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 ~contourpy.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 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 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 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.