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HDR:
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HDRI - a brief overview |
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Well, most of you will know about the principal advantages
of HDR Images. But what exactly means HDR? Whatīs the
difference between HDRI and common RGB Images?
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What is HDRI ? |
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As you might know each Pixel of a digitized Image normally
is represented by three color-values known as RGB. RGB
stands for Red, Green and Blue, the three base colors
of the additive color space used by computers. With these
three colors it is possible to mix any color within this
colorspace. Each of these colors is represented by an
integer number with
a range of 0-255. That means, it is possible to use 256
different brightnesses for each of these colors respectively
for the Image. Image-formats based on this RGB range are
known as LDRIs, Low Dynamic Ranges Images because they
are bound to the 0-255 integer range.
Now, when you take a look at the real world, mother nature
isnīt bound to a range when presenting itīs beauty to
the human eye. There you have the darkness of a cave with
almost no light (at least in the visible spectrum) but
also the brightness of an object like the sun which is
millions of times brighter than normal daylight, filtered
through the atmosphere.
In 1985 Greg Ward invented the RGBE format to be able
to work with real world ligthing in his rendering system
RADIANCE. The RGBE format basically uses a 4th component
known as the shared exponent (E). With the help of this
exponent it was possible to describe the actuall brightness
of a pixel in a real world brightness range (High Dynamic
Range).
In the years after that invention there where developed
several different formats to represent the real world
brightness information inside of a digital picture. E.g.
Bill Reeves of Pixar created a 33-bit long RGB format
, Dan McCoy added it in 1996 to his free TIFF library
(libtiff). Greg Ward later added the so called LogLuv
format to this library which captures 5 orders of magnitude
and the full visible gammut in 24 bits using a perceptual
color encoding (There is also a 32-bit version f this
format which s able to hold up to 38 orders of magnitude
and often results in smaller files due to run-length encoding).
The HDR format of the pictures in this database uses the
RGBE format explained above.
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Comparision
of ligthing information in RGB and HDR images |
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What happens if you take an image of a desklamp and try
to darken it so that you can see more information inside
of the bulb?
| F-Stop
range within a RGB-image |
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RGB picture suffers from having not enough ligthing
information. |
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| F-Stop
range within a HDR-image |
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the HDR-image you can change the fstop from
the bulb without sacrificing contrast and detail.. |
In the example above you can see that HDR definitely contains
much more information about the lighting of your scene
than a standard RGB image. With this information it is
possible to create much more realistic Lighting not only
in 3D scenes but in every artwork which is affected by
lighting situations.
The next big advantage especially for the 3D artist is
the ability to create much more convincing Motion Blur
effects and Reflections on your objects.
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Comparision of lightness and
shadow detail in RGB and HDR images |
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In
the daker f-stop steps you can clearly see the detail in
the lightness areas. When pushing the f-stop higher you
can see the firstly missing detail in the shadow areas.
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HDR-images and Reflections |
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This
reflecting Sphere was rendered with the
help of a RGB image as reflecting environment.
As you can see, the reflections of the
brighter parts are also very dull and the
reflection doesnīt look real. |
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This
reflecting Sphere was rendered with the help
of a HDR image as reflecting environment. ->
Now the brighter parts differ nicely from the other
parts. The image has much more life in it than the
dull standard version. |
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HDR-images and Motion
Blur |
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This
reflecting Sphere was rendered with the help of
a RGB image as reflecting environment.
As you can see, the reflections of the brighter
parts look very dull when motion blurred. It is
not possible to e.g. render really bright reflections
on top of car laquer in a city environment by night
without HDR Images. |
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This
reflecting Sphere was rendered with the help
of a HDR image as reflecting environment. ->
Now the brighter parts differ nicely from the other
parts. The image has much more life in it than the
dull standard version. |
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OpenEXR, a new HDR image format |
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OpenEXR is a High Dynamic Range image format just like the
HDR format. Nethertheless there are big differencies between
these two formats:
Althought the OpenEXR format also was developed in response
to the demand for higher color fidelity, it is much more
sophisticated than the HDR format which more originates
from academical needs.
With OpenEXR, the goal was to develop a format which is
more efficient than the standard 32-bit floating point formats.
Because the traditional formats do store more dynamic information
than needed in the daily work, the files tend to explode
in size. When working with motion pictures, you got huge
amounts of data that way without really needing all the
stored information. OpenEXR uses clever sorting of needed
data and also several lossless compression methods, some
of which can achieve compression ratios of about 2:1 for
images with film grain.
Further on OpenEXR images can store an arbitrary amount
of channels, each with different datatypes and they are
able to store different attributes like color balance information
of digital cameras, etc.
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Why do we support OpenEXR? |
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We want to enable our customers to always work with State of the art technology. We are pretty sure that OpenEXR will be THE standard for HighDynamicRange images in the near future.
Because of that, the HDRIbase is available also as OpenEXR-Version.
For the case that you are not sure, if your favourite application will recognize the OpenEXR format in the near future, the bundle of both versions (HDR+OpenEXR) may be of interest for you. Because of that, we decided to sell a bundle of them also to give you the unique chance to profit from the momentary standard in conjunction with the future standard.
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Where to get more information? |
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Please visit Sachform Technology to get more information about the technology and our spherical mapping products.
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