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[filmscanners] Re: 8bits vs. 16bits/channel:cantheeyeseethedifference



on 3/18/03 11:28 PM, Paul D. DeRocco at pderocco@ix.netcom.com wrote:

>> From: Austin Franklin
>>
>> Can you give a concrete example of what you believe it does?  Do
>> you believe
>> it takes 0x1289 and converts it to 0x13?  Let's say the entire image is
>> completely 0x1280...are you believing that ~half the 8 bit data
>> is 0x13 and
>> ~half 0x12?  Yes, if you believe that, and can demonstrate it, I'd like to
>> see it, and I would agree that is dithering.  But really, no
>> entire field in
>> real life would consist of 0x1280...there would be a range, and
>> in reality,
>> simply converting by lopping off the 8 LSBs will give you the same
>> "fidelity", from an analysis standpoint.
>
> Photoshop doesn't do this. It optionally adds dither noise when creating an
> artificial gradient, but a simple experiment proved to my own satisfaction
> that merely converting from 16bpc to 8bpc does nothing more than truncating
> each 16-bit value.
>
> --
>
> Ciao,               Paul D. DeRocco
> Paul                mailto:pderocco@ix.netcom.com
>

Paul,

Did you see my example showing that PS does in fact do more than just
truncating 16 bits to 8 bits?   PS in a 16 to 8 bit mode change simulates
intermediate values by dithering exactly like the gradient tool does.
I don't know whether it's done by adding noise and truncating or a more
elaborate algorithm but PS does make the average 8 bit gray value keep as
much of the 16bit data that it can.

Roy

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


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