Re: Digicam

[Kennedy McEwen <rkm@xxxxxxxxxxxxxxxxxxxx>]

In article <FAEBJHPJNNGCAGDNGLNPIEIHEIAA.gsellani@accesscom.com>, gary 
<gsellani@accesscom.com> writes
>Those familiar with audio sampled data systems know that the sample time is
>infinitely small. Equating this to imaging, the pixel should not have size,
>but rather a sample density.
>
No.  Whilst the audio analogy certainly has a very short sample 
acquisition period, the signal is band limited by the pre-filter - which 
has the same effect as convolving the time domain waveform with an 
aperture of a specific shape and size.  (multiplication in frequency 
space corresponds to convolution in time).  The finite aperture of the 
pixel or spot performs a similar presampling filter function in image 
sampling, and the shape and size determine the exact spatial bandwidth 
that is samples.

>Going back to the audio analogy, if you take the  sample impulses and play
>them back on a DAC making staircase waveforms, the high frequency response
>will be incorrect. The nature of the staircase implies a sinc filter was
>used, so the playback must incorporate an inverse sinc filter. Since the
>image sampling is not infinitely small, I wonder if the use of an unsharp
>mask is analogous to the inverse sinc filter.
>
No - an unsharp mask has a completely different spatial frequency 
response from a sinc or inverse sinc, so it isn't doing anything similar 
to it.

Remember that in order to see (or hear) the staircase in audio you must 
examine the waveform in sufficient detail to see individual samples as 
they are converted by the DAC.  In imaging, this is the same as seeing 
teh individual pixelation.  So the equivalent of filtering the 
transition between filters is to resize the image with pixel replication 
and then apply an inverse sinc filter of the appropriate size.  As it 
turns out, for zoom ratios up to around 2-3x, this is closely 
approximated by bicubic interpolation, not unsharp masking.

Also remember that when viewing at normal size, the CRT spot or LCD 
pixels perform a similar spatial filter on the final image.
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers
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