Detection stage is described in a documentation of ``objdetect`` module of general OpenCV documentation. Documentation gives some basic information about cascade classifier.
Current guide is describing how to train a cascade classifier: preparation of a training data and running the training application.
There are two applications in OpenCV to train cascade classifier: ``opencv_haartraining`` and ``opencv_traincascade``. ``opencv_traincascade`` is a newer version, written in C++ in accordance to OpenCV 2.x API. But the main difference between this two applications is that ``opencv_traincascade`` supports both Haar [Viola2001]_ and LBP [Liao2007]_ (Local Binary Patterns) features. LBP features are integer in contrast to Haar features, so both training and detection with LBP are several times faster then with Haar features. Regarding the LBP and Haar detection quality, it depends on training: the quality of training dataset first of all and training parameters too. It's possible to train a LBP-based classifier that will provide almost the same quality as Haar-based one.
``opencv_traincascade`` and ``opencv_haartraining`` store the trained classifier in different file formats. Note, the newer cascade detection interface (see ``CascadeClassifier`` class in ``objdetect`` module) support both formats. ``opencv_traincascade`` can save (export) a trained cascade in the older format. But ``opencv_traincascade`` and ``opencv_haartraining`` can not load (import) a classifier in another format for the futher training after interruption.
*``opencv_createsamples`` is used to prepare a training dataset of positive and test samples. ``opencv_createsamples`` produces dataset of positive samples in a format that is supported by both ``opencv_haartraining`` and ``opencv_traincascade`` applications. The output is a file with \*.vec extension, it is a binary format which contains images.
*``opencv_performance`` may be used to evaluate the quality of classifiers, but for trained by ``opencv_haartraining`` only. It takes a collection of marked up images, runs the classifier and reports the performance, i.e. number of found objects, number of missed objects, number of false alarms and other information.
Since ``opencv_haartraining`` is an obsolete application, only ``opencv_traincascade`` will be described futher. ``opencv_createsamples`` utility is needed to prepare a training data for ``opencv_traincascade``, so it will be described too.
For training we need a set of samples. There are two types of samples: negative and positive. Negative samples correspond to non-object images. Positive samples correspond to images with detected objects. Set of negative samples must be prepared manually, whereas set of positive samples is created using ``opencv_createsamples`` utility.
Negative samples are taken from arbitrary images. These images must not contain detected objects. Negative samples are enumerated in a special file. It is a text file in which each line contains an image filename (relative to the directory of the description file) of negative sample image. This file must be created manually. Note that negative samples and sample images are also called background samples or background samples images, and are used interchangeably in this document. Described images may be of different sizes. But each image should be (but not nessesarily) larger then a training window size, because these images are used to subsample negative image to the training size.
Positive samples are created by ``opencv_createsamples`` utility. They may be created from a single image with object or from a collection of previously marked up images.
Please note that you need a large dataset of positive samples before you give it to the mentioned utility, because it only applies perspective transformation. For example you may need only one positive sample for absolutely rigid object like an OpenCV logo, but you definetely need hundreds and even thousands of positive samples for faces. In the case of faces you should consider all the race and age groups, emotions and perhaps beard styles.
So, a single object image may contain a company logo. Then a large set of positive samples is created from the given object image by random rotating, changing the logo intensity as well as placing the logo on arbitrary background. The amount and range of randomness can be controlled by command line arguments of ``opencv_createsamples`` utility.
Background color (currently grayscale images are assumed); the background color denotes the transparent color. Since there might be compression artifacts, the amount of color tolerance can be specified by ``-bgthresh``. All pixels withing ``bgcolor-bgthresh`` and ``bgcolor+bgthresh`` range are interpreted as transparent.
The source image is rotated randomly around all three axes. The chosen angle is limited my ``-max?angle``. Then pixels having the intensity from [``bg_color-bg_color_threshold``; ``bg_color+bg_color_threshold``] range are interpreted as transparent. White noise is added to the intensities of the foreground. If the ``-inv`` key is specified then foreground pixel intensities are inverted. If ``-randinv`` key is specified then algorithm randomly selects whether inversion should be applied to this sample. Finally, the obtained image is placed onto an arbitrary background from the background description file, resized to the desired size specified by ``-w`` and ``-h`` and stored to the vec-file, specified by the ``-vec`` command line option.
Positive samples also may be obtained from a collection of previously marked up images. This collection is described by a text file similar to background description file. Each line of this file corresponds to an image. The first element of the line is the filename. It is followed by the number of object instances. The following numbers are the coordinates of objects bounding rectangles (x, y, width, height).
Image img1.jpg contains single object instance with the following coordinates of bounding rectangle: (140, 100, 45, 45). Image img2.jpg contains two object instances.
The scheme of samples creation in this case is as follows. The object instances are taken from images. Then they are resized to target samples size and stored in output vec-file. No distortion is applied, so the only affecting arguments are ``-w``, ``-h``, ``-show`` and ``-num``.
``opencv_createsamples`` utility may be used for examining samples stored in positive samples file. In order to do this only ``-vec``, ``-w`` and ``-h`` parameters should be specified.
Note that for training, it does not matter how vec-files with positive samples are generated. But ``opencv_createsamples`` utility is the only one way to collect/create a vector file of positive samples, provided by OpenCV.
Example of vec-file is available here ``opencv/data/vec_files/trainingfaces_24-24.vec``. It can be used to train a face detector with the following window size: ``-w 24 -h 24``.
The next step is the training of classifier. As mentioned above ``opencv_traincascade`` or ``opencv_haartraining`` may be used to train a cascade classifier, but only the newer ``opencv_traincascade`` will be described futher.
Maximal desired false alarm rate for each stage of the classifier. Overall false alarm rate may be estimated as (max_false_alarm_rate^number_of_stages).
Maximal count of weak trees for every cascade stage. The boosted classifier (stage) will have so many weak trees (``<=maxWeakCount``), as needed to achieve the given ``-maxFalseAlarmRate``.
Selects the type of Haar features set used in training. ``BASIC`` use only upright features, while ``ALL`` uses the full set of upright and 45 degree rotated feature set. See [Rainer2002]_ for more details.
After the ``opencv_traincascade`` application has finished its work, the trained cascade will be saved in cascade.xml file in the folder, which was passed as ``-data`` parameter. Other files in this folder are created for the case of interrupted training, so you may delete them after completion of training.
..[Viola2001] Paul Viola, Michael Jones. *Rapid Object Detection using a Boosted Cascade of Simple Features*. Conference on Computer Vision and Pattern Recognition (CVPR), 2001, pp. 511-518.
..[Rainer2002] Rainer Lienhart and Jochen Maydt. *An Extended Set of Haar-like Features for Rapid Object Detection*. Submitted to ICIP2002.
..[Liao2007] Shengcai Liao, Xiangxin Zhu, Zhen Lei, Lun Zhang and Stan Z. Li. *Learning Multi-scale Block Local Binary Patterns for Face Recognition*. International Conference on Biometrics (ICB), 2007, pp. 828-837.
