Scratch interfaces are a great mean to enable gestures for occluded areas which cannot be seen by a camera. Therefore, a basic requirement is to enable gestural actions via scratching sounds . The following video explains the actual idea: https://youtu.be/2E8vsQB4pug
This thesis consists of thee main parts:
The implemention is preferably done in C or Java in order to run the application on embedded devices (e.g. Raspberry Pi or Pic32) in a later stage.
 Harrison, Chris and Hudson, Scott E. Scratch Input: Creating Large, Inexpensive, Unpowered and Mobile Finger Input Surfaces. In Proceedings of the 21st Annual ACM Symposium on User interface Software and Technology (UIST 2008), pp. 205-208.