Graphics Reference
In-Depth Information
Occluder
Light source
Object
Pixel
i
in the same position
L
min
L
−
L
max
ɹ
L
ʴ
Figure 8.46
Moving a thin occluder over a scene while capturing images can provide high frequency
illumination.
occluder, such as a long thin pole, to cast an appropriate shadow on the objects.
Images are captured as the pole is incrementally moved in small steps across
the scene. (The authors actually used a video camera and moved the pole
across the image continuously.) The pixels are then compared across the images.
The largest pixel value is taken as
L
+
; the smallest, as
L
−
. Figure 8.46 illustrates
the idea.
The sun is an area light source, so it casts soft shadows. The umbra is the part
of the shadow where the sun appears completely blocked; the penumbra is the
part where the sun appears only partially blocked. The relative size of the umbra
and penumbra depend on how far the pole is above the scene. If the pole is very
close to the ground, the umbra is nearly the size of the pole and the penumbra is an
almost unnoticeable narrow region around the umbra. As the pole gets higher off
the ground the umbra narrows and the penumbra widens. Eventually the umbra
disappears altogether and only the penumbra remains as a diffuse bright shadow.
The frequency of the light pattern produced by the sun with the shadow of a pole
thus decreases as the pole gets further from the scene. In order for the separation
to work, the pole must be sufficiently close to the objects that each scene point
is in the umbra in some captured image, and the penumbra is not too wide. An
added constraint of course is that the pole itself does not block the scene objects.
The idea that simply recording a movie of a pole being swept over an object gives
enough information to separate the lighting into direct and indirect components
