## 2 Related Work
### 3D Vision Technologies  
A 3D data acquisition is possible with the use of the following technologies:  
* Structured-light 3D scanners scan a 3D shape by emitting precisely defined light patters such as stripes and recording distortions of light on objects [14].
* Light Detection and Ranging (LIDAR) measures distances to a series of distant points by aiming a laser in different directions [15].
* Time-of-flight cameras (TOF) operate on a similar principle as LIDAR however instead of redirecting a measuring laser the entire measurement of distances is performed using a single light beam [16].
* Structure from motion and multi-view stereo technologies record a 3D shape of an object by analyzing numerous images taken from different points of view located around the object [17] or by moving the object in front of a camera [18].
* Stereo cameras resemble 3D imaging with the use of a pair of eyes. Stereo cameras record relative differences between locations of objects in images taken from two constituent cameras. The extent of these disparities depends on distances between a stereo camera and objects located in the field of view. The closer the object is, the greater is the disparity [3].
* Camera arrays operate on similar principles as stereo cameras, however it takes advantage of a greater number of cameras [19; 20].  
3D imaging technologies have different features. None of these methods is the most suitable one in all circumstances. This paper contributes to development of methods for acquiring 3D data on the basis of a set of images obtained from a camera array. As far as camera based systems are concerned there are applications in which this equipment is especially valuable. Main advantages of a camera array are the following.  
* It can be used in highly illuminated areas. It is not possible to use structured-light 3D scanners in intensive natural light because external light sources interfere with the measurement performed by this kind of a sensor.
* An array provides dense data concerning distances to parts of objects visible in the 3D space. In contrast to LIDARs and TOF cameras which can provide only sparse depth maps.
* The technology of camera arrays can be flexibly used for measuring both small and large distances depending on a size of used cameras and types of their lens. Such a functionality is very limited when other kinds of 3D imaging devices are used.
* 3D imaging with the use of array does not require relocating the imaging device to different positions. It is necessary when the technology of structure from motion or multi-view stereo (MVS) is used.
* An array is a compact device which can be inexpensive if low-cost cameras are used.
* The weight of an array constructed from small-sized cameras is low, therefore it can be mounted on moving parts of an autonomous robot (e.g., robotic arms) without putting much load on servos or other mechanisms driving the robot.  
These advantages apply to both arrays and stereo cameras. The major disadvantage of a stereo camera is such that the quality of its results is lower than the quality obtained from other 3D imaging devices. This problem is reduced if camera arrays is used, in particular the kind of an array described in this paper (Section 3).  
Another disadvantage of camera-based systems is such that it is time-consuming to process images from cameras in order to retrieve 3D data. This problem is even more serious in the case of using an array, as the number of cameras is greater. However, taking into account a rising speed of computer devices a greater number of computations is a reasonable cost of improving the quality of 3D imaging. A disadvantage of an array in comparison to a stereo camera is also the price of a greater number of constituent cameras. However, as in case of greater computational requirements, it is not a significant cost of obtaining a higher quality of 3D data.