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Author Topic: Is 8-bit enough for OUTPUT quality?  (Read 8209 times)
Guillermo Luijk
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« on: August 30, 2009, 05:04:45 AM »
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A couple of times it has been discussed whether if 8 bits (i.e. 256 values) is enough or not for output quality, even assuming the processing was perfectly done in 16 bits and the conversion to 8 bit took place in the last step.

In plain colour areas with a good exposure, i.e. where the dithering effect of noise is minimised, some images display banding. Some people claims if this happens is only because of a bad processing, but the true is that with just one 8-bit level increments from one band to another, I can actually see the bands in the following image (look for the bands in the first image):



In the second version I enhanced these bands so that they could be easily identified.
The level increment in the first image was the finest achievable with an 8-bit encoding.

This also happens sometimes in the skies, which have the same properties as this wall (plain colour and absence of noise).

What do you think?

1. Is 8-bit (and therefore JPEG) enough for output quality? or it dangerously nears the perceivable limit?
2. Is this very slight banding visible when printing?
3. Could I be being able to see these bands because of a bad calibration of my screen?

Regards.
« Last Edit: August 30, 2009, 11:37:48 AM by GLuijk » Logged

Graeme Nattress
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« Reply #1 on: August 30, 2009, 08:22:11 AM »
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8bit, properly dithered, is probably good enough, but not optimum. It looks like your image was not dithered, or if it was lossy compressed with JPEG, that lossy codec will see the dither as noise and probably remove most of it.
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« Reply #2 on: August 30, 2009, 09:10:16 AM »
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Quote from: GLuijk
What do you think?

1. Is 8-bit (and therefore JPEG) enough for output quality? or it dangerously nears the perceivable limit?
2. Is this very slight banding visible when printing?
3. Could I be being able to see these bands because of a bad calibration of my screen?

Regards.

Guillermo,

A thought provoking post. Your demonstration image appears to have a very steep contrast curve in the areas where the banding is apparent, which accentuates the banding. Whether or not banding would be present with more normal contrast is debatable. Before attempting to answer your question, I performed a Google search and reviewed some literature on the topic.

Norman Koren uses a theoretical approach based on the Weber-Fechner law, which deals with just perceptible differences in vision, and concludes that an 8 bit work flow for the final image is acceptable, but just so.

A Previous Thread on LL discussed 16 bit printing, and some photographers claimed to see a difference. However, 16 bit printer drivers at that time appeared to be available only on the Mac with more recent versions of the Mac operating system. For us Windows users, the question may be academic unless special drivers supporting 16 bit are available.

Mike Chaney discusses 16 bit printing with the newer ink jet printers offering an expanded gamut and concludes that 16 bit printing might offer a small advantage for those using 16 bit ProPhotoRGB.

Since I use 16 bit ProPhotoRGB for my more critical work with my Epson 2200 printer, it would make sense to have a 16 bit driver, but I have not noted banding in my output and do not think that 16 bit output is necessary with this printer. With the more recent Epsons with still wider gamuts, it could help, but I can't comment on this. From a leterature review, I conclude that the improvement would be small. I would be interested in hearing from users of wider gamut printers, since I may upgrade printers in the near future. Also, I would like to know if 16 bit output would be possible with Windows 7 with Photoshop.

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feppe
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« Reply #3 on: August 30, 2009, 09:22:46 AM »
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Very good post, indeed, looking forward to insights from the more technically oriented. My gut feeling is that there are surely theoretical and technical demonstrations to show there is a difference, but in 99.999% of real-world photo cases you won't see a difference.

The banding you show in the original picture is not visible on my non-calibrated* monitor. The banding is clearly visible in the exaggerated version, but I'm not sure what the point of showing that is since that's not the on you'll print?

I've only seen banding in one of my photos in the sky, but that was on monitor.

* had to reinstall Win7 since my WD Raptor died last week, thanks for reminding to calibrate.
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digitaldog
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« Reply #4 on: August 30, 2009, 09:38:51 AM »
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Quote from: GLuijk
1. Is 8-bit (and therefore JPEG) enough for output quality? or it dangerously nears the perceivable limit?
2. Is this very slight banding visible when printing?
3. Could I be being able to see these bands because of a bad calibration of my screen?

1. I suspect its possible, it depends on the output device, but most as mentioned are handing off 8-bits of data anyway. 8-bit good bits have been fine for years but output devices are getting better. Epson claims that while you'll be hard pressed to see a difference sending more than 8-bits through there newer drivers, it gives them more ammo in the future to deal with the data, dither etc and, since you started out with more than 8-bits, why truncate the data?

