned unmodified by the dechunk functions.

Individual lines and polygons are grouped together into a single chunk, but they remain as separate lines or polygons; they are not geometrically combined.

As an example, first generate some contour lines that are chunked:

>>> from contourpy import contour_generator, dechunk_lines
>>> cont_gen = contour_generator(z=[[0, 1, 0], [0, 1, 0], [0, 0, 0]],
...                              line_type="ChunkCombinedOffset", chunk_size=1)
>>> lines = cont_gen.lines(0.5)
>>> lines
([array([[0.5, 1. ], [0.5, 0. ]]),
  array([[1.5, 0. ], [1.5, 1. ]]),
  array([[1. , 1.5], [0.5, 1. ]]),
  array([[1.5, 1. ], [1. , 1.5]])],
 [array([0, 2], dtype=uint32),
  array([0, 2], dtype=uint32),
  array([0, 2], dtype=uint32),
  array([0, 2], dtype=uint32)])

There are 4 chunks and each contains a single 2-point line. Now call dechunk_lines():

>>> lines = dechunk_lines(lines, "ChunkCombinedOffset")
>>> lines
([array([[0.5, 1. ], [0.5, 0. ], [1.5, 0. ], [1.5, 1. ], [1. , 1.5], [0.5, 1. ],
         [1.5, 1. ], [1. , 1.5]])],
 [array([0, 2, 4, 6, 8], dtype=uint32)])

This returns a single chunk containing all 4 lines.

These functions are useful if you want to support the option to generate contours using multiple chunks, such as to support the "threaded" algorithm, but your code to process the contours only supports a single chunk.