Colour is a Medium as well as a Message

My research is in “physics-based” vision (Wolff et al. 1992; Healey and Jain 1994). Physicsbased vision takes seriously radiometric, as well as geometric, constraints in image analysis. The approach has led to new computational methods to determine scene parameters from images including photometric stereo (Woodham 1980; Woodham 1981) and multiple light source optical flow (Woodham 1990). The idea exploited is that multiple images acquired under the identical viewing geometry but different conditions of illumination are a principled way to obtain additional local radiometric constraint. The work is both theoretical and experimental. Theoretical results are implemented and tested in real systems. Recently, advances in imaging technology have led to the development of near realtime implementations (Woodham 1994; Siegerist 1996). These implementations, in turn, support further experimentation. Principal applications are to robot vision and to remote sensing. No connection to colour perception and biological vision is evident. Indeed, David Marr’s book (1982, 249) introduced the topic of photometric stereo as follows “Finally, there is a technique for recovering shape from reflectance maps that cannot possibly have any biological significance, but which is so elegant that I cannot resist mentioning it.” I am fond of this quote, especially the word “elegant.” At the same time, the conclusion that photometric stereo “cannot possibly have any biological significance” always struck me as too strong. One could accept, on empirical grounds, a final determination that photometric stereo does not have any biological significance. But, robot and biological vision systems both function in the identical physical world. One must always allow for the possible existence of biological systems that use a method successfully employed in robot vision (and vice versa). Photometric stereo was new at the time of the writing and the subsequent posthumous publication of Marr’s book (1982). Much has happened in the years that have followed. This chapter is a retrospective. I do not claim to demonstrate that photometric stereo has biological significance. For the purpose at hand, however, it is sufficient to suggest ways that it might. This is consistent with the inter-disciplinary nature of these conference proceedings. It also is a way to pay homage to David Marr who saw the process of scientific discovery as forever new and on-going. The objective of this chapter is to document another way to use colour in computational vision and to suggest possible connections to biological vision. The hope is that this will inspire new collaboration between researchers in biological and robot vision. The remainder of Section 1 develops the case that colour is a medium as well as a message. The basics of photometric stereo and multiple light source optical flow are described, including the role the medium of colour plays in current implementations. Section 2 provides an overview of spectral radiometry and the issues that arise in robot vision and in remote sensing. Section 3 bridges to biological systems with particular emphasis on colour vision in aquatic environments. Section 4 concludes with a discussion of implications for colour vision research.