Technologies of calculation of visitors

Automatic calculation of visitors uses great practical and commercial interest. Calculation of visitors gives the chance to control attendance, to make monitoring of efficiency of marketing efforts, allows to increase security of an object.

The systems of calculation of visitors are installed first of all on sphere objects retail (in shopping centers, shops) and also at the enterprises, football stadiums, the subways and other places of accumulation of people. Data on traffic of visitors can be the basis for carrying out marketing efforts and quality evaluation of work of personnel, allow to optimize resource allocation of the enterprise and also are useful to operational acceptance of other management decisions.

There is a number of methods of automatic calculation of visitors: on the basis of tourniquets, sensors (thermal and intersections of an infrared beam), on the basis of the analysis of a video flow. In comparison with other methods, the analysis of a video flow wins because does not delay visitors as mechanical tourniquets, and can work at the bigger density of people in difference from heat and infrared sensors.

It is possible to select several approaches to the solution of a problem of calculation of last people using the analysis of a video flow, in this article two are considered: direct and indirect^[1].

Direct approach. Calculation of visitors on the basis of tracking

This method means creation of trajectories of movement of moving objects of a frame and fixing of intersection of the virtual line of an input and output. Let's consider in more detail an operation algorithm of tracking and methods of creation of trajectories.

Creation of trajectories. Method 1

For creation of trajectories of movement the analysis of the sequence of the personnel of a video flow at which there are moving objects is kept. Generally at one frame there can be several moving objects therefore to the program it is necessary not only to construct trajectories, but also to distinguish objects and their movements. When moving objects cross the line on one, any complexity with calculation is not present: the task comes down to direction finding of intersection of the line.

The calculation method based on the simplest implementation of tracking, analyzing foreground objects (moving objects) on the consecutive personnel can cope with this task. At first the movement areas different from the background image, on the current and previous frame are selected, further, analyzing the speed, the direction of the movement of objects and also their sizes, transition probabilities of objects from one point of a trajectory of the previous frame in other point of current are calculated. The most probable movements of each object develop in a trajectory.

Creation of trajectories. Method 2

Objects in a frame can move differently: their trajectories can be crossed or blocked, and the zones of the movement corresponding to objects - to integrate to one area. In that case the program needs to reveal each object, to separate object groups and it is correct to count the people crossing the virtual line in this or that direction.

In these cases the problem of creation of an exact trajectory of separate objects becomes complicated. The tracking which is based on a method of creation of a trajectory on two frames is not suitable for the analysis of similar difficult movements, it gives a high error. The calculation method using tracking on the basis of technology of the analysis of the sequence of the personnel and continuous post-processing of the received results copes with such situations. The program builds columns - analyzes transitions of objects from one status (provision) in another. Besides, speeds and the directions of the movement, provision, color responses are analyzed. As result a set of the most probable movements of an object forming a trajectory is issued.

The difference of two methods (method 1 and 2) consists what when processing the sequence of the personnel is considered both the current provision of an object, and the history of its transitions that allows to increase accuracy in difficult situations of intersection of the movement, disappearances and emergence of an object.

In more detail methods of creation of trajectories of moving objects are described in article "Tracking of objects in a video flow. Methods of creation of trajectories"

Separately it is necessary to consider a situation when visitors cross the line of an input and output groups. Correctly to consider people, it is necessary to define precisely their quantity in groups. It can be done in several ways. The first of them detects the heads: the program defines how many objects classified as the head crossed the virtual line of calculation. Other method analyzes the area of the object, crossing the line, and compares it to the average area of the person for this video series. Such approach allows to estimate the number of people in a moving object.

The calculation method on the heads is more exact as an object of the bigger area can be not only group of people, but also, for example, the person with a large subject (for example, with the cart).

The most exact result is achieved at small mutual overlapping of moving objects, in a situation when they can be separated easily from each other.

Indirect approach. Calculation of visitors on the basis of the analysis of an optical flow

If the direct method of calculation of visitors finds an object in a video flow and monitors its movements, then the indirect method watches the virtual line of an input and output and analyzes the movement of color pixels through it. The method monitors relocation of the area of a certain brightness and a certain color through the line, calculates characteristics of features of the image (edges, corners, special points, information on texture, etc.). At the same time the method only fixes the fact of movement of some object through the line, but does not define what an object how many people move in this object is. For determination of number of the people who crossed the line methods of detecting of the heads and the analysis of the area of a moving object are also used.

The indirect method is applicable for a dense flow of people when traditional methods of tracking of moving objects (tracking) are unsuitable. The most exact result is achieved when the flux density of people is approximately homogeneous

Calculation accuracy

There are several approaches to calculation of visitors, in this article two main methods were considered. The first of them, a straight line, uses results of algorithms of creation of trajectories of moving objects (tracking); the second, indirect, the method works with an optical flow. For increase in accuracy of calculation the number of people decides on the help of detecting of images of the heads and also the analysis of the area of objects, crossing the line of an input and output is kept.

The large role as operation of modules of calculation is played by arrangement of the video camera with which according to the image calculation is conducted. The most exact calculation can achieve at "zenith" arrangement of the video camera: the device should be placed under a ceiling, is directed or vertically down, or with a small deviation from a vertical.

Notes