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Author Topic: Is the 16bit advantage a bit of a myth?  (Read 16043 times)
pcox
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« on: February 08, 2008, 12:28:59 PM »
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Hi folks -
I know that's a provocative title, but I'm starting to wonder. I've always been a proponent of 16 bit image processing - fully convinced by the math and happy that I've never had a problem with banding or posterization in any of my images.

I teach workshops and have been drilling into my students - 16 bit, 16 bit, 16 bit. I explained the reasons why, and everyone has gone home happy.

However, I am putting together some teaching materials to show the deficiencies of editing in 8 bit mode... and I can't break the images. I took a few RAW files, processed them each into both 16 and 8 bit images and performed the same exact adjustments on each.

I've pushed, pulled, stretched and abused the 8 bit files to the point they are hugely overprocessed and look truly awful - but no posterization. This included taking some very low-contrast images and stretching the histogram out to the full extent, several sharpening iterations, some shadow/highlight, agressive curves etc.

I applied changes to hugely underexposed images as well, trying to break the shadow areas. No joy.

I did the same things to 16 bit images and got the same results - the 16 bit images were a tiny bit better when seen at 100%, but nothing to write home about.

So am I missing something basic here? Or is the 16 bit advantage only apparent on true edge-case images?

Enlightenment appreciated.

Cheers,
Peter
« Last Edit: February 08, 2008, 12:30:05 PM by pcox » Logged

Peter Cox Photography
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jerryrock
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« Reply #1 on: February 08, 2008, 01:01:34 PM »
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There may be greater differences in color gamut that can not be displayed on the monitor you are viewing the images on.
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Gerald J Skrocki
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« Reply #2 on: February 08, 2008, 01:47:33 PM »
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Hi Peter, I can run a 8 bit file through a B&W conversion such as Virtual Photographer resulting in a very chopped up histogram that looks like a comb and then run the 16 bit version through any of my other B&W converters which support 16 bit and the histogram is not all chopped up at all but completely smooth. The finely graduated tones are lost in the 8 bit and some posterization occurs. Not so in the 16 bit file.  

That can make the sky very blotchy when using 8 bit files and I am sure you loose some of the very fine detail and dimensional quality in a important image in all areas of the image. That is what I see on my computer anyway.
« Last Edit: February 08, 2008, 01:47:59 PM by Arizona » Logged

Glen
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« Reply #3 on: February 08, 2008, 01:49:07 PM »
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Jonathan Wienke has a nice demonstration of this on his website. Take a look at Jonathan's website article. Also search on the LuLa forum for his posts relating to 16 bits.
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Jonathan Wienke
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« Reply #4 on: February 08, 2008, 03:08:56 PM »
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You beat me to it, Eric!
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digitaldog
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« Reply #5 on: February 08, 2008, 03:51:49 PM »
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Eample: RGB-to-Lab Quantization Loss

http://www.brucelindbloom.com/index.html?RGB16Million.html
« Last Edit: February 08, 2008, 03:52:46 PM by digitaldog » Logged

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Eric Myrvaagnes
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« Reply #6 on: February 08, 2008, 08:13:56 PM »
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You beat me to it, Eric!
[a href=\"index.php?act=findpost&pid=173380\"][{POST_SNAPBACK}][/a]
I guess I just knew where to look.  
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« Reply #7 on: February 08, 2008, 08:56:45 PM »
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Even to my unprofessional eyes, I do see a visible drop in dynamic range and less color depth when converting from 16 to 8 bits.  I noticed this effect in photoshop when I have to convert  raw files to 8 bits in order to save as jpeg files.  I suspect it has to do with the capability of the display.  I have the NEC 2190uxi.  I don't recall seeing any difference between 16 and 8 bits on my old cheapo monitor.
chuong
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John Sheehy
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« Reply #8 on: February 08, 2008, 10:17:29 PM »
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It really depends on what kind of operations you perform.  Some couldn't care less about bit depth.  If your image is very noisy or very textured, quantization will be harder to see.

