Vision Group @ NASA Ames

Toward a standard observer for spatio-chromatic detection (2002)

The aim of the ColorFest is to extend the original ModelFest (http://vision.arc.nasa.gov/modelfest/) experiments to build a spatio-chromatic standard observer for the detection of static coloured images. The two major issues that need to be addressed are (1) the contrast sensitivity functions for the three chromatic mechanisms and (2) how the output of these channels is combined. We measured detection thresholds for stimuli modulated along different colour directions and for a wide range of spatial frequencies. The three main directions were an achromatic direction, a nominally isoluminant red-green direction, and the tritanopic confusion line. To assess the summation across the different mechanisms 4 intermediate directions were used. These intermediate directions were the vector sums of the thresholds along the main directions. We evaluate two space-colour separable models. Both models assume that the chromatic tuning of the three mechanisms is independent of spatial frequency. Detection performance is described by a linear transformation C defining the chromatic tuning and a diagonal matrix S reflecting the sensitivity of the chromatic mechanisms for a particular spatial frequency. The output of the three chromatic mechanisms is combined according to a Minkowski metric (General Separable Model), or according to a Euclidean Distance measure (Ellipsoidal Separable Model).