International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B3, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
RECONSTRUCTION OF SKY ILLUMINATION DOMES FROM GROUND-BASED
PANORAMAS
Fabien Coubard ?, Lámán Lelégard ?, Mathieu Brédif^, Nicolas Paparoditis *, Xavier Briottet b
? Université Paris-Est, Institut Géographique National, MATIS lab
73, avenue de Paris 94165 SAINT-MANDÉ, FRANCE
? ONERA - DOTA
2, avenue Édouard Belin 31055 TOULOUSE, FRANCE
Commission III/3
KEY WORDS: Sky radiance, illumination, environment map, terrestrial images
ABSTRACT:
The knowledge of the sky illumination is important for radiometric corrections and for computer graphics applications such as relighting
or augmented reality. We propose an approach to compute environment maps, representing the sky radiance, from a set of ground-based
images acquired by a panoramic acquisition system, for instance a mobile-mapping system. These images can be affected by important
radiometric artifacts, such as bloom or overexposure. À Perez radiance model is estimated with the blue sky pixels of the images, and
used to compute additive corrections in order to reduce these radiometric artifacts. The sky pixels are then aggregated in an environment
map, which still suffers from discontinuities on stitching edges. The influence of the quality of estimated sky radiance on the simulated
light signal is measured quantitatively on a simple synthetic urban scene; in our case, the maximal error for the total sensor radiance is
about 10%.
1 INTRODUCTION
1.1 Why does sky illumination matter?
The recent development of on-line viewers of street level images
leads to the acquisition of a huge number of high-resolution, geo-
referenced, urban, terrestrial images. Panoramic images are also
used for architectural or archaeological surveys. Understanding
the physical process of the formation of digital images is a key to
the development of many applications, for professionals as well
as for the general public. For instance, relighting or augmented
reality can be performed from an estimation of a reflectance map
of the scene (Yu et al., 1999). While processing the radiometric
information carried by the pixels values, it is of primary interest
to estimate precisely the light sources of the scene. For the out-
door scenes, the single light source is the sun, and the medium
through which its light reaches the scene is the atmosphere. The
sky is a complex participating medium, that may be highly het-
erogeneous in the presence of clouds, and that combines multiple
physical phenomena (molecular and aerosol scattering and atten-
uation): its modelization for radiative transfer is thus a difficult
task. In this work, we propose an image-based approach to re-
trieve the downward sky radiance from the whole dome above
the scene. The extraction and aggregation of sky pixels from the
images highlight radiometric issues in the acquired images; these
issues are tackled with a non-physical method.
1.2 Related work
A simplified model of sky radiance can be computed by using a
radiative transfer code, such as 6S (Vermote et al., 1997). This
method implies the knowledge of the atmospheric composition at
the time of acquisition. This composition can be characterized by
the optical depth of the different gases and aerosols, that can be
obtained by meteorological measures. But as these measures are
costly, they usually are not available . It can also be characterized
by one simple observable parameter: the optical visibility, which
is easier to measure. However, the radiative transfer computation
is usually limited to atmosphere made of homogeneous parallel
planes (Schanda, 1986). Though it is sufficient for low resolu-
tion applications such as meteorology or climatology, it can be
a strong limitation for processing high resolution terrestrial im-
ages, in which local variations of the sky downward radiance can
have a significant effect, as in the presence of clouds. In a non
physically-based way, there exist empirical parametric models to
represent the sky radiance; they are presented in (?). The model
of (Perez et al., 1993) is the most popular, and its small number
of parameters is valuable for inversion purpose. These models
are valid for clear or continuously veiled skies, but cannot repre-
sent clouds. In computer graphics, (Debevec, 1998) proposes to
retrieve an environment map by imaging a mirrored ball. It can
be used to photo-realistically introduce objects in a scene. How-
ever, the estimated radiance with this method is valid only for the
location of the mirrored ball, and cannot be used for other points
of the scene in the generic case of non-lambertian materials and
participating medium. That is why we prefer an estimation of sky
radiance for the whole dome.
After a formulation of the problem (section 2), two approaches
are presented for estimating a sky radiance map; they are then
used jointly for attenuating the effect of radiometric artifacts (sec-
tion 3). The environment maps resulting of these approaches are
then presented for a urban dataset acquired by a mobile-mapping
vehicle, and the influence of their quality on a simulation problem
is then quantitatively measured (section 4).
2 PROBLEM FORMULATION
2. Light sources model
Performing a complete simulation of the physical interaction be-
tween the sky and the solar light (scattering and attenuation) is
a very costly computation using volume rendering (Pharr and
Humphreys, 2004). Furthermore, it requires the knowledge of
the composition of the atmosphere, that is usually not homoge-
neous, nor made of homogeneous parallel planes (mostly because
of the presence of clouds). The main idea in the proposed paper
