“The plot of They Live is deceptively simple. Roddy Piper’s character, a semi-homeless day laborer named Nada (Spanish for “nothing”) discovers a pair of sunglasses that allow him to identify the extraterrestrials who have infiltrated the Earth’s populace.” (Read more about the story plot on salon.com)
In a fashion very similar to computer vision procedures, the glasses first discard color information. They then convert or translate images into text. More precisely into orders. Watching the images makes you a slave who obeys without knowing it to these injunctions. Using the glasses, you are then able to read instead of watching. It is interesting to note that Carpenter does not choose to display the text as a subtitle or comment to these images, but to replace them. Using the glasses, the modes are switched. They cannot coexist. The glasses are not only a sort of OCR for ideology, they also detect faces. Or more precisely, reverse-detect them. When seeing an alien with the glasses, its face becomes a dead mask, the alien looses its faciality.