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Converting 8 bit images to 16 bit - any benefits?

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  #11  
Old 07-26-2011, 10:18 AM
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Tony W Tony W is offline
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Re: Converting 8 bit images to 16 bit - any benefi

Andrew, thanks for your input. You confirmed my thinking that the data is fixed in its original state and I do appreciate pasting higher bit data or gradients should be undertaken as 16 bit. You are after all adding new data and that should be treated correctly to achieve the best quality.

The bit about editing the data severely intrigues me. Are you saying that if you need to undertake severe editing 8 bit data should be converted to 16 bit? If that is the case how severe would the editing need to be or under what circumstances do you feel 16 bit be better/safer option?

Thanks to all for the comments
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  #12  
Old 07-26-2011, 10:22 AM
ftp-Jeff ftp-Jeff is offline
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Re: Converting 8 bit images to 16 bit - any benefi

No 8 bit to 16 bit conversion is pointless! Start with a processed 16bit file.... Keep it that way until you have finished.
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  #13  
Old 07-26-2011, 11:06 AM
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Re: Converting 8 bit images to 16 bit - any benefi

Quote:
Originally Posted by Tony W View Post
The bit about editing the data severely intrigues me. Are you saying that if you need to undertake severe editing 8 bit data should be converted to 16 bit? If that is the case how severe would the editing need to be or under what circumstances do you feel 16 bit be better/safer option?
There is no free lunch here, and converting the 8-bit to 16-bit isn’t anything like having the original data. That said, try this. Take a 16-bit file, something like a gradient. Duplicate it so you now have two. Take the duplicate and convert to 8-bit per color. Now convert back to 16-bit. You can try running some edits on it, USM, curves etc. Run the same edits on the original 16-bit file and if you wish, on a third dupe that is only 8-bits per color.

What I think you’ll find is all three are different, but deciding on what or how useful the differences are is difficult (you want to have holes in the histogram as your criteria?). What I’ve found is that of course, the best, cleanest data is the original high bit data file. But the image converted from 8-bit to 16-bit and then edited appears a tad less “damaged“ but its difficult to say if using the histogram is a reliable gauge (are the gaps less due to the math or is there just more padding going on with that conversion to 16-bit?)

Bottom line is, don’t worry about it, stick with original high bit data and edit that. If you have 8-bit per color data, that’s what you are ‘stuck’ with.
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  #14  
Old 07-26-2011, 11:38 AM
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Re: Converting 8 bit images to 16 bit - any benefi

ftp-Jeff, thanks I did feel from the outset that from 8 -16 bit conversion likely to be pointless. I would always prefer to work at the higher bit rate and am generally unconcerned about larger file sizes etc. The point is that sometimes you have no control and are stuck with 8 bit.

Andrew, thanks I intend to run your little test. As to the histogram I have to say that I could not care less how it looks as long as the final image meets my criteria. I am not in the business of trying to produce attractive histograms - but was a little surprised by how poor my 8 bit test histogram looked compared to the 16 bit and the fact that I could not find any differences between the 2 images.
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Old 07-26-2011, 11:42 AM
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Re: Converting 8 bit images to 16 bit - any benefi

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Originally Posted by Tony W View Post
Andrew, thanks I intend to run your little test. As to the histogram I have to say that I could not care less how it looks as long as the final image meets my criteria.
Criteria how and where? On a low rez display? On a halftone 133lpi press? A high quality fine art Inkjet printer?

I agree, the histogram doesn’t tell you everything about the qualities of a document. Short of printing it out to lots of different devices and looking real closely, it does tell you something about the edits.

We also don’t know what the data will produce after further editing or conversions to some kind of output color space.
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  #16  
Old 07-26-2011, 12:00 PM
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Re: Converting 8 bit images to 16 bit - any benefi

Point well made and taken. Generally I would be aiming to print on a good quality inkjet such as my own HP B9180 - a little old maybe (and possibly quite short of qualifying as a Fine Art Inkjet) but still IMO capable of producing excellent prints B&W and Colour
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  #17  
Old 07-27-2011, 12:45 AM
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Re: Converting 8 bit images to 16 bit - any benefi

Thanks for posting this thread Tony

All of my post is of course just IMHO

I too agree with the other posts that just converting from 8 bit to 16 bit does not add quality to the image. I don't know of any case where the image quality can be improved by just the conversion from 8 to 16 bit depth. I believe you were looking for something else in your post however:

Quote:
I am merely trying to establish what benefits if any from converting an 8 bit image to 16 bit for editing.
My experience has been that converting an 8 bit image to 16 bit before doing additional processing in Photoshop can reduce the introduction of further degradation in practical situations relative to doing additional Photoshop processing while remaining in 8 bit depth. This degradation when it occurs is not necessarily noticeable in many images for two reasons:

1) The eye is quite forgiving in many cases for small changes in images. An analogy is with high quality JPG compression. There are many subtle changes in JPG compression in the image yet subtle enough that many do not notice them at all.

