Graphics Reference
In-Depth Information
32 bits/pixel
R
G
B
exponent
(145, 215, 87, 149) =
2
(149 -128)
(145, 215, 87)=
(1190000, 1760000, 713000)
(145, 215, 87, 103) =
2
(103 -128)
(145, 215, 87) =
(0.00000432, 0.00000641, 0.00000259)
Figure 6.6
Arrangement of values in a pixel used in the 32-bit RGBE HDR image format (top), and
examples illustrating the meaning of the fields (bottom). (After images courtesy of Greg
Ward. )
Debevec's work was different in that it was concerned with the problem of recon-
structing HDR images from a set of captured images. Ward's format for storing
HDR images uses the usual three-byte RGB value along with an extra
exponent
channel that scales the overall magnitude of the value by a power of 2. The expo-
nent channel is an 8-bit integer stored as a
biased
value: if
e
is the stored value,
the actual integer has the value
e
−
128. The possible range of exponents is thus
128 through 127, which corresponds to a range of about 10
±
38
. This vastly ex-
ceeds the needs of real world HDR images. This format is known as the
RGBE
file format and is a standard part of the RADIANCE system (
Figure 6.6
)
.
−
6.2.2 Human Perception and Image Formats
Representing color in terms of red, green, and blue color components has some
limitations. Notably, it has no independent meaning beyond providing the values
for an RGB display device. Different devices may have different color charac-
teristics, so the actual color displayed from an RGB representation varies by the
device. This is not necessarily a problem for much of computer graphics, because
the human visual system is not very sensitive to absolute colors, just as it is not
very sensitive to absolute brightness. However, in some applications precise color
reproduction is essential, and this requires a
device-independent color
represen-
tation.
Measurement and representation of color is part of
colorimetry
, a subfield of
color science in general. Color science and colorimetry predate computer graph-
ics. The CIE, the Commission Internationale de l'Eclairage (the International
Commission on Illumination) created the standard XYZ color space in 1931. Col-
ors in this space are represented by a set of three
tristimulus
components,
X
,
Y
,
and
Z
, which are computed by integrating the spectrum of the color against three
standard illuminant curves. The XYZ color space includes the full gamut of colors
