The senses of smell and taste are less understood than the other senses. There are on the order of 5,000-10,000
different types of odor receptors in the human nose, and it is thought that each one is trained to sense a
different part of a scent. The two main technical problems with olfactory displays are scent generation and
scent delivery. Much like generating visual cues, which are created by combining pixels of various colors, the goal of scent
generation is the combination of different chemical and natural substances in order to produce the desired scent.
Unlike visual stimuli, however, no "base smells" have been identified. The most common approach to scent
generation, therefore, is to use off-the-shelf substances, instead of mixing several scents together at a lower
level. Once a scent has been selected, it must be delivered to the nose. Both public and private
approaches have been attempted, and more-recent approaches attempt to combine the low-cumber characteristic of
public displays with the individuality of private displays.