2) These subtle changes can also be masked by noise. The degradation effects may not be noticeable if the image contains a reasonable amount pixel noise which is common. In addition, Photoshop actual adds dither (1 bit noise) with default settings in a variety of operations including when 16 bit images are converted to 8 bit. This helps reduce the visibility of banding due to 8 bit quantization. I personally prefer to leave this option turned off and add noise/dither when I think it is needed rather than letting Photoshop put it in automatically.


There are a variety of image types using what I believe are simple non-extreme edits where I have observed image degradation when leaving 8 bit images in 8 bit mode for editing. I will show one of those cases below and post more if I have time. The anomalies do show up when printing as well.

Degradation Case 1

While the eye is forgiving for small degradation in many images, it turns out that the eye can pick up subtle luminosity changes in a field of slowly changing luminosity/color which is usually called banding. So here is a case where you will see banding degradation when an 8 bit image is processed in 8 bit mode yet not when it is processed in 16 bit mode. This occurs when this 8 bit sRGB image is converted to a working space of ProPhoto RGB.

To actually see the degradation there are several conditions that are required:
- You need an image that is slowly changing in luminosity/color
- This image needs to be a low noise image or the banding will be masked by the preexisting noise.
- You need to turn off the Photoshop dithering/noise which is on by default which would otherwise hide the banding in 8 bit mode. Turning off dithering is done by going into Edit > Color Settings and unchecking the dither option (found in "More Options"). The dithering is pretty good at hiding the banding yet you have to then live with the extra noise.
- It is also necessary to view the image at 100% magnification or greater as to not introduce display anomalies (it could show banding when there really is none)

Here is the link to the full resolution 8bit sRGB JPG sunset image I used in this example. It is a low noise version leveraged from a Stock Exchange image (details at end of post):http://s774.photobucket.com/albums/y...RGBFullRes.jpg

Here is a reduced resolution version of that image.

Sunset low noise 8 bit sRGB low res.jpg

The following image was created through these steps:
-Opened image directly into Photoshop. I was flagged that the sRGB space did not match my Working Space of ProPhoto RGB. I took the option to convert it to ProPhoto RGB.
- Leaving the image in 8 bit mode I duplicated the image twice to two additional layers and set the Blend mode to Screen to brighten it up
- Flattened image
- Edit > Convert To Profile…sRGB

This next shot is a 100% view on the left side of the image where the clouds are. You can see a good amount of banding.

Sunset-8-bit-sRGB-roundtrip-to-ProPhoto-RGB-and-Screen-Layers.jpg


Here are the steps taken to create the following image.
- Brought the same 8 bit sRGB image into Photoshop and again was flagged that it did not match my working space of ProPhoto RGB
- This time I took the option to leave it in the Working Space of sRGB
- Converted to 16 bit mode
- did Edit > Convert To Profile…..ProPhoto RGB
- Again duplicated the image to two more Layers with Blend mode set to Screen
- Edit > Convert To Profile…sRGB
-Converted back to 8 bit mode

Here is the resulting 100% magnification image. As you can see by doing the processing in 16 bit mode the banding has been greatly reduced.

Sunset 16 bit sRGB roundtrip to ProPhoto RGB and Screen Layers.jpg

Without going into the details taking the 8bit sRGB image into a much wider Gamut Color Space slightly posterized the data. The screen blends exaggerat that posterization.

This is only one of many examples of degradation when an 8 bit image is left in 8 bit mode for Photoshop processing as opposed to first converting to 16 bit.

If this type of situation is rare enough for you or the banding is already masked by noise, this may not be an issue for your images. So I don't even have to think about it, I just convert to 16 bits if all I have is an 8 bit image to work with.

If I have time, I will post more examples.


