Re: [fluid-dev] Purpose of dither?

[Z F]

--- Miguel Lobo <address@hidden> wrote:

> >
> > It does not matter if you add random float to a float and truncate
> or
> > trancate and add a random bit, the outcome will be exactly the same
> > sequence of bits so it does not matter, you can always add dither
> > later.
> >
> > Write it out...
> >
> > trun(signal + noise) = trun(signal) + noise'
> >
> > notice that noise' is not the same as noise, but the equality holds
> > so one can do what can not be done -- remove undesired harmonic
> > after truncation.
> 
> 
> The problem is that there is no way of calculating noise' after the
> truncation.  You would need the information that was lost in the
> truncation.
> 
> I think a visual example might be easier to understand.  In the
> Wikipedia
> page for dither http://en.wikipedia.org/wiki/Dither they have a
> series of
> pictures of a cat.  The first one is the original high-depth
> photograph.  In
> the second picture, truncation has been applied without dithering and
> you
> can see big flat areas of uniform color.  How can you know which was
> the
> original shade of a pixel in those flat areas?  You can't.  In the
> third
> picture, truncation has been applied with dithering.  You can see
> that
> although the palette of the third picture is the same as that of the
> second,
> the "average" color of each area has been much better preserved. 
> There are
> more bright pixels where the original picture was brighter, and fewer
> where
> it was darker.  To do that you need the information in the original
> picture.


No, you do not... :) Ever heard of texture synthesis? All you need
is statistical properties, not the signal itself to generate which
looks like a cat. So, it can be done without the original :) Since the
main features of the signal are preserved after truncation (otherwise
it would have been useless), it should be enough to synthesize noise'.

ZF



       
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