Digital BW, The Print

It's called quatization noise. There is extensive literature in the 
digital signal processing, image processing, SAR processing, Tomographic 
processing, etc. discussing this issue. In order to maintain a full 8 
bits accuracy one must allow additional bits in the processing else the 
quantization noise will impact the final signal to noise ration.

It's in the math - has nothing to do with the image. Once you have your 
final image an 8 bit image is sufficient since that is probably the 
limit of the eye.

Truman

Martin Wesley wrote:

>
>
> * -----Original Message-----
> * From: Tom Baker [mailto:tbaker1328@...]
> * Sent: Saturday, November 29, 2003 9:28 AM
> * To: DigitalBlackandWhiteThePrint@yahoogroups.com
> * Subject: RE: [Digital BW] 16 bit printing
> *
> *
> * Martin  -
> * 
> * I know all of that.  But, it 'feels' like something is going
> * on in the end-to-end process that seems to fly in the face of
> * the numbers/theories.  In the end we may not need 16 bit
> * throughout the process to get max print quality.  Or, at
> * least, not have to work in 16 bit all of the time.  Maybe
> * there are improvements in the 8bit processing that the image
> * processors do.
>
> Tom,
>
> I agree. I think 8-bit is probably plenty as long as you can preserve the
> image data during adjustments. I don't think the problem can be addressed
> with better 8-bit software though. The problem is the limitation of the
> math. If you change the gamma of an image from 1 to say 2 the only way you
> can do this if all 256 shades are in use is to reassign pixels to values
> already in use and leave adjacent values empty with no pixels assigned to
> them. The number of shades of gray then is reduced from 256. I don't 
> see any
> way around this.
> * 
> * If it is, in fact, true that the print engines all work in 8
> * bit, then:  1.) maybe we can drop back to 8 bit sooner in the
> * workflow without degradation, or 2.) what kind of quality
> * increase could we see if the print engines were 16 bit.
>
> In answer to 1.) use Photoshop CS, do all your work in 8-bit on layers and
> at the end apply the layers to the original 16-bit file.  The end result
> will be a image that has negligible degradation whether you choose to save
> the end product in 8 or 16-bit mode. You can even save the layer set 
> without
> the image for future revision of your adjustments.
>
> As far as 2.) goes I don't think you would see much if any improvement 
> with
> a 16-bit printer. I suspect that our ability to differentiate shades 
> of gray
> may be lower than what you can achieve in 8-bit. If you wanted to increase
> the bit depth of a print engine I  think you would only need to go up 
> to 10
> or 12-bit. The highest value I have ever seen published for human 
> perception
> of shades of gray was in the 900 to 1025 step range and I am skeptical of
> that. So beyond 10-bit differences would definitely not be discernable to
> the human eye.
>
> Martin
> * 
> *
> *
> * Martin Wesley <mwesley250@...> wrote:
> *
> *
> * * -----Original Message-----
> * * From: Tom Baker [mailto:tbaker1328@...]
> * * Sent: Friday, November 28, 2003 9:46 PM
> * * To: DigitalBlackandWhiteThePrint@yahoogroups.com
> * * Subject: Re: [Digital BW] 16 bit printing
> * *
> * *
> * * Paul  -
> * * 
> * * What would be the result if you had started with a 16bit
> * * image, done all of your work, including the curves
> * * application, then converted to 8bit just before printing? 
> * * Similarly, what happens if you apply the curve before
> * * converting to 16 bit in your most recent scenario?  The
> * * reason I ask is that I'm still wondering if, when, and how it
> * * makes a difference if you print an 8 or 16bit gray file. 
> * * I've printed some 8bit files that look just fine.  But, that
> * * seems illogical.
> * * 
> * Tom,
> *
> * I'll put my 2 cents in here if you don't mind. For the most
> * part you can make great prints working and printing in 8-bit
> * mode. If you couldn't then we would not have gotten as far
> * with this as we have.
> *
> * However, it is desirable to do everything in a higher bit
> * mode. As you adjust an image you can start to chop up the
> * data in 8-bit to the extent that not all of the 256 shades of
> * gray are used and your histogram appears "combed" and/or
> * fuzzy. You can actually have quite a lot of this and still
> * get a good print but you frequently wind up with problems of
> * posterization and tonal flat spots in areas like skin or
> * skies. In 16-bit with 65,536 shades of gray you can lose
> * thousands of shades and it will never be visible in the print.
> *
> * Many people have always worked their prints from beginning to
> * end in 16-bit mode but for the majority the lack of layers
> * and other tools in 16-bit mode was not acceptable and we
> * dropped down to 8-bit after making some initial adjustments.
> *
> * The great thing about Photoshop CS is that you now have all
> * of the tools and layers available in 16-bit that you had in
> * 8-bit. The bad news is that the working file sizes get really
> * out of hand as you start adding layers in 16-bit mode.
> * However, the good news is that you can save an initial 16-bit
> * version of your file and then an 8-bit version. Work out all
> * of your layers and adjustments in the 8-bit file. When you
> * are done put all the layers in a layer folder, open up the
> * 16-bit version of the image you saved, drag the layer folder
> * from the 8-bit to the 16-bit image and presto you have a
> * 16-bit file that is identical to what you would have had if
> * you had done all the work in 16-bit. Very cool!
> *
> * In PS7 and earlier if you did all of your adjustments in
> * 16-bit and then converted to 8-bit before applying the RGB
> * separation curves you would "damage" the image data when the
> * curves were applied. Might or might not effect image quality
> * depending upon the image. Same thing if you applied the
> * curves to an 8-bit file prior to converting to 16-bit. Once
> * the damage has been done in 8-bit converting to 16-bit does not help.
> *
> * If you were not using RGB separation curves, then converting
> * from 16-bit to 8-bit prior to printing would not cause you
> * any problems. However, there is no reason to convert a 16-bit
> * file to 8-bit prior to applying the curve or prior to
> * printing. Just print directly from 16-bit.
> *
> * Martin Wesley
> * http://www.carolyn.cc/Guests/MartinWesley/pages/MW_01.html
> * http://www.borderless-photos.de/guests.html
> *
> *
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