• orthogonal clipping 共to the boundary of the gamut兲
• cusp clipping or chord clipping
• mapping functions, e.g., sigmoid lightness mapping
function and chroma mapping function
• categorical mapping.
Next are some recently developed clipping techniques.
Ito, Katoh, and Ohno
7,8
proposed to clip the out-of-
gamut color to the reproduction gamut boundary. The
smallest E value calculated by a weighted color difference
formula CIELAB is employed. Other color difference for-
mulas, ⌬E
ab
*
,
9
⌬E
uv
*
,
9
⌬E
94
,
10
⌬E
CMC
,
11
⌬E
BFD
,
4
and
⌬E
wt
,
7
were also studied. Braun and Fairchild investigated
the hue nonuniformities in CIELAB
1
and developed the
sigmoidal lightness mapping functions to boost the image
contrast selectively when the original gamut is mapped into
the reproduced gamut.
12
Shimizu et al.
13
proposed the
chroma proportional clipping 共CPC兲 method with the Inter-
national Color Consortium 共ICC兲 color management frame-
work. MacDonald and Morovic
14
compared several meth-
ods of gamut mapping on fine art painting using La
*
b
*
color space. These methods are the paper cast removal,
lightness mapping, orthogonal clipping, chord clipping, and
gamut compression. For different paintings with different
percentages of out-of-gamut, different gamut mapping
methods are reported to have the best performance.
Laihanen
15
presented two approaches of color mapping us-
ing LABNU, a color space similar to CIELAB. These ap-
proaches are gamut compressing with constant hue and
lightness and gamut compression toward the center of light-
ness axis.
We develop the efficient technique of color gamut clip-
ping for color image processing using luminance, hue, and
saturation 共LHS兲 and YIQ. RGB, luminance, and saturation
clippings are compared. Efficiency of the clipping tech-
niques is also investigated.
2 Color Coordinate Systems
Many three-dimensional color spaces have been proposed
and adopted in different applications in the last few de-
cades. All of them take different approaches to represent
the trichromatic nature of light to satisfy their objectives.
For example, the CIELAB and CIELUV spaces
11
promote
uniformity of practice. In CIELUV, if two colored lights,
C
1
and C
2
, are mixed additively to produce a third color
C
3
, C
3
lies on a straight line on a chromaticity diagram
joining C
1
and C
2
at a position that can be calculated from
the relative amounts of C
1
and C
2
.
The LHS color space
16,17
is derived based on the Max-
well triangle in the RGB color cube. The LHS color space
is a cylindrical coordinate system where the vertical axis
corresponds to the luminance and the horizontal plane cor-
responds to the chromatic perceptions. The hue, saturation,
and value 共HSV兲 and hue, lightness, and saturation 共HLS兲
color spaces,
18
which are also known as the hexcone model
and double hexcone model, respectively, are similar to the
LHS color space except that the transformations are simpli-
fied. The generalized lightness, hue, and saturation 共GLHS兲
color model,
19
developed by Levkowitz and Herman, is a
generalization of HSL, HSV, HLS, and other special mod-
els.
The National Television Systems Committee 共NTSC兲
develops the YIQ color space for commercial color televi-
sion broadcasting and other applications. YIQ is a record-
ing of RGB for transmission efficiency. LHS and YIQ are
utilized here due to their efficiency of transformation.
2.1
LHS Color Coordinate System
The LHS color coordinate system has three components,
luminance 共L兲, hue 共H兲, and saturation 共S兲. Its development
is based on the Maxwell triangle in the RGB color cube as
shown in Fig. 1. The corners of the Maxwell triangle are
关1,0,0兴, 关0,1,0兴, and 关0,0,1兴 in the RGB cube. Given a vec-
tor in the RGB cube, the length of the vector OP is propor-
tional to the luminance, and the intersection of the vector
OP and the Maxwell triangle represent the attributes of hue
and saturation. Given the intersection point P
⬘
on the Max-
well triangle in Fig. 2, the hue is the angle between the CP
⬘
and CR, and the saturation is the ratio of the length CP
⬘
to
the length CQ, where C is the center of the Maxwell tri-
angle.
2.1.1
Transformation from RGB to LHS
The transformation from the RGB color coordinate system
to the LHS color coordinate system
17,20
is as follows:
Fig. 1 Maxwell triangle in the RGB cube.
Fig. 2 The intersection point OP
⬘
in the Maxwell triangle.
Yang and Kwok: Efficient gamut clipping...
702 Optical Engineering, Vol. 42 No. 3, March 2003