technical reportA computer driven display system was used to study brightness contrast phenomena, in a project motivated by research in digital picture processing. The modeling approach was that of Stockham and Davidson: the visual system is modeled as the cascade of a linear system (eye optics) and a multiplicative homomorphic system?that is, a logarithmic transformation (retinal receptors), followed by a linear system (neural interaction). In order to test the linearity of neural interaction, smooth stimulus patterns were utilized, containing only a few sinusoidal components within the low frequency band, and exhibiting classical brightness contrast effects (Mach bands, simultaneous brightness contrast, Hermann grid effect). Data were collected from brightness matching experiments with these smooth patterns. The data were verified in preliminary experiments on similar patterns digitally processed by the inverse of the model, in order to obtain cancellation of the brightness contrast effects. The experimental results showed to be in agreement with Davidson's data, obtained by a fundamentally different method. This new experimental approach indicated that the hypothesis of linearity of neural interaction is justified for smooth patterns. Further studies suggested that intensity edges and contours cause strong departure from linearity. Some steps were also taken toward extending the homomorphic model for color contrast phenomena. Conclusions are drawn about the implications of these experiments in the fields of computer image processing and visual psychophysics. The advantages of computer techniques in visual experiments are presented; the applications of the homomorphic model of brightness perception to digital picture processing are reviewed, and the implications of the experimental findings are discussed
