*2.2. Image Preprocessing*

As mentioned above, dorsal hand vein images captured by the same person on different devices also have great differences in brightness, noise, size and rotation angle [12]. These factors will have a great impact on the recognition results, and simple scale normalization is not conducive to extract texture features of samples. In this paper, a centroid adaptive method was used to determine the ROI region of dorsal hand vein images. Find the centroid *C*(*x*0, *y*0) of the ROI hand vein image through the length and width of the vein area, and the centroid was taken as the center of the maximum inscribed circle of dorsal hand vein area which is shown in Figure 3a. The diameter (d) of the maximum inscribed circle, was taken as the standard of size normalization. After scale normalization, the ROI region with a size of 400 × 400 is intercepted, as shown in Figure 3b.

**Figure 3.** Captures the region of interest (ROI) area. (**a**) Maximum inscribed circle; (**b**) ROI area.

In addition, because of the difference about lighting condition and each thickness of hand, distribution of gray level in images can't be equal. Thus, we need to normalize gray level from 0 to 255 by Formula (1).

$$N(\mathbf{x}, y) = ((R(\mathbf{x}, y) - \min) \times 255) / (\max - \min) \tag{1}$$

where *R*(*x*, *y*) represents image gray level of the ROI region, max and min represent respectively the maximum and minimum gray value of images, *N*(*x*, *y*) represents the normalized gray level. The result after gray normalization is shown in Figure 4.

**Figure 4.** Gray-normalized ROI dorsal hand vein image.

In order to obtain the texture contour of dorsal hand vein, gradient based image segmentation method [13] was adopted in this paper. The segmented binary image is shown in Figure 5.

**Figure 5.** The segmented binary image.

However, the binary image may lose lots of gray information, therefore, multiplying inverted binary image and normalized gray image to obtain the gray image that only retains the contour of dorsal hand vein, as shown in Figure 6.

**Figure 6.** The gray image that only retains the contour of dorsal hand vein.
