The effects of compressing to 8 bits

[t024484]

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There are some facts worth to know to avoid a babylonian confusion.

1) Leica compresses their DNG files to 8 bits, just to save on SD card memory.

In that way twice as many pictures can be stored on a memory card.

It also saves time to write 8 bits instead of 16 bits to the memory card, although this is probably of minor importance.

Anyhow, SD cards can hold many more gigabytes nowadays, so saving on memory is not what we really need. The price we have to pay is a DNG file that has more noise because of compressing.

It would be nice to have the option in the menu, Compress: yes/no.

2) When uploading the 8 bit file to a raw converter, it is automatically decompressed to 16 bit, the decompression table is within the DNG file,but the extra noise remains.

So when uploading to Lightroom, you get a 16 bit file with only 256 different values per pixel.

Then comes the demosaicing, where for every single pixel a RGB value is reconstructed from the Bayer file.

Since all surrounding pixels used for this reconstruction also are infected with this noise, it adds up to even more noise.

After this proces, instead of having one pixel value that is either Red Green or Blue, we have now a full set of RGB values for each individual pixel.

3) Then comes the next step, which is gamma correction. This is yet another compression taking place on the 16 bit RGB values. The values are best left in 16 bits up to the point where all picture corrections have been made to avoid extra noise being added because of the correctional calculations.

As a last step it is safe without visable loss of information, to reduce the demosaiced, gamma corrected and processed image to 8 bits RGB values,.

 

Hans

[carstenw]

The price we have to pay is a DNG file that has more noise because of compressing.

 

Not really, there is no more noise, but rather a greater possibility of posterization. There might even be less noise, since it smoothes out differences between similar neighbours.

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[plasticman]

Not really, there is no more noise, but rather a greater possibility of posterization. There might even be less noise, since it smoothes out differences between similar neighbours.

 

YES exactly! One thing I'm noticing photographing silky-smooth baby-skin (yep baby pics!) in different lighting conditions is exactly this posterization in tonal transitions. It won't be visible in a landscape shot with a lot of detail, but smooth gradients falling into shadow clearly show it.

 

Most of the time this isn't a problem, and my baby pics don't suffer unduly of course, but there are many occasions when this will be a significant problem - particularly if any further color manipulation needs to be done to an image in post.

 

I also wonder whether the time saved writing to disc is wasted on the compression process anyway. Something of a lose+lose situation.

 

Anyway, I never cease to wonder at the people on this forum that harp on about no AA filter and the world's best optics, and so on. But if anyone suggests there's something wrong with throwing away a massive amount of color information to save space on your SD card, then suddenly it's "nothing to worry about".

 

Oh well.

[jaapv]

You clearly need a DMR, Mani. I've been flamed for it, but this is imo exactly the reason the DMR files are better than the M8.

[marknorton]

I think it's time Leica revisited the decision to force the 8 bit compression on us.

 

It's easy to see how the original specification for the camera might have specified 10 images in an uninterrupted sequence, 100 images on a 1Gb card and (say) 250 images per battery charge. Compressing the images to 8 bits helps meet all those requirements - less buffer space, less card space, lower power consumption.

 

As it's turned out, most of us don't need to take 10 shots over 5 seconds, some of us think C mode is unnecessary, 2, 4, 8 Gb cards are now the norm and, as I've shown elsewhere, much of the battery charge is wasted keeping the camera alive, waiting for a shot which never comes anyway.

 

I'd certainly like to see the option to bypass the compression and we could then make up our own minds.

[rami G]

I think it's time Leica revisited the decision to force the 8 bit compression on us.

 

It's easy to see how the original specification for the camera might have specified 10 images in an uninterrupted sequence, 100 images on a 1Gb card and (say) 250 images per battery charge. Compressing the images to 8 bits helps meet all those requirements - less buffer space, less card space, lower power consumption.

 

As it's turned out, most of us don't need to take 10 shots over 5 seconds, some of us think C mode is unnecessary, 2, 4, 8 Gb cards are now the norm and, as I've shown elsewhere, much of the battery charge is wasted keeping the camera alive, waiting for a shot which never comes anyway.

 

I'd certainly like to see the option to bypass the compression and we could then make up our own minds.

 

 

I'd second that. I understand some, but not all of the great technical explanations here, but I did have a chance to print both DMR and M8 images, and I could discern a clear difference, at least in some, which I would assume are related to the compression. It would be great if Leica would allow us to utilize the full potential of the sensor and would enable a no-compression option via firmware upgrade.

