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Many papers have been published on the problems of motion
estimation and frame alignment; for review see [1].
Most of these assume fixed gain. In practice, however, camera
gain varies to compensate for varying quantity of light, by way of
Automatic Gain Control (AGC), automatic level control, or some similar form
of automatic exposure.
In fact almost all modern cameras incorporate some form of automatic
exposure control. Moreover next generation cameras,
such as EyeTap devices (http://eyetap.org)
that cause the eye itself to function, in effect,
as if it were both a camera and display,
also feature an automatic exposure control system to make possible
a hands free gaze activated wearable system operable without conscious
thought or effort.
Indeed, the human eye itself incorporates many features
akin to the automatic exposure or AGC of modern cameras.
Figure 1 illustrates how such a camera takes in a typical
scene.
Figure 1:
Automatic exposure as the cause of differently exposed pictures
of the same (overlapping) subject matter:
(a) Looking from inside Hart House Soldier's Tower,
out through an open doorway,
when the sky is dominant in the picture, the exposure is
automatically reduced, and we can see the texture (clouds, etc.)
in the sky. We can also see University College and the CN Tower
to the left.
(b) As we look up and to the right, to take in subject matter
not so well illuminated, the exposure automatically increases
somewhat. We can no longer see detail in the sky, but new
architectural details inside the doorway start to become visible.
(c) As we look further up and to the right, the dimly lit
interiour dominates the scene, and the exposure is automatically
increased dramatically. We can no longer see any detail in the
sky, and even the University College building, outside, is
washed out (overexposed). However, the inscriptions on the wall
(names of soldiers killed in the war) now become visible.
(a,b,c) The differently exposed pictures of overlapping subject
matter can be combined to extend dynamic range and tonal definition,
or to provide a true photographic quantity ``lightspace'' for
intelligent vision systems.
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As we look straight ahead we see mostly sky, and the exposure is quite
small. Looking to the right, at darker subject matter, the exposure is
automatically increased. Since the differently exposed pictures depict
overlapping subject matter, we have (once the images are registered,
in regions of overlap) differently exposed pictures of identical
subject matter. (Registration typically also includes correction of barrel
distortion, correction for darkening at the corners of the image
such as by , etc., to make the camera become
a truly quantimetric instrument.)
In this example, we have three very differently exposed pictures depicting
parts of the University College building and surroundings.
Subsections
Next: Previous work on variable
Up: Quantigraphic Imaging: Estimating the
Previous: Abstract
Steve Mann
2002-05-25