As-projective-as-possible bias correction for illumination estimation algorithms

Mahmoud Afifi, Abhijith Punnappurath, Graham Finlayson, Michael S. Brown

Research output: Contribution to journalArticlepeer-review

20 Citations (Scopus)


Illumination estimation is the key routine in a camera’s onboard auto-white-balance (AWB) function. Illumination estimation algorithms estimate the color of the scene’s illumination from an image in the form of an R, G, B vector in the sensor’s raw-RGB color space. While learning-based methods have demonstrated impressive performance for illumination estimation, cameras still rely on simple statistical-based algorithms that are less accurate but capable of executing quickly on the camera’s hardware. An effective strategy to improve the accuracy of these fast statistical-based algorithms is to apply a post-estimate bias-correction function to transform the estimated R, G, B vector such that it lies closer to the correct solution. Recent work by Finlayson [Interface Focus 8, 20180008 (2018)] showed that a bias-correction function can be formulated as a projective transform because the magnitude of the R, G, B illumination vector does not matter to the AWB procedure. This paper builds on this finding and shows that further improvements can be obtained by using an as-projective-as-possible (APAP) projective transform that locally adapts the projective transform to the input R, G, B vector. We demonstrate the effectiveness of the proposed APAP bias correction on several well-known statistical illumination estimation methods. We also describe a fast lookup method that allows the APAP transform to be performed with only a few lookup operations.

Original languageEnglish
Pages (from-to)71-78
Number of pages8
JournalJournal of the Optical Society of America A: Optics and Image Science, and Vision
Issue number1
Early online date12 Dec 2018
Publication statusPublished - 1 Jan 2019

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