such as "Peekaboom" and "Phetch", were discontinued at that point. Cheating Ahn has described countermeasures which prevent players from "cheating" the game, and introducing false data into the system. By giving players occasional test images for which common labels are known, it is possible to check that players are answering honestly, and a player's guesses are only stored if they successfully label the test images. Furthermore, a label is only stored after a certain number of players (N) have agreed on it. At this point, all of the taboo lists for the images are deleted and the image is returned to the game pool as if it were a fresh image. If X is the probability of a label being incorrect despite a player having successfully labelled test images, then after N repetitions the probability of corruption is , assuming that end repetitions are independent of each other. Image selection The choice of images used by the ESP game makes a difference in the player's experience. The game would be less entertaining if all the images were chosen from a single site and were all extremely similar. The first run of the ESP game used a collection of 350,000 images chosen by the developers. Later versions selected images at random from the web, using a small amount of filtering. Such images are reintroduced into the game several times until they are fully labeled. The random images were chosen using "Random Bounce Me", a website that selects a page at random from the Google database. "Random Bounce Me" was queried repeatedly, each time collecting all JPEG and GIF images in the random page, except for images that did not fit the criteria: blank images, images that consist of a single color, images that are smaller than 20 pixels on either dimension, and images with an aspect ratio greater than 4.5 or smaller than 1/4.5. This process was repeated until 350,000 images were collected. The images were then rescaled to fit the game's display. Fifteen different images from the 350,000 are chosen for each session of the game. References External links The ESP Game Guessing games Human-based computation games Video games developed in the United States