Views from 'crabworld': the spatial distribution of light in a tropical mudflat

Natural scene analysis has been extensively used to understand how the invariant structure of the visual environment may have shaped biological image processing strategies. This paper deals with four crucial, but hitherto largely neglected aspects of natural scenes: (1) the viewpoint of specific animals; (2) the fact that image statistics are not independent of the position within the visual field; (3) the influence of the direction of illumination on luminance, spectral and polarization contrast in a scene; and (4) the biologically relevant information content of natural scenes. To address these issues, I recorded the spatial distribution of light in a tropical mudflat with a spectrographic imager equipped with a polarizing filter in an attempt to describe quantitatively the visual environment of fiddler crabs. The environment viewed by the crabs has a distinct structure. Depending on the position of the sun, the luminance, the spectral composition, and the polarization characteristics of horizontal light distribution are not uniform. This is true for both skylight and for reflections from the mudflat surface. The high-contrast feature of the line of horizon dominates the vertical distribution of light and is a discontinuity in terms of luminance, spectral distribution and of image statistics. On a clear day, skylight intensity increases towards the horizon due to multiple scattering, and its spectral composition increasingly resembles that of sunlight. Sky-substratum contrast is highest at short wavelengths. I discuss the consequences of this extreme example of the topography of vision for extracting biologically relevant information from natural scenes.

Automated summary

This paper addresses four crucial but largely neglected aspects of natural scenes - the viewpoint of specific animals, the position-dependent relationship between image statistics and the visual field, the influence of illumination direction on luminance and polarization contrast, and the biologically relevant information content of natural scenes. The author used a spectrographic imager equipped with a polarizing filter to record the spatial distribution of light in a tropical mudflat, aiming to quantitatively describe the visual environment of fiddler crabs. The results showed that the crabs' environment has a distinct structure, with non-uniform luminance, spectral composition, and polarization characteristics depending on the position of the sun. The findings have implications for understanding how animals extract relevant information from natural scenes.