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Re: [OM] (OM) Competition

Subject: Re: [OM] (OM) Competition
From: Moose <olymoose@xxxxxxxxx>
Date: Tue, 30 Aug 2011 01:55:43 -0700
On 8/29/2011 5:04 PM, usher99@xxxxxxx wrote:
> ...
> I subscribe to this list and for no extra charge get PS pointers from
> PS Moose. :-)

Thanks -- I think.

> Appreciate the discussion and the link  as it  clarifies  the issues.
> I did a forensics on the impression I had from some saved links I had
> files  and it actually  did refer to LR brightness.
> It was transferred in my long term portable  storage to apply to PS
> which may or may not be correct. Gotta watch that.

Nomenclature in image manipulation is notoriously sloppy. Imagine if the 
nomenclature in your profession were so poor!

> I suppose that if one crunches up the highlight area w/o moving the white 
> point though not actually clipping to pure white, so much info is lost as to 
> render the non-clipping a matter of semantics.

First, I hope you are referring to some control other than the PS Brightness 
tool - It does indeed move the white point 
down or black point up, irretrievably losing data to clipping.

Second, not necessarily so. If working in 16 bit or more, there are so many 
numerical values available that compressing 
toward the WP without clipping will make the image appear to have lost all that 
data. However, a reverse process, such 
as moving the midpoint in Levels or use of the Highlight tool will usually 
restore the lost tonal detail, at least to 
the eye.

Consider an 8 bit image. That's enough for printing or web viewing without 
visual degradation. Convert to 16 bit. The 
number of steps from 127 to 255 on the top was 128. In 16 bit, it's become 
32,768 steps. You can push all the original 
tonal information way up in the top of that range. Look at it on screen, and it 
looks all white. Pull it back down, and 
nothing need be lost.

If you start with a true 16 bit image*, data will indeed be lost, but probably 
not noticeable to our human vision 
systems, depending on how extreme the compression is.

It's different at the other end. Convert to 16 bit and the first step, 0 to 1, 
becomes just 4 values. The next step, 
from 2 to 3, becomes 16 values, and so on. Compress at the bottom end, and you 
can't get it back.

Moose

* I haven't kept up, but early camera D/A converters were 12 bit, then 14 bit 
showed up.  I haven't heard of 16 bit. So 
we probably never have true 16 bit images to work with. I've sort of assumed, 
for the reasons above, that the cameras 
map their output into the top of the 16 bit range, leaving the bottom empty.
-- 
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