Re: [CinePaint] Really basic script for batch converting.

On Tue, 24 Nov 2009 13:29:19 -0000 (GMT)
[email protected] wrote:

> 
> Kevin Connery , Jun 21, 2006; 06:41 a.m.
> 
> Photoshop's 16-bit mode is really 15-bits + 1 bit of noise.
> 
> 
> Kevin Connery , Jun 21, 2006; 06:48 a.m.
> 
> Oops. Minor correction. 'Native' 16 bit for Photoshop before CS2 was a 
> 16-bit file, but > with 15 bits of precision. CS2 supposedly--but I haven't
> tested it--16-bit precision.
> The conversion from 8 to 16 adds noise.

At the risk of sounding a bit arrogant and condescending, these people
seem to be Photoshop dilettantes who don't really understand image
processing.

Added noise during signal conversions is called "dithering" and it serves
various functions.  During sample rate conversions of audio signals,
for example, dithering is used to prevent spurious frequency production
due to the imperfect data truncation.  But such added noise is not enough
to be otherwise perceptible or to have a large effect on the original
signal data.  Dithering is also used for image color conversions between
different color depths (a form of sample rate conversion.)  Again, the
same considerations apply.  However, dithering is never used to eliminate
the gaps in a histogram that result from conversion to simply represent
bits using a higher bit depth.

You should not use forums as a primary source of information.  Consult
professional texts (either online or hard copy) and limit forums only
to supplemental information where a clear cut problem is addressed.

Does Photoshop have technical manuals that discuss their specific
conversion operations?  If so, then consult those.


> 
> This thread was also interesting:
> 
> http://www.northlight-images.co.uk/article_pages/16_bit_black_and_white.html
> 
> 
> 20/20/20 goes to 5120/5120/5120 > L=5120
> 21/20/20 goes to 5376/5120/5120 > L=5197
> 20/21/20 goes to 5120/5376/5120 > L=5274
> 20/20/21 goes to 5120/5120/5376 > L=5147
> 
> That's 4 levels of grey where in 8 bit you would have two (20/21/20 goes
> to L=21).

But relative to the total scale of 65536 levels these differences are
minuscule and totally insignificant.  On a histogram they would cluster
imperceptibly if not be totally within the same bin.

This is a commercial site.  The author wants to attract readers
for his enterprise.  Notice his remark:

> I've ignored the fact that Photoshop '16 bit' is actually signed
> 15 bit i.e. +-32k rather than 0-64k)

So what?  The overall range is exactly the same regardless if we use
signed or unsigned integer representations.

Or check this:

> 
> The difference comes from the fact that an 8 bit RGB file represents 24
> bits of information which, depending on your choice of conversion
> technique, can represent more levels of greyscale that the 256 allowed in
> an 8 bit greyscale image.
> 

What is meant by "choice of conversion technique?"  Presumable he refers
to the determination of luminosity from the specific RGB values which
can be done in more than one way.  Indeed, both the GIMP and Cinepaint
equate luminosity in some functions with the max(r,g,b), or the greatest
value of the three.  But for practical purposes it make little difference.

Frank Peters

------------------------------------------------------------------------------
Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day 
trial. Simplify your report design, integration and deployment - and focus on 
what you do best, core application coding. Discover what's new with
Crystal Reports now.  http://p.sf.net/sfu/bobj-july