Digital BW, The Print

-----Original Message-----
From: DigitalBlackandWhiteThePrint@yahoogroups.com
[mailto:DigitalBlackandWhiteThePrint@yahoogroups.com]On Behalf Of Manuel
Toledo Quinones
Sent: Tuesday, April 10, 2007 12:12 PM
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: [Digital BW] Re: 8bit to 16 bit


So, if my understanding is correct,

8-bit rgb = 256 levels x 3 channels = 768 levels (if converted to b&w)

or the equivalent of 9.5 greyscale bits.

Converting the 8-bit file to a 16-bit file produces 64k possible
levels, which will be initially discretely distributed into 256
levels, i.e. each pixel color will have a value equal to 256*x,
where x is the original 8-bit pixel value. So just after the
conversion the information is the same in both the 8- and 16-bit
files. The numbers are just bigger but you still have use 256 levels
per color.

However, after the conversion, you can adjust the pixel values more
finely because you have the 64k levels.  So processing the image in
16-bits should create less artifacts that doing so in 8-bits. The
difference might be visible depending on how good your sight is
and on the amount of processing.

Right?

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "dealy663"
<dealy663@...> wrote:
>
> A little bit of computer science here.
>
> An 8 bit BW file can have at most 256 levels of gray tones, a 16 bit
> file can have 65536 independent tones. Converting from 8 to 16 bit
> simply maps those original 256 tones into the 65536 space. If you had
> a linear gradient going from black to white at the start it would
> appear exactly the same after the conversion. You would still have 256
> independent tones. However you would also have room between each tone,
> 256 blank spots with no gray vales on either side. By this I mean that
> for the gray value that was originally 128 in the 8 bit file it would
> now be 32768. The difference being that in the 8 bit file there were
> picture elements represented by values 127 and 129. While in the 16
> bit file those values would be represented by 32512 and 330224, there
> would be no pixels in the image with values between 32768 and these
> other two numbers.
>
> Next lets say that you adjust the global contrast of your image. In a
> simplified algorithm, an increase in contrast makes the values below
> the midpoint (128 8-bit) darker and the values above the midpoint
> lighter. In an 8 bit file if we subtract 1 from each value less than
> 128 then we now have 126 tones where we had 127 tones before the
> subtraction. This is because the pixels that were formerly at value 1
> are now at zero along with all the pixels that were previously at
> zero. The same logic applies above the midpoint, so where we
> originally had 256 tones before the contrast adjustment we now have
> 254. In a 16 bit file applying the same type of correction still
> leaves you with the original 256 independent tones, because there was
> 256 bits of headroom between each original pixel value. You would have
> to adjust the contrast by more than 255 points before you started
> reducing the number of tones in your image.
>
> The scenario I've described above leads to the posterization that we
> see when making large adjustments in 8-bit files. It is not true that
> all image editing leads to the loss of data, but large movements
> certainly can. There are other issues with 8 vs 16 bit related to
> rounding errors also.
>
> With all that said, I rarely change my 8-bit files to 16-bit before
> editing. I would only do so if there was something really wrong with
> my exposure and I needed to make large adjustments to the file.
>
> Hope I didn't confuse the issue too much.
>
> Derek
>




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