In order to successfully composite CG elements into live action scenes it is important that the lighting
of the CG object match the lighting of the scene it is being composited into. One technique people have
used to reproduce the incident light in a live action scene is to create a high dynamic range photograph
of a mirrored ball placed in the scene (called a "light probe" in [1]) and then use that light probe as
a source for image based lighting.
Previous Work
Currently, in order to create a high dynamic range image of a mirrored ball one must take an iterative
series of photographs with the exposure value of each image being stopped down by a given increment from
the exposure value of the one before. Later, each of the images are assembled into a single high dynamic
range image using a program such as MakeHDR [2]. If an artist wished to accurately illuminate a CG object
traveling through a complex lighting environment, it would be necessary to capture these iterative
photographs at numerous locations (ideally at every frame) along the object's path. Clearly, this would
be an ambitious task.
Technique
One solution for creating a real time high dynamic range light probe is to develop a system in which
multiple exposures of the same image can be captured within a single video frame. We did this by
modifying a five point Multi-Image Filter (a faceted lens that is commonly used to create photographic
kaleidoscope effects), and applying successively increasing values of neutral density gel to four of
the five facets of the filter (3⅓, 6⅔, 10 and 13⅓ stops). This modified filter effectively produces
a single image that is divided into five identical regions, with the center region capturing a "direct"
view and the four outer regions stopped down to their respective exposure values. This modified filter
is placed on a video camera that is mounted along with a mirrored ball on a span of angle iron (see Figure 1).
Assuming the relation between the camera and the ball never changes, the light probe only needs to be
calibrated once. To compensate for the angle shift introduced by parallax effects from the facets of
the multi-image filter, one can compute the arctangent of the distance between facets divided by the
distance between the lens and the silver ball. By determining the number of degrees each facet is
offset from the center, we are able to warp each region of the filter according to the direction
space of its view of the ball. In our case, each facet's view of the ball was computed to be 2.7
degrees off from center.
More accurate calibration can be done with the help of a light stage [3], which provides a "master key"
for factoring out lens distortion and imperfections in the mirrored ball. However, we found that
simply computing the pixel shift and then overlapping each region of the filter was sufficient for
assembling a usable image.
In order to capture high dynamic range light probe data at every frame along a path, one presses
record' on the video camera and carries the light probe along the desired path. A computer program
then imports each recorded frame, isolates the five distinct images in the frame, aligns them according
to predetermined calibration data, and then assembles the aligned images into a high dynamic range
omnidirectional measurement of incident illumination.
Conclusion
This new technique will permit artists to composite CG objects into dynamic complex lighting environments,
accurately reproducing high dynamic range lighting parameters for each frame. In the future, this technique
would benefit from greater precision in applying the neutral density gels to the multi-image filter, a
smaller camera rig, and higher resolution video cameras.
References
Paul Debevec: Rendering Synthetic Objects Into
Real Scenes: Bridging Traditional and Image-Based Graphics With Global Illumination and High Dynamic
Range Photography, In SIGGRAPH 98, August 1998.
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