Another approach to tone mapping of HDR images is inspired by the anchoring theory of lightness perception. This theory explains many characteristics of the human visual system such as lightness constancy and its failures (as in the checker shadow illusion), which are important in the perception of images. The key concept of this tone mapping method (Lightness Perception in Tone Reproduction) is a decomposition of an HDR image into areas (frameworks) of consistent illumination and the local calculation of the lightness values. The net lightness of an image is calculated by merging of the frameworks proportionally to their strength. Particularly important is the anchoring—relating of the luminance to a known luminance, namely estimating which luminance value is perceived as white in the scene. This approach to tone mapping does not affect the local contrast and preserves the natural colors of an HDR image due to the linear handling of luminance.