Abstract
An illumination compensation system for correcting smooth intensity variations due to illumination changes is based on an assumption that an underlining image reflectance function is approximately a piecewise constant and that an image irradiance function is spatially smooth. The system first takes the logarithm of an image brightness function. Gradient constraints are then computed using a finite difference. Reliable gradient constraints are selected based on a local uniformity test. A process is subsequently applied to estimate the logarithmic irradiance function. A logarithmic irradiance function is subtracted from the logarithmic image brightness function and an exponential operation of the above subtracted image function is taken and an illumination compensated image is outputted from the system.