[Marty]

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It would be nice to have the option in the menu, Compress: yes/no.

Just nitpicking here, but even nicer would be lossy/lossless. To leave the data entirely uncompressed at over 20Mb a shot seems unnecessary.

[sandymc]

Since all surrounding pixels used for this reconstruction also are infected with this noise, it adds up to even more noise.

 

Not really; the demosaicing process actually helps - (a) presuming you're running an AHD type algorithm, noise may well be reduced because of the homogeneity minimization, and ( because the demosaicing is in a 16-bit space, it reduces posterization - if there wasn't the demosaicing process, you'd really see ugly levels of posterization.

 

Sandy

[plasticman]

The conclusion at the end of this article is interesting I thought:

 

KammaGamma Articles Solving the Leica M8 DNG riddle

 

The M8 files are not future-proofed in their current form, and may not be able to take advantage of future digital processing advancements... unless Leica allow us the privilege of choosing for ourselves how much compression we want to apply to our files, that is.

 

Incidentally, a real-world (though not critical) example of image break-up was in a baby image that I posted sometime ago (from an M8 which I actually didn't keep). The magenta issue (no filter at the time) meant that I tried to post-process some skin-tones, but posterized clumps appeared that made the color-change much more difficult.

 

Not a big issue for me - but if the image had been a model for the cover of Vanity Fair...

[robertwright]

 

...it would have been retouched up the wazoo and it would not matter what camera you used, the compression or anything else...

[plasticman]

...it would have been retouched up the wazoo and it would not matter what camera you used, the compression or anything else...

 

So true!

 

Anyways, last night I pulled out the back-number of LFI where they discuss the compression algorithm when it was revealed, and frankly even though the article does its best to put a positive gloss on the whole thing I came away much less convinced than before.

Anyone who feels nothing (or next to nothing) is being lost in the process need only read the article and look at the graphs - which are far more eloquent in uncovering the shortcomings of the system than the author probably intended.

[marknorton]

Mani

 

Which issue of LFI is that please?

[jaapv]

Iirc 2007/1

[plasticman]

Thanks Jaap.

 

Technicians in the field may be able to write convincingly that the color data loss is insignificant, but the article itself really left me wondering. Statements such as the conversion back to 16bit for editing left large enough space between discrete color information for post-processing not to cause cross-contamination just left me even more dubious about the algorithm.

 

The saddest thing is that I really like the natural color of M8 files - and when I look at the posterization of shading in midtone areas I'm constantly left wondering how much better they could be if only we could save all the information that's captured.

 

What a strange decision really: this camera that's turning-out to be really beautiful in use and producing lovely images. But the first thing it does is throw away a massive chunk of the image information that it's just captured.

 

Makes no sense to me, now that we can use much larger SD cards. I can't understand why everyone's not angrily clamoring to get back the information their camera is throwing away as soon as the shutter is released.

 

Amazing how people will defend whatever decision Leica makes - good or bad.

[carstenw]

Mani, please read the entire thread again carefully. No one here is saying that there is no difference, and no loss. The closest is Georg who says he hasn't seen a difference, a very different statement. In fact, there is almost universal lamentation that the 16-bit processing isn't even an option.

 

The reason you don't see anyone complaining much about it is partly that there was an earlier thread where we already went through all this, and complained out loud about it, and nothing happened. Partly, it is also just that there is so little difference, that almost no one has ever seen it. You have, or at least you claim to, but if most people don't see it, most people don't complain.

 

It would be interesting if you could post a raw file, as well as a processed final result, where one could see the problem. We tried last time to provoke a difference, but were unable to. Most of us still want the peace of mind of real 16-bits, but we were not able to show that there was a difference.

 

Your accusation of people mindlessly defending Leica is misplaced here. No one is doing this in this thread. There is general agreement, with a little skepticism mixed in, nothing more.

[SJP]

If I recall the previous thread in fact showed that the compression scheme is remarkably clever and indeed little or no difference was observed. There was a suggestion that a slightly improved compression table could remove the remaining small difference between 8 and 16 bit. This was even implemented by some reverse engineering of the firmware. The thread then disappeared waiting for the Leica verdict on this sort of tinkering, never to be heard of since. Did they in fact change the table in the FW upgrades? That would have been easy enough.