Steps to Create the Low Noise version from the Stock Exchange image
--------------------------------------------------------------------
The low noise version of the Stock Exchange sunset image was created by:
- Copy Stock Exchange image to computer. Link to Stock Exchange image: http://www.sxc.hu/photo/1347672
- Open JPG into ACR
- Luminosity noise reduction to 100%
- Luminosity detail set to 50%
- Go into Photoshop as 16 bit sRGB (this was done because ACR actually introduces dithering noise if you bring it into Photoshop directly in 8 bit mode)
- Convert to 8 bit mode (remember to have dithering option turned off in Color Settings)
- Re-save as 8bit sRGB JPG at quality level 12
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  #18  
Old 07-27-2011, 01:03 AM
kav kav is offline
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Re: Converting 8 bit images to 16 bit - any benefi

Quote:
Originally Posted by John Wheeler View Post
Thanks for posting this thread Tony

This occurs when this 8 bit sRGB image is converted to a working space of ProPhoto RGB.
Was this just for testing? I can't imagine why you'd want to convert an image from sRGB to ProPhoto. There's literally nothing to gain there. I thought it was only made an opyion because it's the space ACR uses to make corrections. The most realistic use I can think of would be in product photography. If the color of a product was out of gamut/losing detail to single channel clipping, it could make sense as a stop gap assuming it prevents single channel clipping or compression. If you're importing into sRGB though it has already clipped/compressed whatever colors did not fit within this gamut.
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  #19  
Old 07-27-2011, 01:15 AM
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Re: Converting 8 bit images to 16 bit - any benefi

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Originally Posted by kav View Post
Was this just for testing? I can't imagine why you'd want to convert an image from sRGB to ProPhoto. There's literally nothing to gain there. I thought it was only made an opyion because it's the space ACR uses to make corrections. The most realistic use I can think of would be in product photography. If the color of a product was out of gamut/losing detail to single channel clipping, it could make sense as a stop gap assuming it prevents single channel clipping or compression. If you're importing into sRGB though it has already clipped/compressed whatever colors did not fit within this gamut.
Thanks for the comment Kav. I agree that there is no gain in taking an sRGB image to ProPhoto RGB. My personal approach is to leave the image in the color space that is embedded with the image. That said, there are many books out there that tout using ProPhoto RGB for everything and there are many folks that have Photoshop set up to with their default Working Space set to ProPhoto RGB with no warning flags set on Color Space mismatch. With those settings, all images are brought into Photoshop and are automatically converted to ProPhoto RGB with no flags.

So, since I know of quite a number of Photoshop users that have those types of settings (including some members on ReTouch Pro and NAPP) this was not just a test yet an example how those folks can inadvertently end up with banding.

There are many other cases where visible degradation can occur and unfortunately, there is not a long listing of these cases anywhere for folks to refer to make sure they avoid. Editing in 16 bit mode dramatically avoids the degradation. So with that one step, you do not have to remember a long list of gotchas. That is more of my point.

Good question Kav. Please keep adding more comments as I post more cases

I will be posting another example shortly of another case.
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  #20  
Old 07-27-2011, 01:39 AM
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Re: Converting 8 bit images to 16 bit - any benefi

Degradation Case #2

I will leverage the same Sunset image from my previous post for this next case where you get degradation if you edit an 8 bit image in 8 bit mode instead of converting to 16 bit mode.

In this case:
- Open 8 bit sRGB image into Photoshop and keep sRGB as the same working space
- Convert to Lab mode (still in 8 bit mode)
- Duplicate two layers and set blend mode to multiply
- Flatten
- Convert to Profile back to sRGB

Here is a 100% view around the sun of the image. Notice that there is some banding. The banding is visible in prints.

Sunset 8 bit sRGB rountrip to Lab two multiply layers.jpg

Next image was created with these steps:
- Open 8 bit sRGB image into Photoshop and keep sRGB as the same working space
- Convert to 16 bit mode
- Convert to Lab mode (still in 8 bit mode)
- Duplicate two layers and set blend mode to multiply
- Flatten
- Convert to Profile back to sRGB
- Convert back to 8 bit mode

Here is the resulting image going roundtrip to Lab in 16 bit mode. Note that there is less banding:

Sunset 16 bit sRGB rountrip to Lab two multiply layers.jpg

The same conditions I mentioned from my prior post need to occur to see this banding e.g. low noise image and turn off dithering, viewing at 100%.

If your images either are not low noise to begin with or you leave dithering on, you probably will not see the banding which is great as long as you are fine with the higher noise.

I presented the above case because there have been statements that generically, you don't see degradation in 8 bit mode going back and forth to Lab. That may be a good general statement yet in my practical experience, I do see images that will visibly show degradation with 8 bit roundtrip to LAB. Similar to the last post, you are getting a little posterization going to a much wider Gamut Color Space which is amplified by some simple blends.

I will post some more examples yet may run out of steam before too long so the next post may be later (when there is daylight)
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