Table 1. Contactless palmprint preprocessing.

Reference	Background	Segmentation	Description
[6]			It possibly fails in complex background.
[7]	Controllable	Skin color model	The uniformity of the intra-class features is poor, and it is unsuitable for complex backgrounds.
[8,9]			It possibly fails in complex background.
[10,11]		Shape model	The computational complexity is high.
[12]			Users’ freedom is low, and the uniformity of the intra-class features is poor.
[13]	Complex	Assisted	The assisted techniques are complicated, which degrades the comfortability.
[14]			DLSP reduces the localization error and computation complexity, but the invalid area is large.
[15]		Improved active shape model	The computational complexity is high.