2. I suspect it will be difficult to see if at all but again, depends on the printer and most importantly, #3, it is possible this is the effect of banding from the display if its just a screen capture. The father you calibrate a display past its native behavior, with panels that don't support high bit (use 8-bit through the graphic system), the more likely you can see banding on screen. The problem is, as we see here, its unknown if the banding is in the document or the result of the display.
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Mark D Segal
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« Reply #5 on: August 30, 2009, 10:44:50 AM »
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Guillermo,

My take on this is as follows:

(1) 8-bit vs 16-bit impact on image quality occurs mainly at the processing stage, not the printing stage. It's best to have the math performed on high-bit data so you don't get the kind of gaps between levels which would show as banding. It makes a hell of a difference if you lose say 20 levels in a 255 level scale versus the same loss an app. 32,000 level scale (Photoshop 16 bit is 15-bit +1). But once all the editing work is done, and you've resized the image to output specification in 16-bit mode, it's baked into the image at that point, and if you then convert it to 8-bits and do nothing but send it to an output device you won't lose any smoothness of tonal gradation.

(2) As Bill Janes observed, the lower half of your illustration has extremes of tonal compression and with adjustments of this radical nature you've lost too many levels in 8-bit mode and bust the barriers so to speak. You can only stretch an image so far with these adjustments before the destruction of levels shows very obviously. However, if you repeat the same experiment in 16-bit it could look less bad.

(3) As Andrew says, the printers are 8-bit devices. Added to that, how they print has nothing to do with the bit depth of your image file. The printer driver is figuring out the best way to dither the CMYK ink dots of your RGB image in order to print a photographic-looking image. This technology is unrelated to the bit depth of the image file - it's the screening and dithering technology of the printer driver, or a RIP if you are using one of those. I don't know what a 16-bit printer would do to improve how this technology works, but what we have now in the new generation of Epson printers (all 8 bit devices) delivers truly astounding image quality. It's always hard to imagine much better unless you actually see it when they invent it!

Mark
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Mark D Segal (formerly MarkDS)
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« Reply #6 on: August 30, 2009, 10:48:36 AM »
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Thing is, with a sufficiently steep tone curve in post processing, you could get a 16 bit image to show the same banding.  

The question of whether 8-bits is enough is really two questions - is 8-bits enough for your processing, and is 8 bits enough for your output?

In this case, I think it's clear that 8 bits is not enough for your processing.

Sandy
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« Reply #7 on: August 30, 2009, 11:08:22 AM »
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Quote from: MarkDS
Guillermo,

My take on this is as follows:

(1) 8-bit vs 16-bit impact on image quality occurs mainly at the processing stage, not the printing stage. It's best to have the math performed on high-bit data so you don't get the kind of gaps between levels which would show as banding. It makes a hell of a difference if you lose say 20 levels in a 255 level scale versus the same loss an app. 32,000 level scale (Photoshop 16 bit is 15-bit +1). But once all the editing work is done, and you've resized the image to output specification in 16-bit mode, it's baked into the image at that point, and if you then convert it to 8-bits and do nothing but send it to an output device you won't lose any smoothness of tonal gradation.

Mark,

Reading your comment (1), I now understand why I had banding on the sky of a 40x40" (100x100 cm) b&w print, I first converted from 16 to 8 bit, then made the  resize. After printing the Epson technician said that the banding problem came from the image and not the printer. I opened the file at 16 bit, zoom to 1000% and showed him that my file had no banding, but never checked the 8 bit version.

I will check it again.

Gonzalo
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Guillermo Luijk
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« Reply #8 on: August 30, 2009, 11:17:30 AM »
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Quote from: MarkDS
(2) As Bill Janes observed, the lower half of your illustration has extremes of tonal compression and with adjustments of this radical nature you've lost too many levels in 8-bit mode and bust the barriers so to speak. You can only stretch an image so far with these adjustments before the destruction of levels shows very obviously. However, if you repeat the same experiment in 16-bit it could look less bad.

Hi Mark and all, I am not sure if my images are being correctly understood. The image where I can see the banding is the first one. It's hard to see it, but still can be perceived at least in my monitor. The second version was just a pp to clearly display the bands actually found in the first image, where the level jumps could not be smoother in a 8-bit scale.

So I wonder what's the solution for this: ignore it since the banding is very slight and hardly seen, add noise to dither banding, or completely forget about it because it will not be a problem when printing.

