## 2 Vein Pattern Identification System
### Feature Extraction Process
#### 2.3.2 Local Dorsal Hand Vein Feature Extraction  
There are many different types of veins. [PERSON] et al. [47] classified veins into seven types, namely termination, bifurcation, lake, independent ridge, dot, spur and crossover. With their unique characteristics across individuals, vein patterns can potentially serve as biometrics. Human recognition using vein pattern biometrics can be roughly classified into two main categories: (i) minutia based and (ii) non-minutia based. For minutia-based vein pattern identification [23; 41; 42; 43; 44; 45; 46; 48; 49; 50; 51; 52; 53], minutia points, such as the point of vein termination or bifurcation, are extracted and further used for human recognition. Non-minutia-based vein pattern identification is based on matching between the vein patterns of unknown identity with that of the reference directly, without extracting minutia point information [27]. In this study, we have opted for a minutia extraction approach. The minutia points used here are the vein terminations and bifurcations. With the extracted minutia points, a set of geometric invariant features are constructed and further used to establish the correspondence between two sets of minutiae, using the one from the query image and the other from the reference image. Once the correspondence is established, the geometric transformation parameters are computed to align the query with the reference image. The quantitative similarity criterion is then computed and used for human recognition. Our process of minutia-based feature extraction from vein patterns is divided into two subprocesses: (i) vein extraction, and (ii) minutia extraction, as explained in more detail below. Figure 5: Robustness of hand centroid point extraction of distance transform image. Figure 6: (**a**) Distance transform, (**b**) radius distance plot, (**c**) extracted fiducial points. A. Vein Pattern Extraction  
The vein pattern extraction process is started by converting the color palm image from the image acquisition system into a grayscale image. Veins in the grayscale image appear dark, as deoxyhemoglobin contained within the blood inside the veins absorbs more infrared light. As a result, less infrared light is transmitted to the IR-sensitive camera. Vein extraction from the grayscale image is subdivided into two steps: (i) region of interest (ROI) extraction and (ii) vein extraction. The process of vein extraction is shown in Figure 7.  
1 ROI Extraction  
To improve the efficiency of human recognition using vein patterns, rather than using the whole vein image, a defined ROI is used. From the hand geometric feature extraction process, we have determined U2 to be the point on the contour from T2 with equal distance when traversing from T2 to V2 and, similarly, U3 to be the point on the contour from T5 with an equal distance when traversing from T5 to V4. P1 can then be marked as the middle point between U2 and V2. P2 can be marked as the middle point between V4 and U3, as shown in Figure 8a. The coordinates of P3 and P4 are then calculated as in Equations (2) and (3). \[P3\{x,y\}=P3\{\text{xp2}+(\text{yp2}-\text{yp1}),\text{yp2}+(\text{xp2}-\text{ xp1})\}, \tag{2}\]  
and  
\[P4\{x,y\}=P4\{\text{xp1}+(\text{yp2}-\text{yp1}),\text{yp1}+(\text{xp2}-\text{ xp1})\}. \tag{3}\]  
This ROI bounds a section of the vein pattern inside a square box, having P1, P2, P3 and P4 as the vertices. An identified ROI is shown in Figure 8b, with the corresponding derived vein pattern shown in Figure 8c. Figure 8: Vein pattern ROI extraction: (**a**) reference point for ROI; (**b**) ROI for vein extraction; and (**c**) result of ROI. Figure 7: Vein extraction process, including (i) region of interest (ROI) extraction and (ii) vein extraction. Step 3: Vein Detection  
Figure 11 depicts the vein extraction process. The intensity profile is shown in Figure 11a; after applying Equation (4), the graph of curvature was produced, as shown in Figure 11c. To mitigate noise disturbance, the curvature threshold is set as annotated with solid lines in Figure 11c.