Towards a spatio-chromatic standard observer for detection

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 (an achromatic direction, a nominally isoluminant red-green direction, and the tritanopic confusion line) and four intermediate colour directions were used. These intermediate directions were the vector sums of the thresholds along the main directions. We evaluate two models. 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). For all three observers the ellipsoidal model fits as well as the general separable model. Estimating the chromatic tuning improves the model fit for one observer.