
where M
target
is the target mean of L (i.e., predefined
or user-defined) and M
output
is the mean luminance of
the tone-mapped image.
While Leap helps maintain consistent brightness
in Flash, its reliance on a user-defined target mean lu-
minance introduces complexity and reduces flexibil-
ity. This dependency may not be optimal for all im-
ages or lighting conditions, particularly in challeng-
ing scenarios such as varying illuminant sources or
dynamic lighting environments. In these cases, man-
ual adjustment of the target mean can exacerbate in-
consistencies, leading to suboptimal tone mapping re-
sults.
2.2 Dawn: Robust and Adaptive Scaling
for Flash
To address the limitations of Flash and Storm in com-
plex lighting conditions, we propose Dawn, a novel
Tone Mapping Operator (TMO) that introduces an
adaptive scaling mechanism into the core equation.
Unlike the static or user-defined scaling parameters
used by Flash, Dawn dynamically adjusts the scaling
parameter a based on the luminance statistics of the
image. This adaptive approach eliminates the need for
Leap and inherently handles brightness normalization
throughout the tone mapping process.
The adaptive scaling parameter a for Dawn is
computed using the following equation
a = k
1
· µ
L
+ k
2
· σ
L
+ k
3
(3)
where µ
L
and σ
L
represent the mean and variance of
the luminance values, respectively. The constants k
1
,
k
2
, and k
3
play a crucial role in this computation, as
they control the influence of brightness and contrast
on the scaling mechanism. Specifically, k
1
adjusts the
contribution of the mean luminance µ
L
, affecting the
overall brightness response, while k
2
determines how
much variance σ
L
affects contrast adaptation. The
constant k
3
serves as a base value, ensuring stability in
different luminance ranges. By fine-tuning these con-
stants, Dawn can be tailored to provide optimal tone
mapping in a wide range of lighting conditions.
By leveraging image statistics and sweeping post-
processing corrections away, Dawn continuously
adapts the tone mapping process to each image’s lu-
minance distribution, which ensures smooth transi-
tions in sudden brightness changes, minimized arti-
facts, and optimal brightness. This dynamic scaling
mechanism allows Dawn to handle varying bright-
ness levels and contrasts more effectively than static
parameters. This makes this approach more robust
in delivering higher-quality outputs in both low-light
and multi-illuminant scenes.
2.3 Why Adaptive Scaling Improves
Quality
The adaptive scaling mechanism in Dawn offers sig-
nificant advantages over static parameters by dynamic
adjustment with respect to the luminance statistics of
each image. In low-light conditions, the Leap op-
eration proposed in (Banic and Loncaric, 2018) fre-
quently amplifies noise as it tries to globally adjust
brightness and enhance contrast. In contrast, Dawn
adapts to luminance variance locally, selectively in-
creasing contrast and recovering details by inject-
ing less amount of noise. Next, in multi-illuminant
scenes, where static scaling often causes color shifts
or haloing, Dawn leads to adjusting to brightness vari-
ations in different regions, which tailors the tone map-
ping to specific lighting conditions and minimizing
these artifacts. Moreover, Dawn ensures consistent
tone mapping across regions with varying brightness,
such as shadows, midtones, and highlights, maintain-
ing balanced exposure throughout the scene. This
adaptability, which does not require manual adjust-
ments or predefined parameters, enables Dawn to
handle a wide range of lighting scenarios, from high-
contrast daylight to complex, low-light environments,
with ease and reliability.
2.4 Nonlinear Scaling for Complex
Scenarios
In more extreme lighting environments, Dawn can
employ an optional nonlinear scaling variant to fur-
ther enhance performance. The scaling parameter in
this case is computed as
a = exp(k
1
· µ
L
) + k
2
· log(1 + σ
L
) (4)
where µ
L
represents the mean luminance of the image,
and σ
L
is the variance of the luminance values, which
captures the contrast within the image. The constants
k
1
and k
2
control the contribution of the mean and
variance to the scaling process, respectively. Specif-
ically, k
1
governs the degree to which the mean lu-
minance influences the exponential adjustment, while
k
2
determines the impact of the variance on the loga-
rithmic correction. The addition of 1 to the logarith-
mic function ensures numerical stability when han-
dling low contrast values.
This nonlinear approach emphasizes the dynamic
response to rapid changes in luminance, offering
greater flexibility in complex scenarios. By applying
exponential and logarithmic transformations, Dawn
can adapt more aggressively to scenes with large
variations in brightness or contrast, ensuring better
preservation of detail and consistency of tone.
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