ProLab: A Perceptually Uniform Projective Color Coordinate System

The construction of perceptually uniform color space is an important task of color theory. Many image processing applications requires the color difference to replicate human perception. The perceptual difference is poorly approximated by Euclidean distance within linear color coordinate systems, in particular, within CIE XYZ. Almost all uniform color spaces are constructed as some nonlinear transformation from CIE XYZ. Such transformations do not preserve the straightness of the color tone curves and other collinearity properties which are important for high dynamic range and wide color gamut imagery and required for linear color analysis, including separation of homogeneous backgrounds. In this work, we propose proLab, a new color coordinate system derived as a 3D projective transformation of CIE XYZ that preserves the linearity of manifolds by definition. We show that proLab is far superior to the widely used CIELAB coordinate system (though inferior to the modern CAM16-UCS) in terms of perceptual uniformity, evaluated by the STRESS metric in reference to the CIEDE2000 color difference formula. We also demonstrate that shot noise in proLab is more homoscedastic than in CAM16-UCS or other standard color spaces. This makes proLab a convenient coordinate system for linear color analysis.

Automated summary

The construction of perceptually uniform color space is crucial for color theory, as many image processing applications require color difference to mimic human perception. It is difficult to approximate perceptual difference using Euclidean distance within linear color coordinate systems. ProLab, a new color coordinate system, performs far better than widely used CIELAB system in terms of perceptual uniformity.