Depth Cameras: Structured Light and Time of Flight
A depth camera gives a distance for every pixel. Structured light cameras project a pattern and see how it bends. Time of flight cameras time how long light takes to bounce back. Both are cheap and fast indoors, and each has its own blind spots.
Two ways to measure depth
Structured light projects a known pattern from a projector placed a baseline \(b\) away from the camera, then triangulates, like stereo. Its error grows with the square of depth. Time of flight sends modulated light and measures the phase \(\Delta\varphi\) of the return at modulation frequency \(f_m\):
What to notice
Glass is invisible to both, since the light passes through and the camera sees whatever is behind it. Dark surfaces absorb light, so far points drop out. Turn on sunlight and structured light loses most of its points, because the sun washes out the projected pattern. Time of flight gets noisier but keeps working.
Structured light also leaves holes beside the edges of objects, where the camera can see a surface that the projector cannot light. Time of flight reads too long near the inside corner, because light bounces off both walls before it returns. This is called multipath.