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ra_xyze - convert between RADIANCE RGBE and XYZE formats
ra_xyze [ -r ][ -e exposure ][ -c | -u ][ -p xr yr xg yg xb
yb xw yw ] [ input [ output ] ]
Ra_xyze converts between RADIANCE RGBE
(red,green,blue,exponent) and XYZE (CIE X,Y,Z,exponent)
formats. The -e option specifies an exposure compensation,
which may be given as a decimal multiplier or in f-stops
(powers of two). By default, ra_xyze produces a flat XYZE
RADIANCE picture file from any type of RADIANCE input
picture. To override these defaults, the -c option may be
used to specify run-length encoded output, or the -u option
may be used to specify a flat output.
The -r option causes ra_rgbe to produce a run-length encoded
RGBE file instead. The -p option may be used to override
the standard RADIANCE RGB primary colors to tailor the image
for a particular output device or representation. The eight
floating-point arguments to this option are the 1931 CIE
(x,y) chromaticity coordinates of the three RGB primaries
plus the white point, in that order. The new primaries will
be recorded in the header of the output file, so that the
original information may be fully recovered later. It is
not necessary that the input file by in XYZE format. Th -r
option may therefore be used to convert from one RGB primary
representation to another using the -p option.
If the output file is missing, the standard output is used.
If the input file is missing as well, the standard input is
used.
The CIE standard used is the 1931 2-degree observer, and the
correct output representation relies on the original
RADIANCE input description being defined properly in terms
of the standard RADIANCE RGB primaries, whose CIE (x,y)
chromaticity values are defined in the header file in
src/common/color.h. In this same file is a standard for the
luminous efficacy of white light (WHTEFFICACY), which is
used as a conversion between lumens and watts throughout
RADIANCE. This same factor is applied by ra_xyze when
converting between the radiometric units of the RGBE format
and the photometric units of the XYZE format. The purpose
of this factor is to ensure that the Y component of the CIE
representation is luminance in units of candelas/meter^2.
Although the white point calibration of monitors is often
off-white, we recommend using equal-energy white as the
primary value because a person viewing the monitor will
automatically adjust his or her perception such that the
display appears white. Using a measured white point, one
may get the absolute CIE color up on the screen, but it will
appear to be shifted due to the viewer's transient
adaptation.
Most of the RADIANCE picture filters should work uniformly
on either RGBE or XYZE files, so it is not necessary to
convert back to RGBE format except for conversion or
display, in which case the correct primaries for the chosen
output device should be specified with the -p option if they
are known.
To convert RGBE output from rpict(1) into run-length encoded
XYZE format:
rpict [options] scene.oct | ra_xyze -c > scene_xyz.pic
To prepare a RADIANCE picture for display on a calibrated
NTSC monitor:
ra_xyze -r -p .670 .330 .210 .710 .140 .080 .333 .333
stand.pic ntsc.pic
Greg Ward
Any color correction applied to the original image is not
removed or translated by ra_xyze, and it may result in color
shifts in the output. If color preservation is important
and the correction is unwanted, it is best to remove it with
pfilt(1) using the -er, -eg and -eb options first. (Simply
look at the header and apply the reciprocal primaries of all
COLORCORR= lines multiplied together.) Better still, get
the picture before color correction is applied.
pfilt(1), ra_rgbe(1), rpict(1)
Header and Footer
RA_XYZE(1) RADIANCE (5/30/96) RA_XYZE(1)
Page 1 (printed 7/17/96)
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