Features

By features we mean, basically, numerical functions of the image. That is, any method that gives me a number from the image, I can call it a feature. Ideally, these should be meaningful.

We can classify features into two types:

global
These are a function of the whole image.
local
These have a position and are a function of a local image region.

Mahotas supports both types.

The classification tutorial illustrates the usefulness of feature computation.

If you simply want to compute features from images (without any further processing), you can also use the mahotas-features.py script, which is installed with mahotas (since version 1.4).

Global features

Haralick features

These are texture features, based on the adjacency matrix (the adjacency matrix stores in position (i,j) the number of times that a pixel takes the value i next to a pixel with the value j. Given different ways to define next to, you obtain slightly different variations of the features. Standard practice is to average them out across the directions to get some rotational invariance.

They can be computed for 2-D or 3-D images and are available in the mahotas.features.haralick module.

Only the first 13 features are implemented. The last (14th) feature is normally considered to be unstable, although it is not clear to me why this is. (See this unanswered question on Cross-validated).

Local Binary Patterns

Local binary patterns (LBP) are a more recent set of features. Each pixel is looked at individually. Its neighbourhood is analysed and summarised by a single numeric code. The normalised histogram across all the pixels in the image is the final set of features.

Again, this is an attempt at capturing texture. LBPs are insensitive to orientation and to illumination (scaling).

Threshold Adjacency Statistics

Threshold adjacency statistics (TAS) are a recent innovation too. In the original version, they have fixed parameters, but we have adapted them to parameter-free versions (see Structured Literature Image Finder: Extracting Information from Text and Images in Biomedical Literature by Coelho et al. for a reference). Mahotas supports both.

Zernike Moments

Zernike moments are not a texture feature, but rather a global measure of how the mass is distributed.

Local features

SURF: Speeded-Up Robust Features

Speeded-Up Robust Features (SURF) have both a location (pixel coordinates) and a scale (natural size) as well as a descriptor (the local features).

Read more about SURF.