Digital BW, The Print

Roy,

A good post and I concur, for what it is worth. I am sorry I dropped out of
the discussion but it did not seem to be leading anywhere constructive.

I favor the simple explanation when it will do and the classic view of the
dynamic range in photography as being the ratio of the useable/meaningful
max and min seems to me to suffice quite nicely.

The one thing that I will add is that in both the audio and the imagining
models there is no mention of discrete or finite numbers of volume levels or
brightness levels. The implied assumption is that any level in between can
be used or achieved. Even if theory suggest a finite number of levels
actually exists, this seems totally irrelevant due to the factor of human
perception. If we could clearly hear or see distinct level changes in the
signal we would simply reject the audio or the image as being of
unacceptable quality.

Thanks,
Martin

----- Original Message -----
From: "royvharrington" <roy@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Monday, April 08, 2002 10:30 PM
Subject: [Digital BW] Final(ish) Ranges about Imaging


>
> Hi Austin (is anybody else still listening?)
>
> This discussion has gotten badly out of hand.  We seem to
> be on totally different wavelengths of thought.  It certainly
> hasn't been all bad, though.  No doubt, I've learned a few things
> and its stimulated my thinking on the whole matter.
>
> But:
> We just barely agree what DynRange of audio is.  I'm not really sure!
> We don't agree on DynRange of Imaging. (DROI)
> We don't agree on whether DROI is one specify thing.
> We don't agree on DROI has been used in the past.
> We don't agree on DROI if used really is a misuse.
> We don't agree on DROI changed from correct use to misuse
>    because of the internet.
> We don't agree on if there is a common use of DROI.
> We don't agree on whether common use of DROI is a misuse.
> etc. etc.
>
> The dialogue seems to be bogging down in such esoteric and semantic
> details that we can't see the forest from the trees.
>
> I don't have the long history of working with the DynRange
> concept you have.  But ... I've got a good math and science
> background.  I have a pretty idea of the audio/hearing and
> image/sight perception issues.  I'm not a super expert guru
> in all this stuff, but the math is well within my experience
> and the perception issues are simple enough.  For whatever its
> worth my approach has been to look and research the whole
> picture without an emphasis from one specific direction.
>
> So. The only thing that makes a lot of sense to me now is to
> spell out my logic.  I really don't we're in the realm of
> proofs one way or the other.  I think you've said several times
> that you can take DynRange of any variable you like.  Right?
> If we start with "dynamic range of an image", no variable is
> stated so who's to be the dictator of what the variable is?
>
> What follows is basically by definition just my opinion.
> I've made choices based on what I think is important and
> just plain: how I see it.  The whole package goes together.
> There are many choices but they all relate in some way.
>
> ---------------------------------------------
>
> There's a lot written about DynRange of audio and sound.
> Most of my approach is in moving the concept from the audio
> realm to the imaging realm.  I'm very influenced by the
> parallelism of the physical properties, of the perception, and of
> the math involved.   The parallelism is so compelling that
> I place very high importance to it.  Modelling in the
> abstract is used extensively in scientific research.  Here
> we actually have two physical systems that can share a
> model or even model each other.
>
> First some of the physical properties.  Both sound and
> light are wavelike phenomena.  Sound is a pressure wave
> moving through the air, there is an energy associated with
> it the amplitude of the waveform determines the energy.
> Likewise, light is an electromagnetic wave which has
> an energy as well.  In both cases some source generates
> the respective waves and they propagate in space.
> We have ears and eyes to detect these waveforms.  A key
> aspect is that both senses perceive linearly increasing
> intensity when the underlying waveform has an
> exponentially increasing energy level. Or mathemtically
> speaking the perception intensity is proportional to
> the log of the waveform energy. This parallelism between
> energy of the two waveforms and the perception of the
> two waveforms is so strong that I believe it only makes
> sense to apply the mathematical constructs in a similarly
> parallel fashion.
>
> In the audio realm we have:
> The decibel dB is used extensively as a measure of a power ratio.
>
> db = 10*log10(power1/power2)
> If there are two power levels they differ by this many dB
> given the levels power1 and power2.
>
> There is one special dB measurement called dynamic range.
> It has the same formula structure but uses two specific
