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HDRI Files 663
HDR I Fi les
HDRI is a file format used for high-dynamic-range
images. Most cameras don’t have the capability
to capture the dynamic r ange (the gamut of
luminances between dark and bright regions) that
is present in the real world. However, the range
can be recovered by taking a series of pictures of
thesamesubjectwithdifferentexposuresettings,
and combining them into one image fi le.
This type of image is called a
high dynamic range
image
(HDRI) or radiance map .HDRIfileshave
an
.hdr extension and radiance maps have a .pic
extension.Thefileactuallycontainsallthedata
from all the pictures, so a w ide luminance range
is present, from bright, white highlights to the
darkest black.
HDR files are par t icularly useful as backgrounds
for compositing, and as reflection maps on
comp osited objects. When using a HDR image
as a skylight, use the parameters in the Exposure
group to control the brightness of the scene.
Procedure
To use a HDR image as a back ground, or as a diffuse
or r ef le ction ma p:
1.
In the Material Editor, choose Bitmap as the
map t ype.
2. On the Select Bitmap Image File dialog, under
Files Of Type, choose Radiance Image File
(HDRI). Open the HDR file you would want
to use.
The HDRI L oad Settings dialog appears, with
the i mage displayed in its preview window.
3. On the HDRI Load Settings dialog, look at the
Measured Min/Max values to see the luminance
rangefortheimage.
4. Turn on B lac k Point.
5. AdjusttheBlackPointandWhitePointvalues
until the red lines on the histogram encompass
the majority of the graph, and t he preview
image is satisfactory.
Tip: For the Internal Storage option, use the
default choice of 1 6 bit/chan Linear (48bpp )
unless you have a specific reason for doing
otherwise.
6. When you have finished adjusting values, note
the Linear White Point value, and click OK to
accept the settings.
7. In the Material Editor, expand the Output
rollout. Set the RGB Level to the same value
as the Linear White Point value on the HDRI
Load Settings dialog.
The result is a map with a wide range of deep
blacks and very white hi g h lights. If such an
imageisusedasbothabackgroundinthe
rendering and a reflection map on an object,
the object will appear to be extremely shiny and
reflective.
Interfa ce
HDR Load Settings dialog
When you open a HDR file as a bitmap, the HDR
Load Settings dialog appears. This dialog a l lows
youtospecifytheluminancerangetousefromthe
image, and the method for storing the data.