Color Flows and Color Mixing

In recent work, we demonstrated a strong interaction between color flows and luminance flows in the perception of complex shaded surfaces: When shading spatially covaries with spectral information, the shape from shading percept is strongly suppressed (an example of the color-shading effect). In explaining this phenomenon, we suggested a role for double-opponent orientation-selective operators. These operators can be interpreted as signaling spatial derivative information in the red-green and blue-yellow chromatic planes. Here we investigate the conditions under which these red-green and blue-yellow double-opponent flows align in orientation. We demonstrate theoretically that this alignment occurs when gradients of hue and chroma are themselves locally aligned. In this case, the double-opponent operators signal directions of constant hue ("iso-hue"). This situation arises for example in mixing between two colors, e.g., along a gradient. However, we show that the flows generically diverge when mixing between three or more color sources: In this case, directions of iso-hue and iso-chroma emerge as linear combinations of iso-RG and iso-BY directions. These observations facilitate the construction of stimuli that can identify whether directions of iso-hue play a causal role in the color-shading effect, or whether the effect can be explained solely in terms of covariation between red-green or blue-yellow double-opponent flows and the shading isophote structure. Meeting abstract presented at VSS 2014