Off the Planckian Locus: Using 2D Chromaticity to Improve In-Camera Color

Traditional in-camera colorimetric mapping relies on corre-lated color temperature (CCT)–based interpolation between pre-calibratedtransforms optimized for Planckian illuminants such as CIE A and D65.However, modern lighting technologies such as LEDs can deviate sub-stantially from the Planckian locus, exposing the limitations of rely-ing on conventional one-dimensional CCT for illumination characteriza-tion. This paper demonstrates that transitioning from 1D CCT (on thePlanckian locus) to a 2D chromaticity space (off the Planckian locus) im-proves colorimetric accuracy across various mapping approaches. In addi-tion, we replace conventional CCT interpolation with a lightweight multi-layer perceptron (MLP) that leverages 2D chromaticity features for ro-bust colorimetric mapping under non-Planckian illuminants. A lightbox-based calibration procedure incorporating representative LED sourcesis used to train our MLP. Validated across diverse LED lighting, ourmethod reduces angular reproduction error by 22% on average in LED-lit scenes, maintains backward compatibility with traditional illuminants,accommodates multi-illuminant scenes, and supports real-time in-cameradeployment with negligible additional computational cost. Code and datacan be found on the project webpage: ccmmlp.github.io