Bad Colour Maps Hide Big Features and Create False Anomalies

Abstract

Many colour maps provided by vendors have highly uneven perceptual contrast over their range. It is not uncommon for colour maps to have perceptual flat spots that can hide a feature as large as one tenth of the total data range. The opposite can also occur whereby perceptual discontinuities in the colour map can induce the appearance of false anomalies. This paper presents a set of design techniques that allow colour maps to be constructed with uniform perceptual contrast across their full range. The most important factor in designing a colour map is to ensure that the magnitude of the incremental change in perceptual lightness of the colours across the map is uniform. The rate of incremental change in hue or saturation of the colours in the map prove to be relatively unimportant when one is seeking to discern features at fine spatial frequencies. Accordingly, the colour maps presented here are designed in CIELAB colour space. In contrast to RGB this colour space provides a perceptual organization of colours in terms of their lightness, hue and chroma/saturation which facilitates the design process. The utility of the colour maps presented here is demonstrated using a simple test image that allows the perceptual uniformity of colour maps to be readily evaluated.