Relativistic raytracing

It does if you are ray tracing. They are trying to find the source of a theoretical photon that hits the screen at the present time, the source will therefore have been in the past so they need to run time backwards as the photon moves away from the camera.

From TFA:

> But wouldn't we want to see the black hole? One could send a ray through every pixel of a screen, and let them go around the black hole and hit its surroundings. And if the rays are sent back in time, it's as if one received the light emitted by the surrounding objects in the past!