MIRAGE, a theory for the primitive coding of the (1D) spatial distribution of luminance changes by the human visual system is developed from a theoretical examination of the practical problems associated with the characterization of such changes. The main novel feature of the theory is that the multiplicity of spatial filters in human vision is assumed to exist principally to transmit a broad bandwidth signal of considerable redundancy: the filters are not assumed to be marked with their centre frequency or bandwidth, and are not analyzed independently. The theory is largely independent of the particular filter transfer function form. MIRAGE is applied to a range of one-dimensional luminance patterns, and demonstrates several well-known brightness illusions, and a structured grouping principle. It is finally shown to be supported by a wide range of psychophysical data.