Modeling of the 3d hand models takes great attention to detail as the hands are one of the most complex body parts. The hands can dramatically change shape with each posture and can perform a wide range of complex movements in a short time.
3d models that are used for still renders tend to be less complex than the ones used in animations or for virtual worlds. The 3d hand models used for animations typically need a specific range of motion.
However, the ones used for virtual worlds are used to interact with the 3D world and a wide range of motion is needed. Also, a more personalized and realistic look is often desired. Therefore, different approaches should be used for modeling for still renders, animations, and virtual worlds.
Using proportions can give you less complex models that can be used for still renders and presentations. However, this approach may not be desirable when looking for very realistic and personalized human hands. See examples of stock 3d hand models.
The first approach is to use the proportions of the human body to size the hand. The length of the hand (from the wrist to the middle finger) should be roughly the length of the inner forearm. The male hands should be longer and wider than female hands. The hand of an adult male is about 189mm long and about 84mm wide, while the female hand is 172mm long and about 74mm wide. Also, the fingers should not be perfectly straight, instead they should have a tendency to curve slightly towards the middle finger.
The second approach is to use approximation to establish the right proportions. When using approximation techniques, the proportions are estimated using the existing features of the hand anatomy. For example, the creases of the palm, hand contours, and the wrinkles on the back of the finger knuckles typically mark the areas where the skin is attached to the tendon beneath the skin.
Other techniques involve generating a series of hand postures from real human hands, which yield hand movements and gestures that are very realistic. For this technique the hand 3d models are made up of both a dynamic and a surface model. The dynamic model is used to generate the hand postures. The surface model is based on 3D shape of a real human hand. This surface model is used to generate the hand shape based on the posture obtained from the dynamic model.
Motion capture and analysis is used to capture and process the hand movements. This can yield very realistic and accurate movements of the human hand. For example, for an animation of a hand making a fist, the 3d hand model fingers would need to curl in a spiral and not a circle. To achieve this, the movement of the joints of a real human (and their relative position in space) would be captured and recorded. Then this recorded movement would be assigned to the fingers so that they curl in a spiral making a fist.
Regardless of the approach you use, modeling of the human hand is complex and requires great attention to the shape, gestures, and movement of the hand.