Is color patchy?

In many natural scenes, shadows and shading, which are primarily luminance-defined features, proliferate. Hence one might expect that the chromatic variations of natural scenes, which more faithfully represent the layout of object surfaces, will contain relatively fewer and larger uniform regions than the luminance variations, i.e., will be more "patchy." This idea was tested using images of natural scenes that were decomposed into chromatic and luminance layers modeled as the responses of the red-green, blue-yellow, and luminance channels of the human visual system. Patchiness was defined as the portion of pixels falling within a +/- threshold in the bandpass-filtered image, averaged across multiple filter scales. The red-green layers were found to be the most patchy, followed by the blue-yellow layers, with the luminance layers the least patchy. The correlation between image-layer patchiness and the slope of the Fourier amplitude spectrum was small and negative for all layers, the maximum value being for red-green (-0.48). We conclude that the chromatic layers of natural scenes contain more uniform areas than the luminance layers and that this is unpredicted by the slope of the Fourier amplitude spectrum.