# The Impact of Pressure on the Fingerprint Impression: Presentation Attack Detection Scheme
## 5 Observations on the Dataset  
In this section, we seek to interpret the influence of pressure on the presented fingerprint trait based on the physical characteristics of genuine and fake fingerprints. The interpretation is conducted by subjectively analyzing the ridge/valley pattern in the sequence of frames for the different presentation types (Figure 2).  
Consequently, assuming that each fingerprint presentation is a sequence of n frames \((F_{i})_{i=1}^{n}\), i.e., \(F_{1},F_{2},\ldots,F_{n}\), the following observations are perceived:  
* In bona fide presentations, the image intensity is consistently increased as i increases. Specifically, once the pressure is performed, the image intensity starts to increase very rapidly, resulting in a darker pattern (Figure 2a). Although pressure causes the rapid increment in the image intensity, it is equally important considering the other fingerprint's phenomena, such as perspiration and elasticity;
* Each PAI species shows a specific behavior during the PAI placement at the sensor. Therefore, each species demonstrates a different reaction to the pressure depending on the PAI characteristics, which are implied by the preparation recipe and used materials (Figure 2b-h);
* A slight shift takes place in the central region of the fingerprint pattern of genuine users after pressure (Figure 3a). Considering elastic and rigid PAI species, Figure 3b,c demonstrate how the ridges/valley shift might be extreme in the gelatin presentation whilst excessively unnoticed in the polish nail attack;
* As successive frames in the fingerprint presentation represent the development of ridge/valley pattern, it is subjectively noticed that the similarity between the successive frames of bona fide presentations slightly varies when pressure is performed. On the contrary, using elastic materials, such as Play-Doh and gelatin, the fingerprint pattern vanishes or degrades after applying pressure (Figure 2b-g). On the other hand, rigid material, such as white glue and polish nails, are likely to demonstrate consistent pattern while i increases, and contrary to bona fide and elastic materials, pressure may enhance the visual pattern in those attacks Figure 2c-e;
* In order to conduct an objective comparison based on the previous point, we calculate the structural similarity index (SSIM) between the successive frames for each presentation type. Figure 4 demonstrates the SSIM vector for each presentation class;
* Despite the fact that attacks may imitate the fingerprint pattern at later frames in the presentation, it is noticed that the early frames show an anomalous development in the ridges/valley pattern (Figure 2c,d,g,h); and,
* Contrary to the latter observation, the size, ridges continuity, and contour's shape of bona fide presentations are developed homogeneously (Figure 2a).  
Figure 3: The impact of pressure on the pattern shape. Each sub-image demonstrates two frames taken from a video and matched. Colors are: initial frame in green, later frame in magenta, and the matching pattern in black. (**a**) Bona fide, (**b**) Gelatin, (**c**) Nail polish.  
Figure 2: The influence of pressure in genuine and attack presentations. Frames are taken to demonstrate the variations at the beginning, mid, end of the presentation (left to right). The videos were captured by an optical sensor.