Contrast in complex images

TLDR

The contrast of simple test images is well defined, but for complex scenes the Michelson definition yields a single-point value, so contrast should incorporate spatial frequency content. We propose a definition of local bandlimited contrast that assigns a contrast value to every image point as a function of spatial frequency band. Contrast is computed at one‑octave intervals by taking the ratio of the bandpass‑filtered image at each frequency to the low‑pass‑filtered image at an octave below.

Abstract

The contrast of test images such as sinusoidal gratings or a single patch of light on a uniformed 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 be included in the definition of contrast. We propose a definition of local bandlimited contrast in images which assigns a contrast value to every point in the image as a function of the spatial frequency band. The contrast is calculated for frequencies at one-octave intervals. For each frequency, the contrast is defined as the ratio of the bandpass filtered image at that frequency to the low-pass filtered image filtered to an octave below the same frequency.