DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

A raised cosine distribution is roughly
similar to a Gaussian. See page 542.
Schreiber and Troxel suggest reconstruction
with a sharpened Gaussian
having σ = 0.3. See their paper cited
in the marginal note on page 192.
structure is likely to be visible. If an external optical
element such as a lens attenuates high spatial frequencies,
then a box distribution might be suitable. A simple
and practical choice for either capture or reconstruction
is a Gaussian having a judiciously chosen halfpower
width. A Gaussian is a compromise that can
achieve reasonably high resolution while minimizing
aliasing and minimizing the visibility of the pixel (or
scan-line) structure.
Spatial (2-D) oversampling
In image capture, as in reconstruction for image display,
ideal theoretical performance would be obtained by
using a PSF with a sinc distribution. However, a sinc
function cannot be used directly in a transducer of light,
because light power cannot be negative: Negative
weights cannot be implemented. As in display reconstruction,
a simple and practical choice for a direct presampling
or reconstruction filter is a Gaussian having
a judiciously chosen half-power width.
I have been describing direct sensors, where samples
are taken directly from sensor elements, and direct
displays, where samples directly energize display
elements. In Oversampling, on page 174, I described
a technique whereby a large number of directly
acquired samples can be filtered to a lower sampling
rate. That section discussed downsampling in one
dimension, with the main goal of reducing the
complexity of analog presampling or reconstruction
filters. The oversampling technique can also be applied
in two dimensions: A sensor can directly acquire a fairly
large number of samples using a crude optical presampling
filter, then use a sophisticated digital spatial filter
to downsample.
The advantage of interlace – reducing scan-line visibility
for a given bandwidth, spatial resolution, and
flicker rate – is built upon the assumption that the
sensor (camera), data transmission, and display all use
identical scanning. If oversampling is feasible, the situation
changes. Consider a receiver that accepts progressive
image data (as in the top left of Figure 6.8, on
page 59), but instead of displaying this data directly, it
CHAPTER 18 IMAGE DIGITIZATION AND RECONSTRUCTION 193