# Aroma Release of Olfactory Displays Based on Audio-Visual Content
## 2 Literature Review
### Olfactory Displays  
Olfactory displays are devices designed to release scents into the environment. They are classified into two types: \"wearable\", which are placed either on-body or on-head, and \"environmental\", which are placed in the physical environment [17].  
A wearable and fashionable olfactory necklace called Essence was designed in [18]. The Essence is able to release scents automatically based on data from the virtual context such as the location and current time of the users as well as on physiological data such as brain activity and heart rate. Moreover, the necklace can be activated manually, and the intensity of scents can be controlled through the stretch necklace thread. The results of the user experience show that the device is small enough and comfortable to be worn in most daily life activities. However, the device was unable to release multilabel scents at a time, and released one scent for one case based on the chosen user.  
A smelling screen is an olfactory display embedded in a Liquid-Crystal Display (LCD) screen to generate and distribute odor along the screen based on the image shown [19]. The proposed device consists of four fans located at the corners of the screen to generate airflow that collides multiple times. Then, the airflow blows toward the user through tubing from the airflow collision point, which is considered as the odor source. However, the time between releasing the scents and its recognition by the user is not synchronized due to the delay of the scent reaching the olfactory organ.  
An olfactory display named inScent [20] can be worn as a necklace that enables the user to receive scented notifications. The device was built to hold eight aromas where the scent is exchangeable through a small cartridge. Scents are triggered either manually by the users, or remotely by the instructor via an Android application based on the correlated scenario like scent that reflects the emotional link of the message sender and generates scents by using heating and a small fan to blow airflow toward the user. However, heating a wearable device may cause discomfort to the wearer.  
Another study [21] proposed a thermal/heating approach to distribute scents from generation of the olfactory aromas. The device unit was built to hold eight aroma dispensers; each one containing a capillary tube, speed control fan, gas sensor to measure the release rate, and temperature to control the heating elements. The user controls the intensity of the aroma and fan speed as well as selects the aroma to be released by a software application. However, the heating approach might destroy the chemical components, which may limit the range of odors.  
The authors in [5] presented a placed-in environment olfactory display that released six scents based on the visible content displayed by using an Inception v3 model for image recognition. However, visual elements were only associated with scents.  
Overall, wearable devices can cause discomfort to users, thus hindering immersion into the virtual world. In contrast, environmental olfactory displays do not share this issue, but tend to have synchronization issues.