Specifications for the Stanford Spherical Gantry

compiled by Marc Levoy
(covering the July 11 and July 12 designs)

General specifications


Other specifications

Design issues and alternatives

Advantages of allowing the camera arm to cover both hemispheres

Digitizing light fields

BRDF and BTDF measurement of flat samples

Backdrops for range scanning


For Light fields

Let us assume a camera with a 40-degree FOV lens looking at a 16-inch object (8-inch radius spherical working volume). The corresponding standoff distance, from the point of projection (typically at the camera aperture) to the focal plane, is 22 inches. Let us also assume that the camera arm is 30 inches long.

Object positioning. The governing factor is sensor resolution. Let us assume a 1024 x 1024 pixel sensor, the highest resolution we envision using in the near future. Each pixel then covers 0.04 degrees of visual arc, which is 0.4mm at the focal plane. Because light fields have a limited depth of field, accuracy at the focal plane presents the strongest constraints. To maintain scan repeatability to +- 1/2 pixel, object positioning (in X, Y, and Z) must be repeatable to +- 0.2mm.

Camera positioning. Since video cameras have small apertures, we want to move the gantry by single aperture widths and then average together multiple views to generate a synthetic aperture. Thus, the governing factor in camera positioning accuracy is not the spacing between views in the final light field, but rather the aperture size of the camera. Let us assume a 6mm aperture. To maintain scan repeatability to +- 1/2 aperture width, camera positioning (in altitude and azimuth) must be repeatable to +- 0.3 degrees, which is +- 3mm at the aperture or +- 4mm at the end of the camera arm. For a 32 x 32 view light field over a 90 x 90 degree sweep, this implies 4 x 4 apertures averaged together to construct the synthetic aperture for one view.

Camera aiming. For shape-from-light fields methods, viewing rays that are supposed to cross at a single point in 3-space must cross there. The governing factor is camera arm rigidity, including the axis joint, arm, and camera mount. To insure that rays cross within +- 1/2 pixel at the focal plane, this assembly must flex no more than 0.015 degrees, or 0.21 mm at the end of the camera arm. This is a difficult requirement. It was the limiting factor on the Apple ObjectMaker. If it cannot be satisfied, a calibration procedure will be necessary, and this procedure must be repeated whenever the weight on the arm (typically the camera) is changed.

For flat sample BRDF and BTDF measurement

For BRDF and BTDF measurement of 3D objects

For range scanning

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Copyright © 1996 Marc Levoy
Last update: Monday, 26-Apr-2004 20:38:13 CDT