"Roy Harrington" <roy@harrington.com> wrote:
>>>>>>>>>>>>>>(Julian)
> So in the first case, the DyR is : max/MDS = (4096 steps) / (1 step),
and
> in the second 256/1. i,e, DyR 4096 vs 256.
>
> Looking at it another way, with an 8 bit file, the bottom step is the same
> level as step 16 was in the 12-bit case. So when you converted from the
> 12-bit to the 8-bit, you lost the 16 lowest steps and combined them all
> into 1, the lowest level of the 8-bit situation. In that conversion you
> lost the 16 lowest shades of gray, permanently. So all that info is gone
> and your MDS is now 16 times larger, and correspondingly your DyR has
> diminished by the same amount, 16 times.
I'm sticking with 0 in both files producing the same black and 255 or 4095
producing the same white paper. So the ratio of the amount of light from
either file will be the same.
<<<<<<<<<<<<
The problem here is that you are arbitrarily reinterpreting the values 0 and
2**n-1, and then claiming that that's what they originally meant.
What any number reported by a scanner "means" is that the density at the
point measured was in the range of density values for which the scanner
reports that value. Since that's a rather circular definition, you need to
provide specifications for the scanner that allows users to understand what
those ranges are.
In particular, the range of densities that a (real) 8-bit scanner will
report "0" for is different than the range of densities that a (real) 12-bit
scanner will report "0" for. (Here "real" means "actually performs as an
n-bit scanner, i.e. that the only noise in the digital output is the
quantization noise inherent in a digital value of that number of bits.)
FWIW, here's my current take on this:
>From an engineering perspective, it takes (at least) _three_ numbers to
characterize a digital (actually, any numerical) measurement: the center
value of the range for the minimum value, the center value of the range for
the maximum value, and the dynamic range. Scanner types fudge this by only
reporting two numbers (Dmax and Dmin) and their ratio. That only works
because the range for 0 can be assumed (a) to include true black (obviously,
for scanners) and (b) to be the same width as all the other ranges*, i.e.,
that there is no significant offset at the low end. If the range of
densities for which the scanner reports 0 was significantly larger than the
range of densities for which it reports other values, the discussion at
would be quite dizzy. Since the offset at Dmax can be ignored, it all works
out, but leaves people quite confused about dynamic range and density range.
*: My intuition here is that this is actually _not_ true for scanners, but I
infer from the definitions used at the above web page, that it _is_ a very
good approximation.
David J. Littleboy
davidjl@gol.com
Tokyo, Japan
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