[plasticman]

Hi Carsten

 

Sorry, I didn't mean only this thread - although at least a couple of responses here were along the lines of "don't worry about it" - I've followed discussions about the issue from when it was uncovered (on this forum?). My interest before owning the camera was more academic, naturally. Now I own (and am immensely enjoying) the camera, my engagement is somewhat more passionate.

 

Because of the very nature of this issue, I have no idea whether I'm seeing the artifacts of the compression algorithm or not. But in the sort of images that I'm shooting at the moment (mostly informal shots of my child and her mother taken with a Noctilux wide open), with soft OOF gradients into shade on subtle skin-tones, I'd say that I see some strangely posterized areas of color in almost every image that I take.

Probably not significant in printing - but then again, I have no idea what I MAY want to do sometime in the future with one of these, or some other image. A pity, I'd say, if those future options are limited by this somewhat arbitrary compression scheme.

 

One other observation: many people claim not to see any difference between film and digital (esp when the digital file is treated with Silver Efex or similar), and yet there are others (I'm not one of them, incidentally) who've consistently proved they can identify which is which even from a jpeg compressed on-screen comparison. There is something inherently different in the complexity or depth or texture of the files that I believe we respond to without a conscious awareness of it.

 

Somehow I think there may be some similarity to this situation - our conscious rational brains may be fooled by interpolated colors, but I hazard a guess that if we maintained the 16bit path (or rather 14 or 12bit in reality), then the files would appeal to us subliminally as even richer and better than we already feel they are.

 

I'll be writing directly to Leica about this incidentally. Not just a moan - but I see no reason why good cannot be better.

[carstenw]

Here is the original l-forum thread, from 2006:

 

http://www.l-camera-forum.com/leica-forum/digital-forum/11560-leica-news-lfi-1-2007-a.html

 

An early external article:

 

KammaGamma Articles Solving the Leica M8 DNG riddle

 

The thread announcing this article:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/23206-8bit-dng-issue-explained.html

 

Another thread:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/33491-8-bit-dngs-how-done.html

 

Yet another thread:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/59198-reality-8-bit-leica.html

 

A thread by Sandy:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/33768-8-bit-versus-14-bit-dngs.html

 

A tangentially related thread on the blue dot:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/39117-secrets-blue-dot-revealed-other-m8.html

 

Here is a conspiracy theory thread by Jaap:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/33282-lfi-exposed.html

 

Here is a later thread by Hans:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/59812-8-bits-versus-16-bits.html

 

Here is a later, investigative thread by Hans:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/60160-8-bits-versus-16-bits-continuing.html

 

There are other threads too, including a series of laundrylist threads of requests to Leica where people wanted to have 16-bit files, and threads complaining directly about the state of affairs, and so on.

 

So you see, lots of discussion has taken place. There is no possible conspiracy theory here about a cover-up, or insufficient discussion, or of too many people defending Leica. There is lots of criticism of the decision in over 100 pages of posts in the above threads alone. There is much technical information.

 

BUT! to my knowledge, no one has ever posted a DNG which others could download and see the problem directly. There are only vague statements of DMR superiority, plot and graph analysis, numerical analysis and so on, but no one has posted verifiable problems. This is really what is missing here, not more discussion. So, Mani, you claim to have some baby shots where you can see the difference. Please post one. Then we can go to Leica and say "See! Please give us 16-bit."

[exile]

I'd like the option of full 14 bit for portraits and low light. I want access to all of the information that beautiful sensor has captured. And despite what some people seem to be saying, I have a strong hunch that 14 bit could make all the difference at ISO2500 under yellow indoor lighting when the blue channel is basically mush.

But for street shooting I would still go with 8 bits - no question. Quicker write times and less card changing.

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[plasticman]

So, Mani, you claim to have some baby shots where you can see the difference. Please post one. Then we can go to Leica and say "See! Please give us 16-bit."

 

I have a feeling our posts 'crossed', as it were. While I'm sure my daughter is destined to achieve great things, convincing Leica to change their minds single-handed is probably not going to be one of them.

 

However, as I said, I'll be onto them about it. I'd hope that if enough of us did the same, they might make the (probably quite easy) change to the firmware and menu which would allow us to choose how much information we want the camera to throw away.

 

Incidentally, I didn't for a moment suggest there was a conspiracy. Just that the attitude is often this:

1. "The M8 is better than a Nikon / Canon because it does no in-camera processing to the raw file"

2. "Don't worry about the immense amount of in-camera processing that the M8 does to the raw file, because it really doesn't make any difference to the image"