You can totally destroy an 8-bit image in two steps if you try; you can do a gamma of 0.1 and then 10, or visa versa.  That will decimate any image that had any kind of smoothness to its gradients.  Of course, that is not something one would normally do, but some real things come close.  Perhaps you want to expand the contrast of the midtones in Curves and compress the shadows and highlights, but then use the Shadow/highlight tool to bring out details in the shadows and highlights.  They will posterize very easily in 8 bit mode.
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JeffKohn
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« Reply #9 on: February 08, 2008, 10:44:49 PM »
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It also depends on your working space. If you work in a large gamut space such as ProPhoto RGB the damage to an 8-bit file will become apparently more quickly than it would working in sRGB.
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papa v2.0
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« Reply #10 on: February 09, 2008, 03:55:45 AM »
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Eample: RGB-to-Lab Quantization Loss

http://www.brucelindbloom.com/index.html?RGB16Million.html
[a href=\"index.php?act=findpost&pid=173389\"][{POST_SNAPBACK}][/a]


whats this link to do with  16 bit v 8 bit ?

Bruce is talking about a different issue.
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Jonathan Wienke
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« Reply #11 on: February 09, 2008, 08:23:25 AM »
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whats this link to do with  16 bit v 8 bit ?

Bruce is talking about a different issue.

No he isn't. Do the conversion in 8-bit mode, and you lose 87% of the unique colors in the image due to quantization errors. Convert to 16-bit mode before making the trip to LAB, and the decrease in unique colors from quantization is far less dramatic.
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digitaldog
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« Reply #12 on: February 09, 2008, 09:31:43 AM »
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whats this link to do with  16 bit v 8 bit ?

Bruce is talking about a different issue.
[a href=\"index.php?act=findpost&pid=173496\"][{POST_SNAPBACK}][/a]

Yes and no. Do the same tests in 16-bit, you get a decidedly different result.
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Andrew Rodney
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jbrembat
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« Reply #13 on: February 09, 2008, 12:42:10 PM »
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pcox,
 there is no chance to see posterization or banding. 16 bit is a true mith.
I investigate it in deep for my editing tools.

People forget to think on difference between bits in a linear space and bits in a compressed space.

People forget that devices work at 8 bit not 16 bit.

People say you must use 16 bit in ProPhoto as it is a huge space. But they forget that for any rendering, on monitor or printer, the colors must be compressed into a smaller space.

People say: the 8 bit histogram is with holes, but they forget that 16 bit histogram is shown compressed. And, in any case, do you look the image or the histogram to judge?

Just a note: apply a curve to see holes in the histogram. save the image as jpg (good quality) and then open the saved image and look at the histogram.

16 bit must be used in raw conversion as starting point is 12/14 bit linear and you are working in a linear space.
As soon as the image is developed you are in a compressed space and 8 bit are more than enough.

EricM wrote:
Jonathan Wienke has a nice demonstration of this on his website. Take a look at Jonathan's website article. Also search on the LuLa forum for his posts relating to 16 bits

No demostration. he worked on linear space.

digitaldog, Bruce Lindbloom is correct, to go to/from Lab I use floating point. PhotoShop use fixed point math.

John wrote:
Perhaps you want to expand the contrast of the midtones in Curves and compress the shadows and highlights, but then use the Shadow/highlight tool to bring out details in the shadows and highlights. They will posterize very easily in 8 bit mode.
There is a very strong confusion on internal algorithm math and image bit dept. Very often the computations must be performed in floating point to have good results.

Jacopo
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pcox
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« Reply #14 on: February 09, 2008, 01:43:44 PM »
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Thanks for the responses, all.

It looks like I wasn't missing anything basic, then - the advantages are not really there for most images. Certainly there are edge cases where keeping 16 bits throughout the workflow is beneficial, but not the majority.

Jonathan - I had found your example prior to my first post and the adjustments you made to the image are so excessive as to not represent a real world example. Perhaps such radical adjustments would be beneficial in a forensic type scenario where you must extract detail no matter what the cost, but I can't see ever selling an image that needed that much in the way of editing.

Again, I'm going to continue using 16 bits as there are clearly cases where it helps, and storage is cheap. But it's an interesting thing to learn...