The source of the image is an overexposed RAW file that had to be underxposed by 4EV to crate the final image, the high SNR is the reason why there is an almost total absence of noise.

Regards.
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Mark D Segal
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« Reply #9 on: August 30, 2009, 11:23:24 AM »
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Guillermo, do you think the enhancement of the bands also contributed to worsening them?

Anyhow, I think what I suggested to you holds. Stay in 16 bit as long as you can. Good insurance.

Mark
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Mark D Segal (formerly MarkDS)
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Guillermo Luijk
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« Reply #10 on: August 30, 2009, 11:30:52 AM »
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Quote from: MarkDS
Guillermo, do you think the enhancement of the bands also contributed to worsening them?
No, the bands are the same on both images, just contrast added on the second image. Note that we could never have smoother transitions since the jump in levels is the minimum for an 8-bit scale: (89,79,70) to (90,80,71) for the sample, just 1 level shift on all 3 channels.

Reading at the comments about the improving printing equipment, it's perhaps the time for a 16-bits lossy compressed format to start becoming popular for demanding applications.

BR
« Last Edit: August 30, 2009, 11:36:40 AM by GLuijk » Logged

Panopeeper
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« Reply #11 on: August 30, 2009, 08:06:47 PM »
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Guillermo,

questions:

1. Is what you posted a 100% crop or a reduced size?

2. In which size do you see the banding - 100%, or displayed in Photoshop 25% or whatever?

3. Falls these are reduced sizes: have you tried reduce the size of the image (not the displayed size by zooming out?

4. How did you "reduce the exposure by 4 EV"?
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Gabor
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« Reply #12 on: August 31, 2009, 07:34:31 AM »
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I generally approach these types of problems quite "unencumbered by the thought process", so if you could make the file available, I would be happy to print it, which would at least be an indication of where the problem lies. I have printed quite a few files where some banding was visible on the display (at any %), but the print itself was fine. I have also had a file with barely visible banding in the sky that showed colored banding in the print. Reducing the image/file size (radically) eliminated the problem. That was an 8-bit image—I don't remember seeing this problem at all working in 16-bit.
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Guillermo Luijk
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« Reply #13 on: August 31, 2009, 02:01:29 PM »
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Quote from: Panopeeper
1. Is what you posted a 100% crop or a reduced size?
2. In which size do you see the banding - 100%, or displayed in Photoshop 25% or whatever?
3. Falls these are reduced sizes: have you tried reduce the size of the image (not the displayed size by zooming out?
4. How did you "reduce the exposure by 4 EV"?

1. It's a resized version of an entire image. Unfortunately I don't have the original nor the RAW files anymore.
2. I see it in that size, don't remember about how it did in the original size but checking similar images I have of the same scene, at full size I cannot distinguish any banding.
4. Dividing the RGB values by a scaling factor close to 16.

I suppose the key you are seeking is in 2. Perhaps for being a reduced size, noise was averaged and SNR improved so much that banding became visible, but was probably not visible in the original image thanks to noise dithering.

Anyway I wouldn't like to focus on this particular image, but in the general case of problems arising with 8-bit outputs, no matter if they come from a photograph or any other source such a synthetic gradation. Image you compose an image with a synthetic background which is a perfect transition of levels, and after converting to 8-bit you see the banding. I think this proves 8 bit could not be enough to cheat our eye in all situations,making some dithering techniques necessary to display without noticing any banding.



BR
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« Reply #14 on: August 31, 2009, 04:15:29 PM »
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Quote from: GLuijk
A couple of times it has been discussed whether if 8 bits (i.e. 256 values) is enough or not for output quality, even assuming the processing was perfectly done in 16 bits and the conversion to 8 bit took place in the last step.

Very easy issue. That 8bit imagery can show visible banding given fine enough color transitions doesnt need to be discussed. If it gonna be an issue depends on the image and the output media. When printing fine gradients, I always put in noise (error diffusion) to hide the banding, seems to be the common way. Also when outputting to TV we often used dithering for some gradients. What I havn't done but heard is output to film, and the consensus back then was that e.g. for the big screen, cinema, 8 bit just wont cut it, so they used 12 (if I remember right) or more bits. So for me, for print, I would personally need to see the difference between 8 bit dithered and 16 bit undithered. Maybe it exists, but these cases have to be rare.

