Chunks

The domain to contour can be divided up into a number of chunks that are processed separately.

Advantages of using chunks:

They produce shorter lines/polygons that may be simpler or faster to render.

They make subsequent spatial queries easier.

They allow the use of multithreaded contouring (see Threads).

Disadvantages:

There is a slight performance cost of using chunks.

Some rendering algorithms show faint lines between neighbouring chunks.

Note

Think of chunks as dividing the quads rather than the points of a domain. A z array of shape (ny, nx) has (ny, nx) points but only (ny-1, nx-1) quads. This can be considered a single chunk of shape (ny-1, nx-1). Neighbouring chunks have neighbouring quads but they share the points that lie on their common boundary.

There are three possible ways of specifying chunks in contour_generator() which are the keyword arguments chunk_size, chunk_count and total_chunk_count. A maximum of one of the three may be specified.

Warning

You may not always receive the chunk sizes or counts that you request. A chunk has a minimum size of 1x1 quad!

chunk_size

chunk_size may be a tuple of (y_chunk_size, x_chunk_size) or a single integer that is used for both x and y chunk sizes.

>>> z = np.ones((5, 10))  # Sample z data.
>>> cont_gen = contour_generator(z=z, chunk_size=(2, 4))
>>> cont_gen.chunk_size
(2, 4)
>>> cont_gen.chunk_count
(2, 3)

The final chunk in each direction may be smaller than the others. Here in the x-direction there are 3 chunks of size 4; the first two x-chunks cover 8 quads leaving the final x-chunk to cover just a single quad.

chunk_count

chunk_count may be a tuple of (y_chunk_count, x_chunk_count) or a single integer that is used for both x and y chunk counts.

Using chunk_count can give more even chunks than using chunk_size.

total_chunk_count

total_chunk_count attempts to give a sensible combination of x and y chunk counts.

It uses a simple algorithm that finds two integer factors that are close as possible to sqrt(total_chunk_count). Do not use a prime number for total_chunk_count as the two factors it will use are total_chunk_count and 1.