Retinex Algorithms: Many spatial processes used to solve many different problems

There are many different Retinex algorithms. They make different assumptions, and attempt to solve different problems. They have different goals, ground truths and output results. This “Retinex at 50 Workshop” session compares the variety of Retinex algorithms, along with their goals, ground truths that measure the success of their results. All Retinex algorithms use spatial comparisons to calculate the appearances of the entire scene. All Retinex algorithms need observer data to quantify human vision, so as to evaluate their accuracy. The most critical component of all Retinex experiments is the observer matches used to characterize human spatial vision. This paper reviews the experiments that have evolved as a result of Retinex Theory. They provide a very challenging data set for algorithms that predict appearance. Introduction Edwin Land coined the word Retinex in 1963. He used it to describe the theoretical need for three independent color channels to explain human color constancy.[1] The word was a contraction of “retina” and “cortex”. A “Retinex” is a theoretical color channel that makes spatial comparisons so as to calculate lightness sensations, namely the range of appearances between light and dark. Land had enthusiastically experimented with two-color projections in the late 1950’s and early 60’s.[2] By that time, he had hundreds of patents on many different photographic systems. He was well aware of the possibilities, and limitations, of silver halide photography. Before his Red and White light projection experiments, he accepted the standard explanation of color. Namely, color was the result of the local quanta catches of receptors with different spectral sensitivities. Human color vision was thought to behave the way that color film did; in that color was a local phenomenon that resulted from spectral responses within each very small image segment. The quanta catches of the triplet of retinal cones in a small retinal region generated color appearances. An accidental observation made a colleague in a late-night experiment changed all. The colleague remarked that there was more color than expected from mixtures of photographic separations using red and white lights. Land responded: “ Oh yes, that is adaptation.” At two o’clock in the morning, Land sat up in bed, and said : “Adaptation, what adaptation?” He immediately returned to the lab to repeat the experiment. For the rest of his life, human color vision was a favorite research area. What was it that Land had seen, so briefly, that made him return to the lab in the middle of the night? Human Trichromatic Color Theory and film have always been linked. When Thomas Young made his famous suggestion of human trichromacy in 1802, his colleague at the Royal Institution, Humphrey Davy, was studying a black and white photographic system. Young was the editor of the Institutions journal that described the work.[3] Young was well aware of silver halide’s response to light. That night, Land realized there was nothing he could do with a locally-responsive silver-halide system to make film behave the way that vision did. The color appearances in those projections could not be understood from the quanta catches of receptors in a tiny local region. Human color appearances are fundamentally different. It is spatial comparisons that control color sensations. Silver halide film uses quanta catches in a very small area which includes a small fraction of all the light-sensitive grains. Distant objects cannot influence the film’s response to its quanta catch of each tiny segment. Figure 1 illustrates the human visual pathway that begins with the visual pigments located in the distal tips of the cone and rod receptors in the retina (red ellipse). The quanta catch of these visual pigments initiates the spectral response to light. The receptors provide only the first response to the image on the retina. Appearance is the result of spatial processing along the entire visual pathway. Figure 1. Illustration of the many stages of spatial comparisons in the visual pathway. John Dowling, greatly expanded the work of Hecht and Wald, by describing the complex retinal spatial interactions.[4] Berson has recently shown spatial modulation from Melanopsin photopigment in ganglion cells.[5] In 1953, Kuffler [6] and Barlow [7] showed ganglion cells in the optic nerve make spatial comparisons. Hubel and Wiesel [8], DeValois [9] found spatial comparison cells in the cortex. Semir Zeki [10] found color constancy cells in V4 cortical cells. The dominant theme in research on the human visual pathway over the past 80 years has been the documentation of human spatial mechanisms at every stage along the visual pathway. level. Vision is a spatial process. Vision’s Ratio-making Sense In 1974 Land wrote in his Friday Evening Discourse at the Royal Institution: “This Discourse is about a generally unrecognized animal sense-the ratio-making sense. It is the ratiomaking sense which processes the radiation reaching our eyes in such a way as to discover the constant properties of objects in relation to the radiation falling on them.”[11] Figure 2 illustrated the papers surrounding the “Lightness and Retinex” article.Use reference [12] for download with links to papers. 1971 Land, & McCann Designator 1983 Land 1986 Land 1970 McCann, Land, Tatnall Gradients • 74aMcCanns.pdf • 75SavoyMcCann.pdf • 78cMcCann et al.pdf • 78a McCann.pdf • 78bSavoy.pdf • 80McCannHall.pdf 1971 Horn 1976 McCann, McKee,Taylor 1977 Land Sci Amer 2012 McCann, Parraman, Rizzi 1978 Land, Royal Institute 1970 Land, Ferrari, Kagan, McCann Reset 1968 Land. Ives Medal Address 1968 Land, McCann 1980 Frankle, McCann 2001, Sobol, McCann HP camera 1999, McCann, Hubel Gamut Retinex 1972 Stockham Spatial Filters Milano Retinex 1973 McCann 1999 McCann 2004 McCann Retinex @ 40