Contrast In Images

The contrast of simple images such as sinusoidal gratings or a single patch of light on a uniform background is well defined, but this is not the case for complex images. When the Michelson definition, used for sinusoidal test patterns, is applied to complex scenes, the contrast of the whole picture may be defined based on only one point in the image. Human contrast sensitivity is known to be a function of the spatial frequency; therefore, the spatial frequency content of an image should also be considered in the definition of contrast. We propose a definition of local band-limited contrast in images that assigns a contrast value to every point in the image as a function of the spatial frequency band. For each frequency band, the contrast is defined as the ratio of the band-pass filtered image at that frequency to the low-pass filtered image filtered to an octave below the same frequency. This definition is useful for understanding the effects of enhancement techniques on image contrast. The definition can be implemented in the design of recognizable, minimal-contrast images, thus enabling optimal use of the available dynamic range of the display, and more efficient coding algorithms.