**4.3 ROI extraction for PolyU Palmprint Database**

For PolyU database, which contains 7752 images, the line-scan-based method is utilized to localize the ROI. At last, 42 samples failed to be localized. As is shown in **Figure 9**, most of them are caused by small finger valleys (palm pose) and unideal palm region segmentations (only grayscale information can be utilized). In **Table 2**, only the user ID is listed for the PolyU database.

region can be seen in **Figure 6**. For palm skin patches, both the bright and dark regions are selected to learn a precise classification plane. In the color space, the palm can be easily segmented from the unicolor background. After palm region segmentation, *vp*<sup>1</sup> and *vp*<sup>2</sup> are detected by the local-extremum-based method. **Results: Figure 6** shows the IITD samples that are hard to process. If we cannot detect five fingertips and four finger valley points after palm segmentation, the system will directly return by giving an error code (as is shown in **Figure 6(a)** and **(c)**). If false keypoints are detected due to difficult palm poses, the finally extracted

*Images cannot be localized in IITD database. (a) and (c) are the finger detection failed samples;*

ROI images are abnormal images which should be discarded in real-world

The line-scan-based keypoint detection method is used for COEP. **Figure 7** shows the ROI localization results on COEP database. Since the pegs used in their imaging setup may interfere the keypoint detection algorithm, we should delete

applications (as is shown in **Figure 6(b)** and **(d)**).

*(b) and (d) are the ROI localization error samples.*

**Figure 6.**

*Biometric Systems*

**Figure 7.**

**60**

*ROI localization result of the COEP database.*

**4.2 ROI extraction for COEP Palmprint Database**

#### **Figure 8.**

*Images cannot be localized of COEP database. (a)–(d), (e)–(h), and (i)–(l) are the original, binary, and ROI localization images, respectively. The image ID of (a), (e), and (i) is 0103\_0004; the image ID of (b), (f), and (j) is 0145\_0003; the image ID of (c), (g), and (k) is 0159\_0006; and the image ID of (d), (h), and (l) is 0164\_0002.*

**Figure 9.** *Hard samples of PolyU.*
