ic model (the effect of the atmosphere is eliminated) (section
4.2).
By inserting into (6), each sample contributes one equation
Loi; — 04i ( Eat Eust En) (9)
to a system of n linear equations which is redundant for
n > 3. E,;;, E,;; and E,,, are the irradiances due to the
incident sunlight, ambient light and skylight. Since the scene
geometry and the illumination geometry are known, they can
be calculated according to (3), (4) and (5). For each wave-
length within the visible spectrum, the system of equations
(9) is solved for the unknowns which are the weights k;, ku
and k, of the daylight components. Finally, the weights are
averaged within the visible spectrum. Since o,; is known up
to a constant factor only, k,, k, and k, are the relative
weights of the daylight components.
Nakamae et al. (Nakamae, 1986) and Thirion (Thirion, 1992)
determine the ratio of illumination by direct sunlight and am-
bient light. Skylight, wavelength dependency and the effect
of the atmosphere are not considered.
The second inversion of the illumination model involves
solving the equation of the illumination model (6) for the dif-
fuse reflectance o,, of a material. This inversion will be used
during rendering (section 6).
Given are the scene geometry (section 4.1) and the illumina-
tion geometry d;, Q,, d, and m, (sections 3.3 and 4.1). The
radiances k, In k, L;, k, L, of the daylight components
are known from the first inversion of the illumination model
(see above). p is a known location in object space. It is si-
tuated on the surface of an object of an opaque, diffusely re-
flecting material with material parameter k; > 0. The known
apparent object color L, is the radiance of the light reaching
the camera associated with one of the input images at the
known distance d from p. L, is derived from the RGB triplet
of the reconstructed image function at p' (appendix A). p'
are the image coordinates of p in the selected input image.
The true object color L,; is determined from L; by the sec-
ond inversion of the atmospheric model (the effect of the at-
mosphere is eliminated) (section 4.2).
Inserting into (6) and solving for o, yields
041 = Loi ( k (Ej * E, + Ew) Ts (10)
E,,, E,, and E,; are the irradiances due to the incident
sunlight, ambient light and skylight. Since the scene geome-
try and the illumination geometry are known, they can be cal-
culated according to (3), (4) and (5). Since the relative
weights k,, k, and k, were used, also o,; is known up to a
constant factor only.
Thirion (Thirion, 1992) determines the reflectance of a mate-
rial without considering skylight, wavelength dependency and
the effect of the atmosphere.
438
À
4 interaction
natural
PREPARE | ^99 | env. descr.
preprocessing
artificial | >
env. descr. > RENDER | —»
LIBRARY
| light :
sources mais |
T
param | output |
parameters images
VIEW
modeling
CONVERT|<@——| DXF file =! SA À
LL meee irre —
Fig. 4 System overview.
5. ENVIRONMENT DESCRIPTIONS
The natural environment description (Fig. 4) contains geo-
metrical and non-geometrical data about the natural environ-
ment of the planned building: Atmospheric parameters (Len
VT d,), illumination parameters (ds, ,, k;, ky, m,, d,, k,,
L,) and polygons (vertices, material attribute, image attrib-
ute). Most of this information is retrieved from the input
images during the interactive preprocessing step (section 4).
Additionally, the natural environment description contains the
interior and exterior orientation and the file name of each of
the input images.
The artificial environment description (Fig. 4) contains the
CAD model of the planned building with additional artificial
light sources, if required. The program CONVERT (Fig. 4)
converts data from the widely used DXF format (Autodesk,
1988) to the required data format.
6. RENDERING
During the final rendering step (program RENDER) (Fig. 4),
the input images and the natural and artificial environment
description (section 5) are used to generate the output
images. These show the planned building embedded in the
existing environment from the perspectives of the input
images. The rendering algorithm is an extension to conven-
tic
thi
ti
aA 0.330292 (^
N fr.
amie eam Per = EN INA 8 AA sn PN
