Good Colour Maps: How to Design Them (1509.03700v1)

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. Colour maps may also have perceptual discontinuities that induce the appearance of false features. Previous work in the design of perceptually uniform colour maps has mostly failed to recognise that CIELAB space is only designed to be perceptually uniform at very low spatial frequencies. 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 is uniform. The specific requirements for linear, diverging, rainbow and cyclic colour maps are developed in detail. To support this work two test images for evaluating colour maps are presented. The use of colour maps in combination with relief shading is considered and the conditions under which colour can enhance or disrupt relief shading are identified. Finally, a set of new basis colours for the construction of ternary images are presented. Unlike the RGB primaries these basis colours produce images whereby the salience of structures are consistent irrespective of the assignment of basis colours to data channels.

• The paper proposes a framework for designing perceptually uniform color maps to address the non-uniformity issues common in existing color mapping techniques.
• It proposes design principles for various color map types, emphasizing uniform lightness variation and addressing issues like those found in rainbow maps.
• It recommends using test images to evaluate map performance and discusses applications in fields like geophysical and medical imaging.

Analysis of "Good Colour Maps: How to Design Them"

The paper "Good Colour Maps: How to Design Them" by Peter Kovesi presents a methodological framework for creating effective and perceptually uniform color maps, particularly for applications in geophysical exploration and medical imagery. The author identifies key deficiencies in commonly used color maps supplied by vendors, highlights the importance of perceptual uniformity, and proposes solutions for designing improved color maps.

A fundamental issue addressed is the nonuniform perceptual contrast present in many conventional color maps, which can obscure or falsely highlight features within data sets. The paper shows that previous design efforts have not fully recognized the limitations of the CIELAB color space, primarily its validity only at very low spatial frequencies. Kovesi emphasizes that designing perceptually effective color maps requires maintaining a uniform incremental change in the perceptual lightness of colors across the map.

Detailed Examination of Color Map Types

The paper categorizes color maps into linear, diverging, rainbow, cyclic, and isoluminant types, each with specific requirements and applications. For linear maps, a linear variation in lightness either ascending or descending is critical, facilitating clarity and intuitive data ordering. Diverging maps are structured symmetrically around a central reference point, typically marked by a neutral color, and must be carefully crafted to avoid visually perceived false features caused by lightness gradient reversals.

Rainbow color maps, while popular, are notable for their perceptual pitfalls, particularly due to non-intuitive ordering and uneven contrast. The author argues for a design approach that mitigates these issues through optimizing lightness gradients and smoothing hue transitions, especially around problematic colors such as cyan and yellow. Cyclic maps, vital for circular data representation such as orientations or phases, can suffer from misinterpretation if not perceptually balanced. Kovesi proposes cyclic color maps exhibiting four identifiable regions to reflect principal orientations, enhancing interpretability.

Color Maps in Relief Shading and Ternary Images

An innovative aspect of Kovesi's work is the use of color maps in relief shading. The paper demonstrates that such maps, combined with shading techniques, can enhance depth perception in data visualization. Isoluminant color maps, which are inherently constant in lightness, are recommended when used alongside relief shading to prevent the color overlay from interfering with shape perception.

In ternary imaging where three data channels are mapped to colors, the traditional RGB basis is critiqued for its perceptual imbalance. Kovesi introduces alternative bases aimed at achieving near-isoluminant performance, thus allowing a balanced representation of data channels where structural features are consistently prominent across different color assignments.

Technical Recommendations and Practical Applications

The paper offers concrete methodologies for enhancing existing color maps through perceptual contrast equalization and the application of test images to evaluate a map's performance in revealing data structures. The utility of predefined test images is underscored as a tool for detecting color map deficiencies, allowing researchers and developers to iteratively refine design choices before applying the maps to actual data.

Overall, Kovesi's work provides a comprehensive analytical framework and practical guidelines for the design of color maps. Given its implications, the research stands as a valuable reference for future developments in data visualization, potentially influencing how visualization tools are designed and integrated into applications across various scientific and industrial domains. The work invites further exploration into scale-dependent perceptual uniformity and its influence on digital imaging, inviting experimentation with newer color spaces and visualization strategies.