Simple Analytic Approximations to the CIE XYZ Color Matching Functions

We provide three analytical fits to the CIE x, y, and z color matching curves, commonly used in predictive and spectral renderers as an intermediate between light spectra and RGB colors. Any can replace the standard CIE curves, which come tabulated. Using tabulated curves can introduce typos, encourage crude simplifying approximations, or add opportunities to download curves from sources featuring inconsistent or incorrect data. Our analytic fits are simple to implement and verify. While fitting introduces error, our fits introduce less than the variance between the human subject data aggregated into the CIE standard. Additionally, common rendering approximations, such as coarse spectral binning, introduce significantly more error. We provide simple, analytic fits in Equations 2 and 3, but even our more accurate fit in Equation 4 only requires 10 lines of code.


Peter-Pike Sloan (NVIDIA)
Peter Shirley (NVIDIA)

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