**2.1 Comparisons of the current palmprint databases**

**Table 1** summarizes the current palmprint databases. Some basic information is compared. In **Table 1**, official ROI means whether the official ROI images are provided; localization code means whether the corresponding ROI extraction code is released. Some sample images of these databases are shown in **Figure 1**.

### *2.1.1 The Hong Kong Polytechnic University (PolyU) Palmprint Database*

The PolyU Palmprint Database is the first publicly available palmprint database. It contains 7752 images captured from 386 hands in two sessions; around 10 images are collected for each palm in each session. The palmprint acquisition device is a contact-based device that consists of a high-quality industrial monochrome camera and a well-selected ring light source. The palm pose also is restricted by the pegs. So the captured images have high image qualities. However, the released image resolution, 384 284 pixels, is relatively low.
