1. A computer system comprising: a hardware interface configured to receive depth video of an environment from a camera; a computer-memory device storing a background model of the environment preservable over a reboot of the computer system, the background model including a plurality of trusted coordinates derived from the depth video; an update engine configured to update the background model, including moving a trusted coordinate to greater depth if an observed pixel depth is behind the trusted coordinate over a first duration, but retaining the trusted coordinate if the observed pixel depth is in front of the trusted coordinate over the first duration, and moving the trusted coordinate to lesser depth if the observed pixel depth is in front of the trusted coordinate over a second, longer duration; and a posture-recognition engine configured to ignore a background portion of the depth video but to recognize posture of a user in front of the background portion, the background portion bounded by the plurality of trusted coordinates of the background model.