1. A tangible computer readable storage medium having computer-executable instructions stored thereon that, if executed by a computing system comprising one or more computing devices, cause the computing system to perform operations comprising: generating a slow-changing background model based in part on trends in captured camera images; generating a fast changing-background model based in part on trends in captured camera images; determining a background change in one or more of the captured camera images with respect to the slow-changing background model; and generating a vision image using the fast-changing background model in response to determining that the background change exceeds a threshold associated with the slow-changing background model.