Dappled Photography: Mask Enhanced Cameras for Heterodyned Light Fields and Coded Aperture Refocusing

by A. Veeraraghavan, R. Raskar, A. Agrawal, A. Mohan and J. Tumblin

A light field conveys both spatial and angular distribution of light incident on the camera sensor. The pioneer work to capture a light field in one photographic exposure is the plenoptic camera, a device that uses a microlens array to rearrange a 4D light field and capture it with a 2D sensor. However, the optics of the microlens array defines a fixed resolution tradeoff between spatial and angular sampling of the light field.
In this paper, the authors propose to modulate the light field by shadowing the incoming light with a mask in the optical path. In the Fourier Light Field Space (FLS), the mask creates a train of identical kernels positioned in a slanted slice, and thereby, via convolution, pulls high angular frequencies to the central angular slice, the only slice the camera measures in the FLS. Assuming that the incident light field is band limited, the captured image is the flattened version of the incident light field in the Fourier domain.
Moreover, the slant of the mask kernel, which decides the spatial-angular resolution tradeoff, is determined by the location of the mask. Consequently, the resolution tradeoff can be adjusted by translating the mask. The minimal and the maximal angular resolution are achieved by placing the mask at the aperture and at the conjugate plane respectively.
However, the mask enhanced camera seems to trade reconstruction quality for flexibility. First, the optimal pattern of the mask varies with its location yet in practice the mask pattern is permanent. Second, the mask blocks about half of the incident light and reduces the signal-to-noise ratio of the sensed image. After all, the paper provides a profound analysis on the principle of mask enhanced cameras, a major category of computational camera, making itself influential in the Computation Photography community.