A versatile 3D gamut mapping adapted to image color distribution

We have been proposed a GBD (Gamut Boundary Descriptor) for comparing the gamut between image and device in discrete polar angle segment divided by (Deltatheta, Deltaphi) in CIELAB space. The gamut shell is described as the simple radial distances, called r-image. This GBD represents a monochromatic 2D image whose pixel denotes the radial vector magnitude arranged in discrete integer address. This simple presentation makes it easy to compare the point-to-point gamut sizes between image and device. We presented a method of image-dependent gamut compression using r-image and introduced image-dependent gamut expansion using histogram stretch. This paper examines how to decide which should be applied, compression or expansion, for gamut mapping in reference to the each r-image of image and device. When the most of r-image pixel values for image are greater than those for device in entire region and around middle lightness region, a gamut compression is necessary. On the other hand, if they are smaller than those for device in middle region and mainly located at lower region of lightness, a gamut expansion is desirable. The numerical gamut comparison by r-image to decide compression or expansion is tested for typical image samples. The paper reports the experimental results in both compression for wide gamut and expansion for unsaturated narrow gamut.