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> I use PS7 on a PC. I'm having a problem with 16- to 8-bit conversion
> and wonder if any other PC users have noticed it also.
> Try this:
> 1. Open a new 256x256 RGB image
> 2. Define a gray, e.g. <9, 9, 9>
> 3. Using the paint bucket, fill the image with the gray
> 4. Display the histogram. You should see a single straight line
> for each of the 4 charts with zero standard deviation
> 5. Convert to 16-bits, i.e. Image, Mode, 16-bits
> 6. Convert to 8-bits, i.e. Image, Mode, 8-bits
> What results on my PC is that 237 of the pixels convert to <10,10,10>.
> The deviate pixels are located randomly, with the first occurring at
> 0, 221. I wonder if I'm seeing an "undocumented feature" of PS. If
> this isn't replicable it's probably a hardware bug on my machine.
>
> I noticed this initially in a 16-bit image with large areas of uniform
> color. When I convert it to 8-bits, however, the incorrectly
> converted pixels are noticeable as speckles. On my PC this
> paradoxically means that using 16-bits means a loss of accuracy.

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> By default Photoshop adds noise to the image when converting from 16 bits to
> 8 bits. There is logic behind this. It prevents banding in some
> situations.

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> The good news is you can turn off this feature by un-checking the "Use
> Dither" conversion option in the Color Settings dialog.

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> Mike Russell wrote:
>
>
> With so few pixels changed, about 1 in 277, it's difficult for me to imagine a
> situation where it would accomplish that.

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>
>
> Thanks, that's it. Talk about an obscure and hard to find option!
>
> Dane

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> 2. Define a gray, e.g. <9, 9, 9>
> 3. Using the paint bucket, fill the image with the gray
> 4. Display the histogram. You should see a single straight line for
>each of the 4 charts with zero standard deviation
> 5. Convert to 16-bits, i.e. Image, Mode, 16-bits
> 6. Convert to 8-bits, i.e. Image, Mode, 8-bits

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>
>What would you gain by converting an 8 bit image to 16 bit? and then back
>to 8
>bit. Wouldn't that be like putting perfume on a pig? Sorry, I had to say
>that.
> Working on a 16 bit image is better, but the image should start out as
>16 bit,
>I would think, as in a RAW image, or a 16 bit scan.

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back[QUOTE][color=darkred]
>
> Certainly starting with a 16 bit per channel image (even if it is really

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> 10-12 bits from a digital camera) and working entirely in 16 bit mode

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> final step when you reduce to 8 bits for printing is the best way to

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> when you can. But even if you start with an 8 bit image, there are many
> advantages to promoting it to 16 bits before processing. It prevents a lot

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> loss of precision in processing operations - that includes filters as well

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> gamma adjustments and layer modes. Most workflow experts recommend working

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> 16 bit mode whenever possible, and that was one of the major driving

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> behind Photoshop CS providing complete support for 16 bit images and

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> "DrJohnRuss" <drjohnruss@aol.com> wrote in message
> news:20040127193156.17180.00000946@mb-m03.aol.com...
lot of[QUOTE][color=darkred]
well as[QUOTE][color=darkred]
working in[QUOTE][color=darkred]
factors[QUOTE][color=darkred]
layers.[QUOTE][color=darkred]
>
> Doc, I think she's right. You don't gain anything by stretching out the

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> info; at least, nothing I've read or heard indicates that is the case.

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> "Flycaster" <noyb@nospam.com> wrote in message
> news:40171433_1@corp.newsgroups.com...
> lot of
> well as
> working in
> factors
> layers.
> bit
>
> I suspect that Dr. Russ may know something about numerical analysis,
> unlike most Photoshop users. Numerical analysts tend to be horrified
> by Adobe's programming. Perhaps unduely so. Our customers may
> complain bitterly about an error in the 15th significant digit. Photoshop
> routinely produces errors in the 3rd digit and most people don't notice.
>
> Most Photoshop operations incur numerical error. This error can
> accumulate if you perform multiple commands or use multiple layers.
> Some operations, such as a small Curves adjustment, are nothing to worry
> about. Other operations, such as conversion between color modes with
> large Curves adjustments, can incur noticeable error, especially in
> high-quality prints of images with smooth gradients like a clear blue sky.
> Using 16 bits, the accumulation of error is negligible except in the most
> extreme cases, regardless of whether the image started as 8 or 16 bits.
>
> Here's an example where I converted an 8-bit RGB sky photo to LAB,
> applied a steep curve, and converted back to RGB:
> http://home.nc.rr.com/sarle/Test5-RGB8-Curved.tif
> http://home.nc.rr.com/sarle/Test8-RGB8-Curved.tif
> One of these was done all in 8 bits. The other started with the same 8-bit
> image but I did the conversion and curve in 16 bits. Can you tell which
> is which? Maybe not just by looking at them on a monitor, but you can
> tell from the histograms. If you stacked up several curves layers, the
> difference might become obvious. I'll try that when I get CS.
>
> I have a bad habit of applying curves repeatedly using actions. This
> can produce severe posterization in 8 bits, but the results are not
> intended to look like realistic photos. So I think Dan Margulis is
> right for most practical purposes regarding photo processing.
>
> Man, those Adobe programmers have cushy jobs! Getting away
> with errors in the 3rd digit!

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> "Warren Sarle" <saswss@unx.sas.com> wrote in message
> news:pkHRb.13902$F86.1634489@twister.southeast.rr.com...
8-bit[QUOTE][color=darkred]
> ...
> (also, what the hell are those files? I'm on cable and dropped the

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> after it had been running for 30 seconds, an *executable* started to run,
> and my virus checker clamped up tighter than a drum. A little heads-up
> would've been thoughful.)

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> Here's an example where I converted an 8-bit RGB sky photo to LAB,
> applied a steep curve, and converted back to RGB:
> http://home.nc.rr.com/sarle/Test5-RGB8-Curved.tif
> http://home.nc.rr.com/sarle/Test8-RGB8-Curved.tif
> One of these was done all in 8 bits. The other started with the same
> 8-bit image but I did the conversion and curve in 16 bits. Can you
> tell which is which? Maybe not just by looking at them on a monitor,
> but you can tell from the histograms. If you stacked up several
> curves layers, the difference might become obvious. I'll try that
> when I get CS.

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>
> I suspect the difference is due to the noise added during the 16->8 bit
> conversion.

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> As for your virus checker, I'd say get a new one.

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>
> "Mike Russell" <REgeigyMOVE@pacbellTHIS.net> wrote in message
> news:GcKRb.6951$iU5.2054@newssvr29.news.prodigy.com...
> SNIP

Only a little. Take the difference between the two images and run
Auto Levels. You'll see that the differences have strong systematic
trends related to the direction of the original gradient, as well
as some noise.
[QUOTE][color=darkred]
> We do not know Warren's Color settings, nor do we know how noisy the
> original was. Test5 looks like the 8-bit version to me, coming from a
> digicam.

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> They're TIFF files. You can tell by the .tif extension. TIFF files are
> large,
> but it would have been pointless to provide JPEG files, since JPEG
> compression would have swamped the differences. The executable was
> probably whatever your system uses to display TIFF files.
>
> As for your virus checker, I'd say get a new one.
>

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>
> What would you gain by converting an 8 bit image to 16 bit? and then back to 8
> bit.

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