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Author Topic: Correcting images in LAB vs RGB  (Read 22049 times)
Ishmael.
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« on: February 21, 2010, 06:42:28 PM »
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I've recently discovered the power of the LAB color space and it almost seems too good to be true.I'm sure certain adjustments should be done in RGB, but for the most part I am getting significant more powerful images out of LAB than RGB. Given that I'm relatively new to photoshop I would really like to know from you veterans out there why someone would correct an image in RGB instead of LAB.

Thanks

Ish


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« Reply #1 on: February 21, 2010, 08:07:49 PM »
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Considering that the ideal workflow in terms of quality, flexibility and non destructive workflow would be to do as much tone and color “correction” work at the raw rendering stage, one that can’t support nor needs Lab, one needs to examine just how useful Lab workflows are.

You know the old sayings. If it seems too good to be true, or, if all you know is a hammer, everything looks like a nail, and considering there are no Lab capture or output devices, maybe you could describe your workflow from capture to output in a bit more detail.
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Ishmael.
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« Reply #2 on: February 21, 2010, 08:31:02 PM »
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Quote from: digitaldog
Considering that the ideal workflow in terms of quality, flexibility and non destructive workflow would be to do as much tone and color “correction” work at the raw rendering stage, one that can’t support nor needs Lab, one needs to examine just how useful Lab workflows are.

You know the old sayings. If it seems too good to be true, or, if all you know is a hammer, everything looks like a nail, and considering there are no Lab capture or output devices, maybe you could describe your workflow from capture to output in a bit more detail.


I'm capturing in sRGB with a canon 350d, and processing photos for web output on the photography website I am currently developing. From the reading I've done on LL, I thought that you wanted to work as little as possible with the image in camera RAW....is that incorrect in your experience?
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« Reply #3 on: February 21, 2010, 09:06:46 PM »
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Quote from: Ishmael.
From the reading I've done on LL, I thought that you wanted to work as little as possible with the image in camera RAW....is that incorrect in your experience?

Uh, if you are getting that from reading on LuLa, then you need to check your reading comprehension skills...and the product name is "Camera Raw"...

Who has EVER said do little or nothing in Camera Raw and do everything in Photoshop afterwards...oh, yeah, Dan M. (who also advocates doing as much as possible in Lab cause well, Camera Raw isn't designed for "profession users" and you need to learn esoteric imaging skills to work in Lab–which Dan is happy to teach you).

Seriously, if you are capturing in sRGB maybe you would be better off ignoring Camera Raw–heck you might as well shoot in JPEG cause, well who would EVER want to work in an "ultra-wide theoretical color space such as Pro Photo RGB"?

If you want help, ask...but really, you shouldn't presume to tell us what "we" (here on LuLa) have been advocating unless you actually know what you are talking about...
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Ishmael.
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« Reply #4 on: February 21, 2010, 09:44:24 PM »
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Quote from: Schewe
Uh, if you are getting that from reading on LuLa, then you need to check your reading comprehension skills...and the product name is "Camera Raw"...

Who has EVER said do little or nothing in Camera Raw and do everything in Photoshop afterwards...oh, yeah, Dan M. (who also advocates doing as much as possible in Lab cause well, Camera Raw isn't designed for "profession users" and you need to learn esoteric imaging skills to work in Lab–which Dan is happy to teach you).

Seriously, if you are capturing in sRGB maybe you would be better off ignoring Camera Raw–heck you might as well shoot in JPEG cause, well who would EVER want to work in an "ultra-wide theoretical color space such as Pro Photo RGB"?

If you want help, ask...but really, you shouldn't presume to tell us what "we" (here on LuLa) have been advocating unless you actually know what you are talking about...



Perhaps you should check your reading comprehension skills and double check my original post: "Given that I'm relatively new to photoshop..." I am clearly asking for help and admitting that I dont know a lot about this subject, but apparently you took that as an invitation to demonstrate your vast arrogance.

On another note, if anyone who is slightly more down to earth and helpful would like to clear up this issue for me, I would seriously appreciate it.
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Panopeeper
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« Reply #5 on: February 21, 2010, 09:54:24 PM »
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Quote from: Ishmael.
I've recently discovered the power of the LAB color space and it almost seems too good to be true.I'm sure certain adjustments should be done in RGB, but for the most part I am getting significant more powerful images out of LAB than RGB. Given that I'm relatively new to photoshop I would really like to know from you veterans out there why someone would correct an image in RGB instead of LAB.
1. If you are recording raw data, then the "native" shot is most probably for orientation, normally its quality is not of paramount importance (sometimes is has to be horrendeous). Regarded, that the vast majority of monitors are working in sRGB, some in AdobeRGB, it appears (to me) nonsensical to make the shot in any other color space than in that, which can be seen directly on your monitor.

2. If you are aiming at ETTR, you need a special setup resulting in such raw data, which makes the embedded JPEG pretty much useless anyway, except for judging the exposure.

