Labeled Image Functions¶
Labeled images are integer images where the values correspond to different regions. I.e., region 1 is all of the pixels which have value 1, region two is the pixels with value 2, and so on. By convention, region 0 is the background and often handled differently.
Labeling Images¶
New in version 0.6.5.
The first step is obtaining a labeled function from a binary function:
import mahotas as mh
import numpy as np
from pylab import imshow, show
regions = np.zeros((8,8), bool)
regions[:3,:3] = 1
regions[6:,6:] = 1
labeled, nr_objects = mh.label(regions)
imshow(labeled, interpolation='nearest')
show()
(Source code, png, hires.png, pdf)
This results in an image with 3 values:
 background, where the original image was 0
 for the first region: (0:3, 0:3);
 for the second region: (6:, 6:).
There is an extra argument to label
: the structuring element, which
defaults to a 3x3 cross (or, 4neighbourhood). This defines what it means for
two pixels to be in the same region. You can use 8neighbourhoods by replacing
it with a square:
labeled,nr_objects = mh.label(regions, np.ones((3,3), bool))
New in version 0.7: labeled_size
and labeled_sum
were added in version 0.7
We can now collect a few statistics on the labeled regions. For example, how big are they?
sizes = mh.labeled.labeled_size(labeled)
print 'Background size', sizes[0]
print 'Size of first region', sizes[1]
This size is measured simply as the number of pixels in each region. We can instead measure the total weight in each area:
array = np.random.random_sample(regions.shape)
sums = mh.labeled_sum(array, labeled)
print 'Sum of first region', sums[1]
Filtering Regions¶
New in version 0.9.6: remove_regions & relabel were only added in version 0.9.6
Here is a slightly more complex example. This is in demos
directory as
nuclear.py
. We are going to use this image, a fluorescent microscopy image
from a nuclear segmentation benchmark
This image is available as mahotas.demos.nuclear_image()
import mahotas as mh
import mahotas.demos
import numpy as np
from pylab import imshow, show
f = mh.demos.nuclear_image()
f = f[:,:,0]
imshow(f)
show()
(Source code, png, hires.png, pdf)
First we perform a bit of Gaussian filtering and thresholding:
f = mh.gaussian_filter(f, 4)
f = (f> f.mean())
(Without the Gaussian filter, the resulting thresholded image has very noisy
edges. You can get the image in the demos/
directory and try it out.)
f = mh.gaussian_filter(f, 4)
f = (f> f.mean())
imshow(f)
show()
(Source code, png, hires.png, pdf)
Labeling gets us all of the nuclei:
labeled, n_nucleus = mh.label(f)
print('Found {} nuclei.'.format(n_nucleus))
labeled, n_nucleus = mh.label(f)
print('Found {} nuclei.'.format(n_nucleus))
imshow(labeled)
show()
(Source code, png, hires.png, pdf)
42
nuclei were found. None were missed, but, unfortunately, we also get
some aggregates. In this case, we are going to assume that we wanted to perform
some measurements on the real nuclei, but are willing to filter out anything
that is not a complete nucleus or that is a lump on nuclei. So we measure sizes
and filter:
sizes = mh.labeled.labeled_size(labeled)
too_big = np.where(sizes > 10000)
labeled = mh.labeled.remove_regions(labeled, too_big)
sizes = mh.labeled.labeled_size(labeled)
too_big = np.where(sizes > 10000)
labeled = mh.labeled.remove_regions(labeled, too_big)
imshow(labeled)
show()
(Source code, png, hires.png, pdf)
We can also remove the region touching the border:
labeled = mh.labeled.remove_bordering(labeled)
labeled = mh.labeled.remove_bordering(labeled)
imshow(labeled)
show()
(Source code, png, hires.png, pdf)
This array, labeled
now has values in the range 0
to n_nucleus
, but
with some values missing (e.g., if region 7
was one of the ones touching
the border, then 7
is not used in the labeling). We can relabel
to get
a cleaner version:
relabeled, n_left = mh.labeled.relabel(labeled)
print('After filtering and relabeling, there are {} nuclei left.'.format(n_left))
Now, we have 24
nuclei and relabeled
goes from 0
(background) to 24
.
relabeled, n_left = mh.labeled.relabel(labeled)
print('After filtering and relabeling, there are {} nuclei left.'.format(n_left))
imshow(relabeled)
show()
(Source code, png, hires.png, pdf)
In mahotas after version 1.4
, we can even make many of the same operations
with a single call to mh.labeled.filter_labeled
:
relabeled,n_left = mh.labeled.filter_labeled(labeled, remove_bordering=True, max_size=10000)
Borders¶
A border pixel is one where there is more than one region in its neighbourhood (one of those regions can be the background).
You can retrieve border pixels with either the borders()
function, which
gets all the borders or the border()
(note the singular) which gets only
the border between a single pair of regions. As usual, what neighbour means is
defined by a structuring element, defaulting to a 3x3 cross.
API Documentation¶
The mahotas.labeled
submodule contains the functions mentioned above.
label()
is also available as mahotas.label
.

