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[filmscanners] Re: 8bits vs. 16bits/channel: can the eyeseethe difference



on 3/16/03 8:43 PM, Austin Franklin at austin@darkroom.com wrote:

> Hi Roy,
>
>> I think you missed the rest of Paul's statement.  As he says "finer
>> gradations are indeed represented in an 8-bit image through dithering".
>>
>> If you scan a real image in 16-bit mode and there are more gradations
>> between say 128 and 129, even after converting to 8-bit mode there
>> will still be gradations between a pure 128 and a pure 129 patch.
>> Photoshop creates these extra gradations by dithering the transition
>> between 8-bit values.
>
> Then it really wouldn't matter if they existed or not, as it simply wouldn't
> know...and it would create them anyway, right?

Not sure I fully understand your question/comment.  What I'm saying is that
the in between values that can obviously be there in the 16-bit are actually
preserved on conversion to 8-bit, not that anything new is created.  When I
used the word "creates" I mean PS creates a 8-bit representation of the
existing 16-bit gradation not that it makes up new gradations.

>
>> In fact, by default, even the gradient tool
>> smoothes transitions from one 8-bit values to the next -- you don't
>> get 256 steps you get a smoothed gradient from end-to-end.
>
> What about any other display program (not PS), or printing?  I mean, the 8
> bit data is simply 8 bit data, the "in between" data just doesn't exist.

Actually, the "in between data" does  exist in the form of dithering.  Just
as a print can have more grays than the number of inks, so to can the
PS file represent more grays than the 8-bits might seem to allow.

> Creating this "in between" data has to be something that is deliberately
> done, and it may in fact be creating something that didn't exist in the
> first place!

Again, I'm not talking about CREATING "in between" data I talking about the
representation of it where it already exists.

>
> I did not know PS did this, thanks for the info.  Do you have a pointer to
> where it is documented?

Documentation of all the nitty-gritty is pretty hard to come by.  The
gradient tool has a dither selection box so you can turn the feature
on and off.  What I've done in the past is simply test it and try it out.
Create a gradient and then look at the data by increasing the contrast.
Create some 16 bit data the you know has intermediate values and then
change it to 8-bit and then again see what you got.

One thing to remember about dithering is that its a resolution versus
tonal value tradeoff.  So the more resolution you give the above tests
the better the tonal gradations you'll see.

>
> Regards,
>
> Austin
>

Roy

roy@harrington.com
Black & White Photography Gallery
http://www.harrington.com


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