Cheers,
Peter
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Jonathan Wienke
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« Reply #15 on: February 09, 2008, 02:14:06 PM »
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JBrembat is quite wrong when he says "there is no chance to see posterization or banding. 16 bit is a true mith."Calling 16-bit's usefulness a myth is bullshit. 16-bit drivers for printers are available now, and monitors that support >8 bit video inputs are working their way into the marketplace. And even if you are printing to an 8-bit device, you'll still see less posterization from a 16-bit image, because the color space conversion is done in 16-bit mode and then rounded to the nearest 8-bit value afterward. As a result, all 256 levels are possible, in contrast to 8-bit color space conversion where the number of usable levels per channel may b half or less. The same is true when displaying images on 8-bit displays; not all posterization and banding is caused by gamut issues.

Editing in 16-bit mode will not make a noticeable difference in some images, especially if they only require minimal adjustments to be print-ready. Where 16-bit's advantages are most noticeable is in images that require significant tonal adjustments, such as to bring out shadow detail or correct underexposure. Also, in images with large out-of-focus areas or other smooth tonal gradients, 16-bit editing can be crucial to avoid posterization and banding.

I've provided one example where 16-bit editing makes a huge difference. Admittedly, it is a worst-case scenario, But I've seen enough of a difference in real-world images often enough that I've decided it's worth the overhead.
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pcox
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« Reply #16 on: February 09, 2008, 02:20:51 PM »
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I've provided one example where 16-bit editing makes a huge difference. Admittedly, it is a worst-case scenario, But I've seen enough of a difference in real-world images often enough that I've decided it's worth the overhead.
[a href=\"index.php?act=findpost&pid=173558\"][{POST_SNAPBACK}][/a]

Jonathan -
Can you post an example of where you've seen this in the real world? I deal primarily in images with smooth tonal gradients and I've been using them in my tests. In areas where I have managed to break the 8 bit image, the 16 bit posterized as well.

I also deal with a lot of images where shadow detail needs to be brought out. Again, processing the RAW files twice, once in 8 bit and once in 16, I was unable to break the image without going so far over the top that the image would not be usable anyway.

Peter
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« Reply #17 on: February 09, 2008, 02:27:32 PM »
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Jonathon,

Can you post the raw file for the image you used in your example? I think that would be of interest to people wanting to investigate this further.
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Jonathan Wienke
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« Reply #18 on: February 09, 2008, 02:35:04 PM »
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Jonathan -
Can you post an example of where you've seen this in the real world?

I'm currently at Walter Reed Army Medical Center  being treated for some neurological problems (see my blog for more details if you're so inclined) and most of my photo archive is still in Germany.

Is the posterization you refer to on-screen or in prints? Does your printer support 16-bit printing, and if so, were you using it.

Another point to consider is that when working with RAW images, even if you output to 8-bit from the RAW converter, you're still doing most of the heavy lifting WRT tonal and exposure adjustments with either 16-bit or floating-point math depending on the internal design of the RAW converter, and are rounding to the nearest 8-bit value as the final step. If you want to see a true 8-bit/16-bit comparison, shoot RAW + JPEG in high DR situations or underexpose a stop or two. Then process the RAW and the JPEG side by side and see which one shows the most posterization, especially in shadow areas.
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pcox
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« Reply #19 on: February 09, 2008, 02:39:30 PM »
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Jonathan -
For the purposes of these tests I have done no processing in the RAW converter - in fact in some I even darkened and compressed the tones of the images to see if I could stack the decks.

There may well be a difference when dealing with in-camera JPGs, but that's not what I'm concerned about here - I work entirely in RAW, so I wanted to test starting from that point.

Cheers,
Peter

Quote
I'm currently at Walter Reed Army Medical Center  being treated for some neurological problems (see my blog for more details if you're so inclined) and most of my photo archive is still in Germany.

Is the posterization you refer to on-screen or in prints? Does your printer support 16-bit printing, and if so, were you using it.

Another point to consider is that when working with RAW images, even if you output to 8-bit from the RAW converter, you're still doing most of the heavy lifting WRT tonal and exposure adjustments with either 16-bit or floating-point math depending on the internal design of the RAW converter, and are rounding to the nearest 8-bit value as the final step. If you want to see a true 8-bit/16-bit comparison, shoot RAW + JPEG in high DR situations or underexpose a stop or two. Then process the RAW and the JPEG side by side and see which one shows the most posterization, especially in shadow areas.
[a href=\"index.php?act=findpost&pid=173565\"][{POST_SNAPBACK}][/a]
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