## 3 System Building Blocks
### Acquisition
#### 3.1.1 Studied Camera Types Technologies and Selection  
Plenoptic (or light-field) cameras present a different architecture from conventional cameras. This difference lies in the fact that conventional cameras are composed of the main lens and an image sensor, whereas plenoptic cameras have a microlens array between the image sensor and the main lens of the camera. This microlens array allows the light field to be captured from various points of view, forming a 4D light field with a 2D image, so that it is possible to estimate the depth of the scene [29]. This estimate of the depth is obtained by using the redundancy created by multi-view geometry, where a 3D point is projected onto the image several times.  
There are two kinds of plenoptic cameras: standard and multifocus. These two cameras were studied using the Lytro Illum camera and the Raytrix R42 camera, respectively. The difference is mainly the focal length of the lenses. The focal length of the microlenses, in a standard plenoptic chamber [30], corresponds to the distance between the image sensor and the array of microlenses; thus, all microlenses have the same focal length. Each lens contributes only one pixel value to the final image, with the resolution of this image equal to the number of microlenses. This feature drastically reduces the image resolution compared to sensor capacity; however, the computational power required to process the  
Figure 1: The pipeline of the proposed application.  
image is also lower, making it suitable for compact cameras. An example of this type of camera is the Lytro Illum in Figure 2 (left). On the other hand, the multifocus plenoptic camera has the microlens array placed in front of the image sensor, where each microlens has a different focal length from its neighboring lenses, which are thus classified as different lens types. This design allows a better combination between effective resolution and depth of field size, resulting in higher-resolution images.  
An example of a multifocus plenoptic camera is the R42 camera model developed by Raytrix GmbH, presented in Figure 2 (right). Since they are multifocal (there are micro lenses with three different focal lengths), these cameras have additional calibration issues. As such, there is a large number of parameters that need to be adjusted, making it disadvantageous.  
Two cameras of the time-of-flight camera type were also studied: the DepthSense325 and the RealSense (Figure 2). These camera types employ time-based imaging, processing distance estimation based on the speed of light by measuring the time spent in the round trip for a light signal between the camera and the object for each image point. These cameras use an artificial light (provided by a laser or LED) to estimate this distance for each point in the image. They are also affordable and relatively easy to use. This type of camera, due to the simplicity of its use and the results presented, was selected to be included in this application.  
The technology of stereo cameras was also taken into consideration for the defined purpose. A stereo camera is a type of camera that possesses two lenses, with an image sensor on each lens. This camera calculates the disparity between the two images at different positions, allowing them to simulate human binocular vision. This provides the ability to capture three-dimensional images--a process known as stereoscopy that can be used to create 3D images. The camera employed for the tests was the Stereo ZED, shown in Figure 3. For this camera, the acquired images revealed that the distance between the camera and the object was not adequate for face images, as the minimum distance for acquisition was larger than the distance used to capture a face image, hence the reconstruction process that estimates the depth parameter did not display reliable values.