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Perceptually based tone mapping for low-light conditions

In this paper we present a perceptually based algorithm for modeling the color shift that occurs for human viewers in low-light scenes. Known as the Purkinje effect, this color shift occurs as the eye transitions from photopic, cone-mediated vision in well-lit scenes to scotopic, rod-mediated vision in dark scenes. At intermediate light levels vision is mesopic with both the rods and cones active. Although the rods have a spectral response distinct from the cones, they still share the same neural pathways. As light levels decrease and the rods become increasingly active they cause a perceived shift in color. We model this process so that we can compute perceived colors for mesopic and scotopic scenes from spectral image data. We also describe how the effect can be approximated from standard high dynamic range RGB images. Once we have determined rod and cone responses, we map them to RGB values that can be displayed on a standard monitor to elicit the intended color perception when viewed photopically. Our method focuses on computing the color shift associated with low-light conditions and leverages current HDR techniques to control the image's dynamic range. We include results generated from both spectral and RGB input images.