> power levels: the maximum power and the minimum power.
>
> DynRange (dB) = 10 * log10 (maxpower/minpower)
>
> It is basically a measure of the largest power ratio that
> the audio system can produce.   Power is just energy
> per time unit, so a power ratio is identical to an
> energy ratio ( the "per time" unit whatever it is cancels
> out in the ratio).  So DynRange becomes a measure of the
> energy ratio in the loud sound wave vs. the quieter sound
> wave.
>
> Since sound and light have the same energy vs perception
> characteristics, it makes sense to define DynRange for
> light to also be a measure of the energy ratio of two
> extremes -- called bright and dark. "bright" is the
> max light energy and dark is the min energy. How shall
> we measure light energy and what units?  Since we are
> only ever interested in the ratio of two light energies,
> the unit whatever they are always cancel.  Similarly,
> we don't have to measure absolute values anyway -- just
> get the ratio of the energies.  Well in photography
> we do this all the time.  A one stop difference in light
> is just a 2x difference in light energy i.e. ratio = 2.
>
> As a photographer light ratios measured in stops is
> used all the time.  But how do you get the ratio given
> the number of stops?  1 stop = 2, 2 stops = 4, 3 stops  = 8.
> Ratio = 2^numstops
>
> Putting this all back into the dB formula we have:
>
> dB = 10 * log10 (ratio)
>    = 10 * log10 (2^numstops)
>    = 10 * log10(2) * numstops
>    = 3dB * numstops
>
> Not surprisingly, this is the same as audio 3db = double power.
>
> Now back to the main subject, DynRange.  With an appropriate
> choice or measurement of maxlight value and minlight value,
> we simply have:
> DynRange (dB) = 3dB * (number of stops between min and max)
>
> dB certainly could be the unit we do all calculations,
> however in the imaging realm we have several units that are
> already very well used.  We talk about stops everywhere in
> photography -- making a new unit that is just 3 * stops
> isn't necessary at all.  It makes sense to me that the unit
> we use in imaging is the one that we already have. With
> the  multitude of technologies involved in imaging, we
> actually  have another unit that is very similar and thats
> the density unit -- this is just 0.3 * stops.   It looks
> very much like dB and if it didn't already exist already.
> Looking at these three units they are all dimensionless
> numbers that are the log of an energy ratio.  For historical
> reasons stops are log base 2 whereas the others are log
> base 10, otherwise they are all equivalent units.  Since
> different imaging technologies have long used certain units,
> it makes sense to use the appropriate unit.  The conversion
> is very simple anyway.
>
> This sounds almost trivially simple but the important issue
> is the choice of maxlight value and minlight value.
> Conceptually, you just pick the brightest value and the
> darkest value -- but there's lots of room for tweaking
> the idea -- like does the value need texture or detail or
> go just beyond that or what.
>
> At this point I'm just showing a derivation that produces
> a reasonable concept with an intuitive understanding and a
> potential measuring method.  It has a basic meaning that
> matches the usage of the term in several existing areas.
> Ansel Adams' book The Negative uses the term dynamic
> range which is very similar -- the useful Zones or stops
> for the Zone system.  The term in also used quite a bit
> in the scanner realm and in that context the specification
> "dynamic range" is stated as the range of densities that the
> scanner can handle.
>
> -----------------------------
>
> So that's my take on dynamic range for imaging.  In my opinion
> its the most natural way to define the idea.  I don't
> think there are any large leaps of faith. Also, others have
> used a similar concept.  I'd have to admit that there are
> lots of usages that are not really technically defined.
> But they seem to just mean that good dynamic range
> translates to "looks good tonally".  I can't really see
> this as contradictory.
>
> I haven't defined precise ways to define the max and min because
> I think various people might want different standards for
> that -- just as the audio dynamic range equation doesn't
> specify the standard for max and min.
>
> I'd be glad to clarify anything in this discussion.  Austin,
> if you would like to put your whole story together into
> one package it would be very helpful to see where you stand.
> I imagine this grates against you a bit, but ... I can't help it.
> The discussion, as before, lost most of the semblence of
> what we wanted to define.  I'd like to say I'm open minded
> about the whole thing, but all the above fits so well.
>
> -------------------
>
> Best regards,
>
> Roy
>
> Roy Harrington
> roy@...
> Black & White Photography Gallery
> http://www.harrington.com
>
>
>
>
>
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