Camera Calibration for Daylight Specular-Point Locus
This addresses the problem of accurate illuminant estimation for computer vision applications, but it is incremental as it builds on existing assumptions about daylight and camera sensors.
The paper tackles the problem of camera calibration for illuminant color recovery by proving that specular points under daylight lie on a straight line in a log-chromaticity space, and it demonstrates applications in illuminant detection and image re-lighting with different color temperatures.
In this paper we present a new camera calibration method aimed at finding a straight-line locus, in a special colour feature space, that is traversed by daylights and as well also approximately followed by specular points. The aim of the calibration is to enable recovering the colour of the illuminant in a scene, using the calibrated camera. First we prove theoretically that any candidate specular points, for an image that is generated by a specific camera and taken under a daylight, must lie on a straight line in log-chromaticity space, for a chromaticity that is generated using a geometric-mean denominator. Use is made of the assumptions that daylight illuminants can be approximated using Planckians and that camera sensors are narrowband or can be made so by spectral sharpening. Then we show how a particular camera can be calibrated so as to discover this locus. As applications we use this curve for illuminant detection, and also for re-lighting of images to show they would appear under lighting having a different colour temperature.