Getting started with Bob

The following tutorial constitutes a suitable starting point to get to know how to use Bob’s packages and to learn its fundamental concepts.

They all rely on the lab-like environment which is Python. Using Bob within a Python environment is convenient because:

  • you can easily glue together all of the components of an experiment within a single Python script (which does not require to be compiled),

  • scripts may easily rely on other Python tools like SciPy as well as Bob, and

  • Python bindings are used to transparently run the underlying efficient C++ compiled code for the key features of the library.

Multi-dimensional Arrays

The fundamental data structure of Bob is a multi-dimensional array. In signal processing and machine learning, arrays are a suitable representation for many different types of digital signals such as images, audio data and extracted features. Python is the working environment selected for this library and so when using Python we have relied on the existing NumPy multi-dimensional arrays numpy.ndarray. This provides with greater flexibility within the Python environment.

At the C++ level, the Blitz++ library is used to handle arrays. Bob provides internal conversion routines to transparently and efficiently convert NumPy ndarrays to/from Blitz++. As they are done implicitly, the user has no need to care about this aspect and should just use NumPy ndarrays everywhere while inside Python code.

For an introduction and tutorials about NumPy ndarrays, just visit the NumPy Reference website. For a short tutorial on the bindings from NumPy ndarrays to Blitz++, you can read the documentation of our Blitz++/Python Arrays package.


Many functions in Bob will return multi-dimensional arrays type bob.blitz.array, which are wrapped by as a numpy.ndarray. While you can use these arrays in all contexts inside Bob, NumPy and Scipy, some functionality of the numpy.ndarray are not available. In particular, resizing the arrays with numpy.ndarray.resize will raise an exception. In such cases, please make a copy of the array using numpy.ndarray.copy.

Digital signals as multi-dimensional arrays

For Bob, we have decided to represent digital signals directly as numpy.ndarray rather than having dedicated classes for each type of signals. This implies that some convention has been defined.

Vectors and matrices

A vector is represented as a 1D NumPy array, whereas a matrix is represented by a 2D array whose first dimension corresponds to the rows, and second dimension to the columns.

>>> import numpy
>>> A = numpy.array([[1, 2, 3], [4, 5, 6]], dtype='uint8') # A is a matrix 2x3
>>> print(A)
[[1 2 3]
 [4 5 6]]
>>> b = numpy.array([1, 2, 3], dtype='uint8') # b is a vector of length 3
>>> print(b)
[1 2 3]


Grayscale images are represented as 2D arrays, the first dimension being the height (number of rows) and the second dimension being the width (number of columns). For instance:

>>> img = numpy.ndarray((480,640), dtype='uint8')

img which is a 2D array can be seen as a gray-scale image of dimension 640 (width) by 480 (height). In addition, img can be seen as a matrix with 480 rows and 640 columns. This is the reason why we have decided that for images, the first dimension is the height and the second one the width, such that it matches the matrix convention as well.

Color images are represented as 3D arrays, the first dimension being the number of color planes, the second dimension the height and the third the width. As an image is an array, this is the responsibility of the user to know in which color space the content is stored. Bob’s Color Conversion Routines provides functions to perform color-space conversion:

>>> import bob.ip.color
>>> colored = numpy.ndarray((3,480,640), dtype='uint8')
>>> gray = bob.ip.color.rgb_to_gray(colored)
>>> print (gray.shape)
[480 640]


A video can be seen as a sequence of images over time. By convention, the first dimension is for the frame indices (time index), whereas the remaining ones are related to the corresponding image frame. More information about loading and handling video sources can be found in Bob’s Video I/O Routines.

Audio signals

Audio signals in Bob are represented as 2D arrays: the first dimension being the number of channels and the second dimension corresponding to the time index. For instance:

>>> import
>>> audio ="test.wav")
>>> audio.rate
>>> signal = audio.load()
>>> signal.shape
(1, 268197)

Bob’s Audio I/O Routines supports loading a variety of audio files. Please refer to its documentation for more information.


You can also use to load wav files in Python but the returned data is slightly different compared to In Scipy the first dimension corresponds to the time index rather than the audio channel. Also in Scipy, the loaded signal maybe an int8 or int16 or something else depending on the audio but always returns the data as float arrays. We recommend using since it supports more audio formats and it is more consistent with the rest of Bob packages.

Input and output

The default way to read and write data from and to files with Bob is using the binary HDF5 format which has several tools to inspect those files. Bob’s support for HDF5 files is given through the Bob’s Core I/O Routines package.

On the other hand, loading and writing of different kinds of data is provided in other Packages of Bob using a plug-in strategy. Many image types can be read using Bob’s I/O Routines for Images of Various type, and many video codecs are supported through the Bob’s Video I/O Routines plug-in. Also, a comprehensive support for MatLab files is given through the Matlab(R) I/O Support for Bob interface.

Additionally, Bob’s Core I/O Routines provides two generic functions and to load and save data of various types, based on the filename extension. For example, to load a .jpg image, simply call:

>>> import
>>> import #under the hood: loads Bob plug-in for image files
>>> img ="myimg.jpg")

Image processing

The image processing module is split into several packages, where most functionality is contained in the Bob’s Basic Image Processing Routines module. For an introduction in simple affine image transformations such as scaling and rotating images, as well as for more complex operations like Gaussian or Sobel filtering, please refer to the Bob’s Basic Image Processing Routines. Also, simple texture features like LBP’s can be extracted using bob.ip.base.LBP.

Gabor wavelet functionality has made it into its own package Bob’s Gabor wavelet routines. A tutorial on how to perform a Gabor wavelet transform, extract Gabor jets in grid graphs and compare Gabor jets, please read the Bob’s Gabor wavelet routines.

Machine learning

Machines and Trainers are one of the core components of Bob. Machines represent statistical models or other functions defined by parameters that can be trained or set by using trainers. Two examples of machines are multi-layer perceptrons (MLPs) and Gaussian mixture models (GMMs).

The operation you normally expect from a machine is to be able to feed a feature vector and extract the machine response or output for that input vector. It works, in many ways, similarly to signal processing blocks. Different types of machines will give you a different type of output. Here, we examine a few of the machines and trainers available in Bob.

Database interfaces

Bob provides an API to easily query and interface with well known databases. A database contains information about the organization of the files, functions to query information such as the data which might be used for training a model, but it usually does not contain the data itself (except for some toy examples). Please visit Bob Database for an excellent guide on Bob’s datbases.

Bob includes a (growing) list of supported database interfaces. There are some small toy databases like Iris Flower Data Set and the MNIST Database Interface database can be used to train and evaluate classification experiments. For the former, a detailed example on how to use Bob’s machine learning techniques to classify the Iris flowers is given in Tutorial: Analysis of the Fisher Iris Dataset.

However, most of the databases contain face images, speech data or videos that are used for biometric recognition and presentation attack detection (anti-spoofing). A complete (and growing) list of database packages can be found in our Packages.

Several databases that can be used for biometric recognition share a common interface, which is defined in the package. Generic functionality that is available in all verification database packages is defined in the, while a list of databases that implement this interface can be found in,,, or any other biometric package depending on the modality of the database.

Performance evaluation

Methods in the Bob’s Metric Routines module can be used evaluate error for multi-class or binary classification problems. Several evaluation techniques such as Root Mean Squared Error, F-score, Recognition Rates, False Acceptance and False Rejection Rates, and Equal Error Rates can be computed, but also functionality for plotting CMC, ROC, DET and EPC curves are described in more detail in the Bob’s Metric Routines.