This patent, held by an employee of Canon Information Systems, describes
a gamut mapping strategy, where lightness is mapped so that the central
part of the lightness scale (e.g. from L*=38 to L*=90) is kept unmodified
and only the extremes are compressed in a way which is not disclosed (most
probably a function similar to the soft clipping function used by Stone
and Wallace (1991)).
As in the majority of algorithms, here too the hue angle is kept unchanged.
Finally, chroma is clipped onto the surface of the reproduction gamut boundary
along lines, which are at an angle (a) to the horizontal axis (Figure 1).
Note, that all the colours within 2a of the destination gamutís cusp are
mapped onto the cusp and that the angle of 15 degrees was identified to
give the best results in most cases.
The problems, which were identified in the patent are the changes
in the perceived hue of colours with high chroma, which are due to the
non-uniformity of the lines of constant hab.