U of S | Mailing List Archive | alt-photo-process-l | Re: OT: 16 bit editing myth or reality?

Re: OT: 16 bit editing myth or reality?

You could be right but I (we) don't know if PS works with real number until the data needs to be save or whatever. If it's not the case and this would be surprising, then each editing step would effect the data and each additional editing steps would have a cumulative effect. I don't think what you propose below is correct even if PS works with real number because the starting point would be different.
Let's say I choose arbitraly a normalised [0..1] pixel level of 0.5, lets see what our starting value could be in 8 bit resolution. This would mean we started with a value of 0.5*255=127.5 => 127 or 128, now in 16 bit this would be 0.5*65535=32767.5 => 32767 or 32768. Lets say our original pixel value is 32767, PS would transform this in 32767/65535 = 0.49999237 and we could proceed with editing from this value but if we convert this value to 8 bit and save it for later comparison, PS would need to convert this to a value to an integer value between [0..255], so lets do this by 0.49999237 * 255 = 127.4980545 and to 127 by rounding. If we need to apply the same editing to the 8 bit data file, PS will convert 127 to 127/255 = 0.498039215, a difference of 0.001953155, though very small this difference could be increased by editing the image further. Just out of curiosity say we apply a small gamma correction of say 1/1.45 and see what effect it as.
0.49999237^(1/1.45) = 0.6199955
0.4980392^(1/1.45) = 0.6183242
for a difference of 0.001671312
The difference is now lower, this could be surprising and it's why a full analysis including both color errors and quantization errors migth produce interesting results. I made a search on Google to see if someone did this kind of analysis but as of now (4) I saw only perceptual comparisons and no number crunching but interestingly the few test I've seen seem to indicate the difference are imperceptible from 8 or 16 bit editing on printed image. Food for thought don't you think?
PS I don't recall any discussion of this topic in particular but I have a bad memory so don't take my word for it.
----- Original Message -----
Sent: Sunday, December 16, 2007 3:37 PM
Subject: Re: OT: 16 bit editing myth or reality?


It would be easy to do such a comparison....in the end, convert the 16 bit version to 8 bit, then count how many levels of tone are contained in each file.

In a message dated 12/16/07 2:19:30 PM, gauvreau-yves@cgocable.ca writes:



Happy holidays to you and all others on the list.


In my last message, I hope I was able to tell every one the context in which this (my) experiment was done. Though I think most of the list member had the intuitive or learned knowledge that working in higher bit resolution would minimise quantization errors or simply result in higher quality image, it wasn't obvious to me and probably to most of us here how this improved quality translated into actual quantitative values.


As for doing what you suggest below, it should be obvious that such an exegarated transform would result in poor quality image for both resulution, though the 8 bit would probably suffer the most. (I like very much porcupines...)


If I where to do an in depth analysis of the "benefit" of using 16 bit resolution on B&W image, I would try to recreate a typical set of editing transformation as would be done on an actual image. Any out of the ordinary editing would be counter productive in my opinion and the result wouldn't be that useful since they wouldn't have any connection to real world image and real world editing. In this type of analysis I would also use PS for the simple reason that recreating the math behind some of the editing transform would be way to much work and it wouldn't be relevant to the initial goal. When all four image would be ready to print that is image #1: original unedited 16 bit image, image #2: image #1 converted to 8 bit, image #3: edited from image #1 and image #4: edited using the same editing step as for image #3 but from image #2 instead, to be print ready image #3 would have to be converted to 8 bit. Then I would load image #2 to #4 (image #1 is redoundant) in my program or whatever to count the looses if any. I could also do a little bit better by recording each individual editing step by itself, this would allow one to evaluate the contribution of each type of editing transform and the more global result of applying all of these transform steps together. To be even more complete, this analysis could be done on a color image and the stats should also include CIE delta errors (1976) this would give a more complete picture then just measuring quantization errors as I did.


I'll first seach to see it as been done before and if not, I just might give it a try out of curiosity and provide you with the result.




Best Wishes,
Mark Nelson

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