Christian


//edit: For very large prints I cannot say, maybe there is a point where 8 bit dithered files are just not adequate. Somebody with actual hands on experience printing very large sizes could very easily clarify that.
« Last Edit: September 01, 2009, 07:13:29 AM by Christian Miersch » Logged
NikoJorj
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« Reply #15 on: September 01, 2009, 03:09:52 PM »
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Quote from: GLuijk
In plain colour areas with a good exposure, i.e. where the dithering effect of noise is minimised, some images display banding.
Just to add 2c of Euro : I also can see some slight banding in the first image, but I'd personnally have the opinion that the dithering introduced by inkjet printing could easily hide it in most cases...
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« Reply #16 on: September 01, 2009, 03:27:40 PM »
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My opinion is that the original question needs to be posed more carefully. Guillermo wants to know whether 8 bits is enough for output, not editing. But what, exactly, does the term "output" mean? To me, the term "output" really means the final space after which no additional color transformations are applied by devices (hardware or software) prior to viewing.

If you post an 8-bit file on a web site, as done here, that's not really an "output" file. That 8-bit image may undergo additional color transformations via the viewing application or OS's color management system (via the display profile), and possibly the video card. These can easily introduce banding and other artifacts.

Similarly, you could take an 8-bit file tagged with a standard RGB working space and say it's ready to be printed, no further editing to be done ... but it's still not really an "output" file. The image data will still undergo a transformation via the output profile into the device space, and again, artifacts can be introduced in this process and can sometimes be visible in the final print.
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« Reply #17 on: September 01, 2009, 04:00:09 PM »
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Quote from: madmanchan
My opinion is that the original question needs to be posed more carefully. Guillermo wants to know whether 8 bits is enough for output, not editing. But what, exactly, does the term "output" mean? To me, the term "output" really means the final space after which no additional color transformations are applied by devices (hardware or software) prior to viewing.

If you post an 8-bit file on a web site, as done here, that's not really an "output" file. That 8-bit image may undergo additional color transformations via the viewing application or OS's color management system (via the display profile), and possibly the video card. These can easily introduce banding and other artifacts.

Similarly, you could take an 8-bit file tagged with a standard RGB working space and say it's ready to be printed, no further editing to be done ... but it's still not really an "output" file. The image data will still undergo a transformation via the output profile into the device space, and again, artifacts can be introduced in this process and can sometimes be visible in the final print.

Eric, in your last para above, if I understand correctly, once the file is prepped for print but we haven't pushed the print button yet, eventhough we see a version in the Print Preview, that's not the final set of data going to the printer. To understand the process a bit better- what happens after we click Print? Let's say it's going to an Epson 3800 and we're not using a RIP - we're using the Epson driver, and before we click print we're in 16 bit ProPhoto colour working space. At least three things I'm aware of need to happen - Photoshop (through the PCS) converts the numbers from Photoshop's ProPhoto RGB working color space to the print colour space guided by the printer/paper profile, then the Epson driver converts the data from 16 to 8 bit and it needs to re-interpret the RGB data to CMYK for printing. (1) Is this a correct portrayal of the critical steps? (2) Where in the chain do you think would enter the greatest risk of banding that you could not see on say a "10-bit" display but would show on paper?

Mark
« Last Edit: September 01, 2009, 04:00:31 PM by MarkDS » Logged

Mark D Segal (formerly MarkDS)
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« Reply #18 on: September 01, 2009, 04:40:44 PM »
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Quote from: GLuijk
Hi Mark and all, I am not sure if my images are being correctly understood. The image where I can see the banding is the first one. It's hard to see it, but still can be perceived at least in my monitor. The second version was just a pp to clearly display the bands actually found in the first image, where the level jumps could not be smoother in a 8-bit scale.
Regards.

I, for one, mis-interpreted your original intent. I now underastand that banding was present in the original image (I can't see it on my screen), but was accentuated by the contrast curve. Since 8 bits is just adequate for most images, there must be some cases where 16 bit output would be better. Whenever editing of an image causes the Weber-Fechner limit to be exceeded, banding might be perceptible. However, if it can be perceived only with special curves and is not visible in the final image, then it is of little concern.  With the current low noise of current cameras at low ISO, noise dithering may no longer mask posterization. Emil's web site has examples of how noise can dither out posterization.
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Guillermo Luijk
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« Reply #19 on: September 01, 2009, 04:54:10 PM »
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Quote from: bjanes
With the current low noise of current cameras at low ISO, noise dithering may no longer mask posterization.
I think this is the key of the story. Following this idea, the problem should arise more easily on cameras with high per-pixel SNR. For instance modern FF cameras with relatively low resolutions like the Nikon D700.

BR
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