Image Processing Reference
In-Depth Information
Table 6-7 Comparison of High-Dynamic Range Image Formats
Sample size
Max value
Step size
LogLuv 24
24 bits
10 5
32 bits
10 76
LogLuv 32
32 bits
10 38
TIFF float
32 bits
10 38
Pixar log (TIFF)
33 bits
10 4
48 bits
10 11
Other formats that support HDR imagery include the following:
Alias IFF
Cineon DPX
Mental ray
Lightwave FLX
Real Pixel
The most accurate representation is probably the Industrial Light & Magic OpenEXR
format. Although it represents pixel colors with the maximum number of bits, the range
of values is smaller than in the other formats. The true measure is the step size, which
allows the most accurate gradation of tone across the image. All but the Pixar log format
support the full gamut of colors.
By using a floating-point value to represent a pixel color, the dynamic range is
greatly increased. All kinds of problems related to range limits, rounding errors, and
issues with exposure under varying lighting conditions can be managed more effectively.
Here are some examples, first in Figure 6-19 a short exposure to show the backlight
overexposure from the window, then in Figure 6-20 a flash photograph shot at night to
eliminate the backlight. Finally, Figure 6-21 shows both images combined in Photoshop
to eliminate the burn through and reveal the foreground detail as well as the trees outside
the window.
The result I want is to have both parts of the view perfectly exposed. After all, that is
what I see when I look at it directly. The final image was manipulated by hand with some
careful masking and combining of the two previous images. HDR allows this to happen
HDR support allows photographs inside a darkened building to get the detail of the
interior when the exposure time is so long that the backlight through the windows burns
out the window detail and halos round the shape of the aperture. Setting the exposure
time short enough to get the detail in a stained-glass window greatly underexposes the
interior. HDR techniques allow the tonal range to be adjusted and the exposure setting to
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