![](https://csdnimg.cn/release/download_crawler_static/86725761/bg11.jpg)
10 mantissa,” and this format has been floating around the computer graphics hardware developer
network for some time. (OpenEXR also supports a standard IEEE 32-bit/component format, which we
will not discuss here.)
Because it can represent negative primary values along with positive ones, the OpenEXR format covers
the entire visible gamut and a range of about 10.7 orders of magnitude with a relative precision of 0.1%.
Since humans can simultaneously see no more than 4 orders of magnitude, this makes OpenEXR a good
candidate for archival image storage. Although the basic encoding is 48 bits/pixel as opposed to 32 for
the LogLuv and RGBE formats, the additional precision is valuable when applying multiple blends and
other operations where errors might accumulate. It would be nice if the encoding covered a larger
dynamic range, but 10.7 orders is adequate for most purposes, so long as the exposure is not too
extreme. The OpenEXR specification offers the additional benefit of extra channels, which may be used
for alpha, depth, or spectral sampling. This sort of flexibility is critical for high-end compositing
ipelines, and would have to be supported by some non-standard use of layers in a TIFF image.
We predict that the OpenEXR format will find widespread use in the computer graphics research
community and special effects industry, as it has clear advantages for high quality image processing, and
is supported directly by today’s high-end graphics cards. ILM is to be commended for offering their
excellent library for license-free use, and given the complexity of the most common variant, PIZ lossless
wavelet compression, reimplementing EXR in a private library seems a formidable task.
Microsoft/HP scRGB Encoding
A new set of encodings for HDR image representation have been proposed by Microsoft and Hewlett-
Packard, and accepted as an IEC standard (61966-2-2). The general format is called scRGB, formerly
known as sRGB64. This standard grew out of the sRGB specification developed by Michael Stokes,
Matthew Anderson, Srinivasan Chandrasekar, and Ricardo Motta, at HP and Microsoft. In essence, it is
a logical extension of 24-bit sRGB to 16 linear bits per primary, or 12 bits per primary using a gamma
encoding.
The scRGB standard is broken into two parts, one using 48 bits/pixel in an RGB encoding and the other
employing 36 bits/pixel either as RGB or YCC. In the 48 bits/pixel substandard, scRGB specifies a
linear ramp for each primary. Presumably, a linear ramp is employed to simplify graphics hardware and
image-processing operations. However, a linear encoding spends most of its precision at the high end,
where the eye can detect little difference in adjacent code values. Meanwhile, the low end is
impoverished in such a way that the effective dynamic range of this format is only about 3.5 orders of
magnitude – not really adequate from human perception standpoint, and too limited for light probes
and
HDR environment mapping. The standard does allow for negative primaries, which is an improvement
over earlier RGB standards, but it makes wasteful use of this capability as much of the range is given to
representing imaginary colors outside of the visible gamut, especially at the bottom end of the scale,
where human color perception actually degrades. At the top end of its dynamic range, where people see
colors more clearly, the gamut collapses in on itself again as primaries get clipped to the maximum
representable value. This can be visualized on the colorspace animation page
.
The second part of the standard offers a more sensible and compact 36 bits/pixel specification, which
employs a standard gamma ramp with a linear subsection near zero. Although it uses 25% fewer bits, it
has nearly as great a dynamic range as the 48-bit version, 3.2 orders as opposed to 3.5, and also allows
negative primary values. It still suffers from a collapsing of its gamut at the top end and the unusable
colors at the bottom, but it’s a better encoding overall. Even better than this scRGB-nl substandard is the
scYCC-nl encoding, which benefits from a separate luminance channel and a color space that does not
collapse quite as quickly at the top end. However, there is some strange behavior at peak luminance
where the bri
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