Graphics Reference
In-Depth Information
visible to the average human. Conversion to RGB is accomplished by a device-
dependent linear transformation. However, some XYZ values can get converted
to an RGB value outside the range of the device (i.e., greater than one or less than
zero), in which case the device is not capable of displaying the color exactly. Dis-
cretization, e.g., to 8 bits, adds a further restriction. In general, the set of colors a
particular device can reproduce is known as the
color gamut
of the device.
The value of
Y
in the XYZ color space is the
luminance
of the color, a measure
of overall brightness based loosely on human color perception at a typical level of
brightness adaptation. At the same radiometric radiance level, green light appears
somewhat brighter than red light, and significantly brighter than blue or violet
light. The luminance is often used in conversion from color to grayscale. The
X
and
Z
components of an XYZ color record the color information; the values
x
=
/
(
+
+
)
=
/
(
+
+
)
are the
chromaticity
values (coordi-
nates) of the color. The chromaticity values
x
and
y
are invariant under a linear
scaling of the color, so they represent, in some sense the “hue” of the color in-
dependent of the brightness. However chromaticity is not uniform in the way
humans perceive color: how much
x
and
y
have to change in order for a human
observer to notice a difference in the color depends on the particular values of
x
and
y
. For example, a slight change of values may make a noticeable difference in
the blue range, but the same change is hardly noticeable in the green range. This
is relevant to image storage, because it implies that greater precision is needed to
accurately represent blues than greens.
In 1976, the CIE adopted another color space, the Luv space, that is closer to
being perceptually uniform. The coordinates, properly written as
L
∗
,
u
∗
,and
v
∗
(the
∗
superscript is sometimes omitted), vary in approximately the same manner
as human perceive changes in color. The variable
L
represents the overall appar-
ent brightness or lightness of the color, while
u
and
v
are adjusted chromaticity
values. In this context, the values are sometimes called by the more general name
of
chrominance
values. Ideally, a small change in any of the Luv coordinates cor-
responds to a similar apparent change in color. The values are determined from
X
,
Y
,and
Z
through a nonlinear relationship. Figure 6.7 illustrates a
chromaticity
diagram
in Luv space. Here,
L
is fixed, at
L
X
X
Y
Z
and
y
Y
X
Y
Z
1 and the diagram shows how
the color varies as a function of
u
and
v
. The outer region contains all the col-
ors visible to the human eye, while the inner triangular region is the gamut of a
representative RGB color display device.
After he had some experience with the RGBE format, Ward began to see the
advantages of a file format that contained a device-independent representation of
pixel values. Human vision is much more sensitive to brightness (luminance) than
chromaticity, and brightness covers a wider range. More precisely, human per-
ception of brightness is roughly logarithmic: repeatedly doubling the luminance
=