mahotas.labeled.
bbox
(f, as_slice=False) Bounding boxes of all objects in a labeled array.
After:
bboxes = mh.labeled.bbox(f)
bboxes[34]
will contain the bounding box of(f == 34)
.Parameters: f : integer ndarray
as_slice : boolean, optional
Whether to return slice objects instead of integer coordinates (default: False).
Returns: bboxes : ndarray
See also
mh.bbox
 the binary version of this function

mahotas.labeled.
borders
(labeled, Bc={3x3 cross}, out={np.zeros(labeled.shape, bool)}) Compute border pixels
A pixel is on a border if it has value i and a pixel in its neighbourhood (defined by Bc) has value j, with
i != j
.Parameters: labeled : ndarray of integer type
input labeled array
Bc : structure element, optional
out : ndarray of same shape as labeled, dtype=bool, optional
where to store the output. If
None
, a new array is allocatedmode : {‘reflect’, ‘nearest’, ‘wrap’, ‘mirror’, ‘constant’ [default], ‘ignore’}
How to handle borders
Returns: border_img : boolean ndarray
Pixels are True exactly where there is a border in labeled

mahotas.labeled.
border
(labeled, i, j, Bc={3x3 cross}, out={np.zeros(labeled.shape, bool)}, always_return=True) Compute the border region between i and j regions.
A pixel is on the border if it has value i (or j) and a pixel in its neighbourhood (defined by Bc) has value j (or i).
Parameters: labeled : ndarray of integer type
input labeled array
i : integer
j : integer
Bc : structure element, optional
out : ndarray of same shape as labeled, dtype=bool, optional
where to store the output. If
None
, a new array is allocatedalways_return : bool, optional
if false, then, in the case where there is no pixel on the border, returns
None
. Otherwise (the default), it always returns an array even if it is empty.Returns: border_img : boolean ndarray
Pixels are True exactly where there is a border between i and j in labeled

mahotas.labeled.
bwperim
(bw, n=4) Find the perimeter of objects in binary images.
A pixel is part of an object perimeter if its value is one and there is at least one zerovalued pixel in its neighborhood.
By default the neighborhood of a pixel is 4 nearest pixels, but if n is set to 8 the 8 nearest pixels will be considered.
Parameters: bw : ndarray
A blackandwhite image (any other image will be converted to black & white)
n : int, optional
Connectivity. Must be 4 or 8 (default: 4)
mode : {‘reflect’, ‘nearest’, ‘wrap’, ‘mirror’, ‘constant’ [default], ‘ignore’}
How to handle borders
Returns: perim : ndarray
A boolean image
See also
borders
 function This is a more generic function

mahotas.labeled.
filter_labeled
(labeled, remove_bordering=False, min_size=None, max_size=None) Filter labeled regions based on a series of conditions
New in version 1.4.1.
Parameters: labeled : labeled array
remove_bordering : bool, optional
whether to remove regions that touch the border
min_size : int, optional
Minimum size (in pixels) of objects to keep (default is no minimum)
max_size : int, optional
Maximum size (in pixels) of objects to keep (default is no maximum)
Returns: filtered : labeled array
nr : int
number of new labels

mahotas.labeled.
label
(array, Bc={3x3 cross}, output={new array}) Label the array, which is interpreted as a binary array
This is also called connected component labeled, where the connectivity is defined by the structuring element
Bc
.See: http://en.wikipedia.org/wiki/Connectedcomponent_labeling
Parameters: array : ndarray
This will be interpreted as binary array
Bc : ndarray, optional
This is the structuring element to use
out : ndarray, optional
Output array. Must be a Carray, of type np.int32
Returns: labeled : ndarray
Labeled result
nr_objects : int
Number of objects

mahotas.labeled.
labeled_sum
(array, labeled, minlength=None) Labeled sum. sum will be an array of size
labeled.max() + 1
, wheresum[i]
is equal tonp.sum(array[labeled == i])
.Parameters: array : ndarray of any type
labeled : int ndarray
Label map. This is the same type as returned from
mahotas.label()
minlength : int, optional
Minimum size of return array. If labeled has fewer than
minlength
regions, 0s are added to the result. (optional)Returns: sums : 1d ndarray of
array.dtype

