Camera Calibration for Daylight Specular-Point Locus

TL;DR

This paper introduces a novel camera calibration method that identifies a specific straight-line locus in color space, enabling accurate daylight illuminant detection and image relighting based on specular point analysis.

Contribution

It provides a theoretical proof and practical calibration technique to find the daylight-related locus in log-chromaticity space for improved illuminant estimation.

Findings

The locus is a straight line in log-chromaticity space for daylight conditions.

Calibration enables accurate illuminant detection from images.

Application to relighting demonstrates practical utility.

Abstract

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.