improver.utilities.neighbourhood_tools module
Provides tools for neighbourhood generation
- boxsum(data, boxsize, cumsum=True, **pad_options)[source]
Fast vectorised approach to calculating neighbourhood totals.
This function makes use of the summed-area table method. An input array is accumulated top to bottom and left to right. This accumulated array can then be used to efficiently calculate the total within a neighbourhood about any point. An example input data array:
| 1 | 1 | 1 | 1 | 1 | | 1 | 1 | 1 | 1 | 1 | | 1 | 1 | 1 | 1 | 1 | | 1 | 1 | 1 | 1 | 1 |
is accumulated to become:
| 1 | 2 | 3 | 4 | 5 | | 2 | 4 | 6 | 8 | 10 | | 3 | 6 | 9 | 12 | 15 | | 4 | 8 | 12 | 16 | 20 | | 5 | 10 | 15 | 20 | 25 |
If we wish to calculate the total in a 3x3 neighbourhood about some point (*) of our array we use the following points:
| 1 (C) | 2 | 3 | 4 (D) | 5 | | 2 | 4 | 6 | 8 | 10 | | 3 | 6 | 9 (*) | 12 | 15 | | 4 (A) | 8 | 12 | 16 (B) | 20 | | 5 | 10 | 15 | 20 | 25 |
And the calculation is:
Neighbourhood sum = C - A - D + B = 1 - 4 - 4 + 16 = 9
This is the value we would expect for a 3x3 neighbourhood in an array filled with ones.
- Parameters
data (
ndarray) – The input data array.boxsize (
Union[int,Tuple[int,int]]) – The size of the neighbourhood. Must be an odd number.cumsum (
bool) – If False, assume the input data is already cumulative. If True (default), calculate cumsum along the last two dimensions of the input array.pad_options (
Any) – Additional keyword arguments passed to numpy.pad function. If given, the returned result will have the same shape as the input array.
- Return type
- Returns
Array containing the calculated neighbourhood total.
- Raises
ValueError – If boxsize has non-integer type.
ValueError – If any member of boxsize is not an odd number.
- pad_and_roll(input_array, shape, **kwargs)[source]
Pads the last len(shape) axes of the input array for rolling_window to create ‘neighbourhood’ views of the data of a given shape as the last axes in the returned array. Collapsing over the last len(shape) axes results in a shape of the original input array.
- Parameters
input_array (
ndarray) – The dataset of points to pad and create rolling windows for.Desired shape of the neighbourhood. E.g. if a neighbourhood width of 1 around the point is desired, this shape should be (3, 3):
X X X X O X X X X
Where O is our central point and X represent the neighbour points.
kwargs (
Any) – additional keyword arguments passed to numpy.pad function.
- Return type
- Returns
Contains the views of the input_array, the final dimension of the array will be the specified shape in the input arguments, the leading dimensions will depend on the shape of the input array.
- pad_boxsum(data, boxsize, **pad_options)[source]
Pad an array to shape suitable for boxsum.
Note that padding is not symmetric: there is an extra row/column at the top/left (as required for calculating the boxsum).
- rolling_window(input_array, shape, writeable=False)[source]
Creates a rolling window neighbourhood of the given shape from the last len(shape) axes of the input array. Avoids creating a large output array by constructing a non-continuous view mapped onto the input array.
- Parameters
input_array (
ndarray) – An array from which rolling window neighbourhoods will be created.shape (
Tuple[int,int]) – The neighbourhood shape e.g. if the neighbourhood size is 3, the shape would be (3, 3) to create a 3x3 array around each point in the input_array.writeable (
bool) – If True the returned view will be writeable. This will modify the input array, so use with caution.
- Return type
- Returns
“views” into the data, each view represents a neighbourhood of points.
- Raises
ValueError – If input_array has fewer dimensions than shape.
RuntimeError – If any dimension of shape is larger than the corresponding dimension of input_array.