Active Stereo

A stereo camera pair with an infrared projector adding a speckle pattern to help match the left and right images and compute depth.

Active stereo is a depth-camera design: two infrared cameras triangulate depth from disparity just like ordinary stereo, while an infrared projector simultaneously casts a random dot pattern onto the scene. Passive (ordinary) stereo matches the two images using natural texture, and fails on texture-free surfaces like blank walls or plain tabletops, leaving holes in the depth map; the projected speckle pattern artificially adds texture to these surfaces so matching is no longer ambiguous. Intel RealSense’s R200 and D400 series use this approach; Intel’s technical paper notes that, unlike structured light, the projected pattern doesn’t need to be known in advance — it only needs to be dense and non-repeating along the matching direction. Because the cameras can also exploit natural texture lit by sunlight, this kind of camera still works outdoors. Orbbec’s Gemini 330 series is also marketed as combining active and passive stereo.

ExampleA camera like the RealSense D435 photographing a plain white table gets a depth map full of holes with the infrared projector off; turning the projector on covers the table in a speckle pattern, and the holes disappear.

Also called

IR-Projected Stereo, Projected Texture Stereo

Related

Stereo Camera · Structured Light · Speckle Structured Light · Depth Camera · Stereo Matching · RealSense Depth Camera (D435i / D405)

Sources

Intel RealSense Stereoscopic Depth Cameras (Keselman et al., arXiv:1705.05548)
Orbbec Gemini 335 产品页 (Chinese)