[Leica] aliasing

[pdzwig at summaventures.com (Peter Dzwig)]

Jean,

Thanks for your comments. I think that you have mis-understood what I was 
driving at -and re-reading it I think that I may not have been terribly well.

I started to write a response to you but it rapidly became much more 
technical 
than the LUG would be interested in - no, HONESTLY,guys - can I suggest that 
we 
take this off-list.

We can then produce an agreed note which we can put back on the list.

Yours,

Peter Dzwig

Jean Louchet wrote:

> Hi, it's me again :-)
> 
> 
>>From: Adam Bridge <abridge@gmail.com>
>>Subject: Re: [Leica] Fungus in camera
>>To: Leica Users Group <lug@leica-users.org>
>>Message-ID: <4cfa589b040701233643a4a3ac@mail.gmail.com>
>>Content-Type: text/plain; charset=ISO-8859-1
>>
>>It seems very odd to me that Leica is trying to eliminate the
>>anit-aliasing filter. I distinctly remember my digital signal
>>processing courses that state that before you sample an analog signal
>>you have to limit the bandwidth with a filter to at least half the
>>sampling frequence (and that would be if you had a perfect low-pass
>>filter, you have to go lower for real-world filters.)
> 
> 
> Yes, _before you sample_, not after. The point is that the word "filter"  
> has 2 different meanings. The only filter that will work BEFORE sampling
> is an optical filter (a piece of glass). After sampling, the only possible
> resource is digital filters (image processing algorithms) but the harm has
> already been done, and you just can't recover information that has been
> lost (as I wrote in a recent posting).
> 
> 
>>Moire patterns would be the result because artifacts would be generated
>>by digital sampling.
> 
> 
> YES but how could the algorithm make the distinction between an artefact
> moire and the pattern on one's shirt?
> 
> 
>>I had a real-world example of this where we were
>>measuring ion-acoustic waves in very low-density argon plasmas and got
>>results that were really exciting - until the experimentalist realized
>>he hadn't built-in the low-pass filter. When the filter was installed
>>the "exciting" results vanished and things approached theory. Oh darn.
>>But that wasn't with optical systems and maybe someone who's won the
>>galactic institude prize for extreme cleverness has figured out how to
>>avoid the artifacts.
>>
> 
> I suppose you used an analog filter BEFORE sampling to solve the problem 
> (in acoustics, analog filters are not pieces of glass but probably some 
> mechanical device?). Once sampled it's too late!
> 
> 
>>From: Peter Dzwig <pdzwig@summaventures.com>
>>Subject: Re: [Leica] Fungus now filters
>>To: Leica Users Group <lug@leica-users.org>
>>Message-ID: <40E559B5.6060404@summaventures.com>
>>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>
>>The point of anti-aliasing is to smooth out "the jaggies", the jagged
>>edges in digitised images caused by finite resolution.
> 
> 
> You are referring to what Computer Graphics people call "antialiasing"  
> but the same word does not refer to the same reality. What they call
> antialiasing is the set of rendering techniques to smooth out edges (like
> the Bresenham algorithm). The aliasing problem found in digital cameras is
> another issue, it is a sampling problem as Adam said, not a rendering
> problem.
> 
> 
>>In theory with a large enough number of pixels in a camera you should be
>>able to ignore the effects because the human eye wouldn't be able to
>>resolve them.
> 
> 
> This is not true. Even with as many pixels as you want, if the lens
> resolution is good enough to transmit spatial frequencies higher than half
> the pixel frequency AND the sensor elements are not contiguous, there will
> be aliasing which will generate low- and medium-frequency interferences
> that anybody will be able to see (even without a very good sight).
> 
> 
>>The need for anti-aliasing is a result of the artifacts introduced by
>>the availability of a finite number of pixels to display an image in.
>>For a standard monitor this number is 1.25Mp and here anti-aliasing is
>>necessary without doubt.
> 
> 
> No, you are again mixing up two different problems, one is sampling and 
> the other one is rendering.
> 
>>It MAY be - and I say may because I don't know for certain - that at 14
>>MP, up to certain "reasonable" magnifications the effects are either not
>>noticeable or are swamped by other effects or are cancelled out by the
>>effects of other algorithms, when the digital image is viewed or 
>>printed.
> 
> 
> I don't agree. Of course aliasing will be reduced but this is just because
> lenses act as lowpass filters. With high quality lenses there will still
> be aliasing with 60MP. As I wrote last week, the two only methods are: not
> using a lens "better than the sensor", or designing a sensor where there
> is no gap between adjacent photosensitive elements (this will also act as
> an analog lowpass filter). The computer graphist's antialiasing is just
> digital (algorithmic)  antialiasing and can't do anything to repair a
> violation of Nyquist-Shannon's rule.
> 
> 
>>On Thu, 01 Jul 2004 08:49:02 -0400, Dan C <bladman99@yahoo.ca> wrote:
>>
>>>The other article concerns the digital back for the R8/R9.  They are
>>>aiming for a December launch, but they seem to be having problems
>>>getting the internal image processing software ready.  Leica isn't
>>>happy with it.
> 
> 
> My own guess is that Leica people (who are great optical and mechanical
> engineers but maybe not digital image processing specialists) are
> discovering that digital image processing is hopeless if sampling was not
> good enough. R lenses are so sharp that aliasing problems must be really
> hard and digital sensors not up to the task. Again, non-adjacent
> photosensitive elements are a real problem with good optics. There is no
> aliasing trouble in the Espio at 400x640 pixels with a cheap plastic
> single element lens!
> 
> Jean
> 
>  ------------------------------------------------------------
>  Dr Jean Louchet       COMPLEX Project     INRIA Rocquencourt
>                        BP105   78153 Le Chesnay cedex, France
>  Jean.Louchet@inria.fr     http://fractales.inria.fr/~louchet
>  ------------------------------------------------------------
> 
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