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1.2 HISTORICAL MOTIVATION FOR HDR IMAGING 5
1.1.3 HDR IMAGING TECHNIQUES
HDR imaging is the set of techniques that computationally extend the usual or standard dynamic range
of a signal. HDR imaging has arisen in multiple fields, such as computational photography, computer
graphics, and animation. HDR signals may be produced in several ways: by the combining of multiple
lower dynamic range signals for HDR reconstruction; synthetically, by simulation or raytracing; or by
use of HDR sensors for data acquisition.
1.1.4 HDR FROM MULTIPLE EXPOSURES
According to Robertson et al. (2003), “the first report of digitally combining multiple pictures of the
same scene to improve dynamic range appears to be (Mann, 1993).”
HDR imaging by reconstruction from multiple exposures is defined as follows:
Definition of HDR reconstruction: The estimation of at least one photoquantity from a plurality of
differently exposed images of the same scene or subject matter (Mann, 1993, 2000, 2001; Mann and
Picard, 1995a; Ali and Mann, 2012; Robertson et al., 2003; Reinhard et al., 2005).
Specifically, HDR reconstruction returns an estimate of a photoquantity (or sequence of estimates
in the case of video), q(x, y) (any possibly spatially or temporally varying q or q(x), q(x, y, z), q(x, y, t),
or q(x, y, z, t)), on the b asis of a plurality of exposures f
i
= f (k
i
q(x, y)), at exposure settings k
i
,where
there is also noise in each of these exposures f
i
, through a camera response function, f , which is often
unknown (although it may also be known, o r it may be linear, o r it may be the identity map). The
exposure settings k
i
may also be unknown.
A separate optional step of tone mapping the photoquantigraph, q, may be taken, if desired — for
example, to produce an output image that can be printed or displayed on low dynamic range (LDR)
output media. In situations where there is no need for a human-viewable HDR image (eg, HDR-
based input to a computer vision system such as the wearable face-recognizer Mann, 1996b), the
photoquantigraph may have direct use without the n eed to convert it to an LDR image.
A typical approach to generate q from f
i
is to transform each of the input images f
i
to estimates of
that photoquantity, and then to combine the results with use of a weighted sum (Mann, 1993, 2000,
2001; Mann and Picard, 1995a; Debevec and Malik, 1997; Robertson et al., 2003). Other approaches
are probabilistic in nature, and t ypically use nonlinear optimization (Ali and Mann, 2012; Paletal.,
2004).
1.2 HISTORICAL MOTIVATION FOR HDR IMAGING
HDR reconstruction from m ultiple exposures originated with author S. Mann (described as “the
father of the wearable co mputer,” IEEE Internation a l Solid-State Circuits Conference, February 7,
2000), through a childhood fascination with sensing and metasensing that led to the invention of
the DEG (Fig. 1.3). This includes the use of wearable sensors to process and mediate daily life,
from wearable technologies as a photographic art form (Mann, 1985; Ryals, 1995), to gesture-based
augmented/augmediated reality (AR) (Mann, 1997b) for the capture, enhancement, and rendering of