Photometric Stereo Under Non-uniform Light Intensities and Exposures
This paper studies the effects of non-uniform light intensities and sensor exposures across observed images in photometric stereo. While conventional photometric stereo methods typically assume that light intensities are identical and sensor exposure is c
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KAIST, Daejeon, South Korea [email protected], [email protected] 2 Osaka University, Suita, Japan [email protected] 3 SenseTime Group Limited, Beijing, China [email protected] Abstract. This paper studies the effects of non-uniform light intensities and sensor exposures across observed images in photometric stereo. While conventional photometric stereo methods typically assume that light intensities are identical and sensor exposure is constant across observed images taken under varying lightings, these assumptions easily break down in practical settings due to individual light bulb’s characteristics and limited control over sensors. Our method explicitly models these non-uniformities and develops a method for accurately determining surface normal without affected by these factors. In addition, we show that our method is advantageous for general photometric stereo settings, where auto-exposure control is desirable. We compare our method with conventional least-squares and robust photometric stereo methods, and the experimental result shows superior accuracy of our method in this practical circumstance. Keywords: Photometric stereo · Shape estimation intensity and exposure · Surface normal
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Unknown light
Introduction
Non-uniform light intensities and exposures across observed images are a practical and common circumstance in data acquisition for photometric stereo that uses multiple images under distinct light directions. For example, different light bulbs with different intensity characteristics may be used for illuminating a scene. Even with identical light bulbs, due to that scene radiance is determined by surface normal and light directions, auto-adjusted sensor exposure is desirable depending on the light directions to avoid over-/under-exposures, which results in non-uniform exposures (equivalently, non-uniform light intensities). Therefore, the capability of properly handling varying and unknown light intensities and exposures across observed images is an important feature for making photometric stereo practical. Part of this work was done while the first author was an intern at Microsoft Research Asia. c Springer International Publishing AG 2016 B. Leibe et al. (Eds.): ECCV 2016, Part II, LNCS 9906, pp. 170–186, 2016. DOI: 10.1007/978-3-319-46475-6 11
Photometric Stereo Under Non-uniform Light Intensities and Exposures
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The setting can be regarded as a “semi-calibrated” photometric stereo, where the light directions are known but their intensities are unknown. We argue that accurate light intensity calibration is practically a hard task to perform due to that the light bulb’s luminous efficiency varies over time and quantization error in the measurement even with high-dynamic range imaging. This paper provides a way to bypass the difficult intensity calibration in photometric stereo. In the Lambertian image formation model, a measured intensity m is written as mi,j = Ei ρj n j li ,
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where i and j are indices of light direction and pixel location, li , nj ∈ R3×1 are unit vectors of lig
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