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23.11. Night Tonemapping 619
23.11 Night Tonemapping
The tone reproduction operators discussed so far nearly all assume that the im-
age represents a scene under photopic viewing conditions, i.e., as seen at normal
light levels. For scotopic scenes, i.e., very dark scenes, the human visual system
exhibits distinctly different behavior. In particular, perceived contrast is lower,
visual acuity (i.e., the smallest detail that we can distinguish) is lower, and every-
thing has a slightly blue appearance.
To allow such images to be viewed correctly on monitors placed in photopic
lighting conditions, we may preprocess the image such that it appears as if we
were adapted to a very dark viewing environment. Such preprocessing frequently
takes the form of a reduction in brightness and contrast, desaturation of the image,
blue shift, and a reduction in visual acuity (Thompson et al., 2002).
A typical approach starts by converting the image from RGB to XYZ. Then,
scotopic luminance V may be computed for each pixel:
V = Y
1.33
1+
Y + Z
X
− 1.68
.
This single channel image may then be scaled and multiplied by an em-
pirically chosen bluish gray. An example is shown in Figure 23.30. If some
Figure 23.30. Simulated night scene using
the image shown in Figure 23.12. (See also
Plates XV and XIX.)
pixels are in the photopic range, then
the night image may be created by lin-
early blending the bluish gray image
with the input image. The fraction to
use for each pixel depends on V .
Loss of visual acuity may be mod-
elled by low-pass filtering the night im-
age, although this would give an incor-
rect sense of blurriness. A better ap-
proach is to apply a bilateral filter to re-
tain sharp edges while blurring smaller
details (Tomasi & Manduchi, 1998).
Finally, the color transfer technique
outlined in Section 23.3 may also be used to transform a day-lit image into a
night scene. The effectiveness of this approach depends on the availability of a
suitable night image from which to transfer colors. As an example, the image in
Figure 23.12 is transformed into a night image in Figure 23.31.