mahotas.labeled.
labeled_max
(array, labeled, minlength=None) Labeled minimum.
mins
will be an array of sizelabeled.max() + 1
, wheremins[i]
is equal tonp.min(array[labeled == i])
.Parameters: array : ndarray of any type
labeled : int ndarray
Label map. This is the same type as returned from
mahotas.label()
Returns: mins : 1d ndarray of
array.dtype

mahotas.labeled.
labeled_size
(labeled) Equivalent to:
for i in range(...): sizes[i] = np.sum(labeled == i)
but, naturally, much faster.
Parameters: labeled : int ndarray Returns: sizes : 1d ndarray of int

mahotas.labeled.
relabel
(labeled, inplace=False) Relabeling ensures that
relabeled
is a labeled image such that every label from 1 torelabeled.max()
is used (0 is reserved for the background and is passed through).Example:
labeled,n = label(some_binary_map) for region in xrange(n): if not good_region(labeled, region + 1): # This deletes the region: labeled[labeled == (region + 1)] = 0 relabel(labeled, inplace=True)
Parameters: relabeled : ndarray of int
A labeled array
inplace : boolean, optional
Whether to perform relabeling inplace, erasing the values in
labeled
(default: False)Returns: relabeled: ndarray
nr_objs : int
Number of objects
See also
label
 function

mahotas.labeled.
is_same_labeling
(labeled0, labeled1) Checks whether
labeled0
andlabeled1
represent the same labeling (i.e., whether they are the same except for a possible change of label values).Note that the background (value 0) is treated differently. Namely
is_same_labeling(a, b) implies np.all( (a == 0) == (b == 0) )
Parameters: labeled0 : ndarray of int
A labeled array
labeled1 : ndarray of int
A labeled array
Returns: same : bool
True if the labelings passed as argument are equivalent
See also
label
 function
relabel
 function

mahotas.labeled.
perimeter
(bwimage, n=4, mode="constant") Calculate total perimeter of all objects in binary image.
Parameters: bwimage : array
binary image
n : int, optional
passed to
bwperim
as ismode : str, optional
passed to
bwperim
as isReturns: p : float
total perimeter of all objects in binary image
See also
bwperim
 function Finds the perimeter region
References
[R1] K. Benkrid, D. Crookes. Design and FPGA Implementation of a Perimeter Estimator. The Queen’s University of Belfast. http://www.cs.qub.ac.uk/~d.crookes/webpubs/papers/perimeter.doc

mahotas.labeled.
remove_bordering
(labeled, rsize=1, out={np.empty_like(im)}) Remove objects that are touching the border.
Pass
labeled
asout
to achieve inplace operation.Parameters: labeled : ndarray
Labeled array
rsize : int or tuple, optional
Minimum distance to the border (in Manhatan distance) to allow an object to survive. May be int or tuple with len == labeled.ndim.
out : ndarray, optional
If
im
is passed asout
, then it operates inline.Returns: slabeled : ndarray
Subset of
labeled

mahotas.labeled.
remove_regions
(labeled, regions, inplace=False) removed = remove_regions(labeled, regions, inplace=False):
Removes the regions in
regions
. If an elementwisein
operator existed, this would be equivalent to the following:labeled[ labeled elementwisein regions ] = 0
This function does not relabel its arguments. You can use the
relabel
function for that:removed = relabel(remove_regions(labeled, regions))
Or, saving one image allocation:
removed = relabel(remove_regions(labeled, regions), inplace=True)
This is the same, but reuses the memory in the relabeling operation.
Parameters: relabeled : ndarray of int
A labeled array
regions : sequence of int
These regions will be removed
inplace : boolean, optional
Whether to perform removal inplace, erasing the values in
labeled
(default: False)Returns: removed : ndarray
See also
relabel
 function After removing unecessary regions, it is often a good idea to relabel your label image.

mahotas.labeled.
remove_regions_where
(labeled, conditions, inplace=False) Remove regions based on a boolean array
A region is removed if
conditions[regionid]
evaluates true.This function does not relabel its arguments. You can use the
relabel
function for that:removed = relabel(remove_regions_where(labeled, conditions))
Or, saving one image allocation:
removed = relabel(remove_regions(labeled, conditions), inplace=True)
This is the same, but reuses the memory in the relabeling operation.
See also
remove_regions
 function Variation of this function which uses integer indexing