1. A computer program product, tangibly embodied in a machine readable medium, the computer program product comprising instructions that, when read by a machine, operate to cause data processing apparatus to: detect, using electromagnetic radiation projected proximally to a surface, an object touching the surface; determine whether the detected object is of a predetermined object type using a trained recognition model, the recognition model being trained to determine whether the detected object is of the predetermined object type based on an appearance of the detected object; and generate a user input to a computer application based on the detected object, if it is determined that the detected object is of the predetermined object type.