3. I wonder if anyone can demonstrate, that it is better to develop the image in LAB than in RGB; I don't know of any consideration of basic importance. (If you are working with some special printer requiring LAB color space, then why are you shooting raw?) There are some operations, which are or may be better in LAB, but that is, IMO, mostly not enough to convert the image in LAB and back. Keep in eye, that the vast majority of monitors and printers supports RGB but not LAB.
« Last Edit: February 21, 2010, 09:55:31 PM by Panopeeper » Logged

Gabor
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« Reply #6 on: February 21, 2010, 11:00:32 PM »
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Quote from: Ishmael.
Perhaps you should check your reading comprehension skills and double check my original post: "Given that I'm relatively new to photoshop..."


Uh huh, read that...but I'm pretty darn sure your post indicated that according to YOU, the preponderance of the postings on LuLa seem to indicate a preference to editing in Lab rather that Camera Raw?

If that is what you MEANT to say, then no, you are full of it...
(and there's nothing wrong with my reading comprehension).

Maybe you should spend a little more time around these parts and learn the players...I really and seriously won't apologize for my behavior (here or elsewhere) but I kinda do know what the f$%K I'm taking about...since I kinda write the book on the subject (see: Real World Camera Raw).

So, if you want to learn, ask your questions without editorial comments...that's what kinda works better here.

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« Reply #7 on: February 21, 2010, 11:29:10 PM »
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Hi,

Well, I guess it's about the opposite. What do you mean with sRGB? That setting on the camera is just ignored (AFAIK) if you shoot "RAW" and use ACR or even better LR (Lightroom). Now if you shoot in camera JPEGs most of the processing is done in camera and 94% of the information thrown away (if your camera has 12 bit ACD). If you are concerned about processing your images yourself you should shoot raw.

Regarding color space, digital cameras can capture a very large color space, using a small color space like sRGB will loose all colors registered by the camera that won't fit in the small color space. That's not necessarily bad, if you don't have colors falling outside sRGB in the image. For instance, 23 out of 24 of the color patches on the famous Xrite Color Checker Card would fall within sRGB.

Printers can print colors falling outside sRGB and sRGB also contains colors that cannot be printed.

The best way of manipulating colors is probably in Lightroom, because that gives you a parametric workflow. There are other products offering parametric workflows, Apple's Aperture and Bibble Labs's Bibble Pro 5. With a parametric workflow you don't manipulate an image, just create a "recepie" how it should be handled. The image will be interpreted according to the recepie as needed.

To keep all colors and tonal info in a PS based workflow you need 16 bit-tiffs. DNG (Digital Negative) images from my 24 MP DSLR are about 25 MByte, a 16 bit TIFF would be 6x24 around 144 MByte. The small file contains all the information in the big one. Sony's own file format "ARW" is less efficient than DNG, about 37.5 MByte/image. Some raw converters don't support DNGs, however.

Nothing wrong with working in Lab. One advantage of LAB that it separates tonality (L) from color (ab). A good raw processor would also do that.


Finally, if you want to use Photoshop and Lab as color space, don't forget to use 16bits and tagging with the correct profile. If you convert from Lab to an RGB color space you are going to loose quite a bit of information, and having those 16 bits/color helps you loosing as little as possible.

Best regards
Erik


Quote from: Ishmael.
I'm capturing in sRGB with a canon 350d, and processing photos for web output on the photography website I am currently developing. From the reading I've done on LL, I thought that you wanted to work as little as possible with the image in camera RAW....is that incorrect in your experience?
« Last Edit: February 21, 2010, 11:31:47 PM by ErikKaffehr » Logged

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« Reply #8 on: February 22, 2010, 01:17:50 AM »
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Quote from: Ishmael.
I've recently discovered the power of the LAB color space and it almost seems too good to be true.I'm sure certain adjustments should be done in RGB, but for the most part I am getting significant more powerful images out of LAB than RGB. Given that I'm relatively new to photoshop I would really like to know from you veterans out there why someone would correct an image in RGB instead of LAB.

Hi Ish,

I should mention for the benefit of (the large numbers of) people who do not know me,  that I'm firmly on the Dan Margulis side of the fence.  He's my hero, mainly because every single point he makes is illustrated with specific images that benefit from whatever technique he is discussing.  I think color correction by the numbers is the cat's meow.  

That said, there are advantages to each color space, depending on the original image, the techniques and blending modes, you will be using, and where you want to go with it.  RGB is a great color space for channel blending, apply image, and for removing color casts for images with mixed lighting.

Lab is dynamite, and a large number of the people who take a class in Lab will bear hug it for a year or so while discovering its possibilities.  Lab is not subtle, though, so - time permitting - it's generally good to finish up in RGB after making the big moves in Lab.

There is a universe of possibilities, in Lab and with other techniques involving blending layers, and I hope you will ignore the noise that, sadly, is inevitable in these forums, and experiment and look at as wide a variety of techniques as possible.

Mike Russell
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« Reply #9 on: February 22, 2010, 03:01:11 AM »
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I don't understand why you (and many other) newbies get the cold shoulder or even attacks at them for having misconceptions - we should do a better job of embracing new members and talent rather than going on rants against them.

I think the confusion starts from the capture comments. If you "capture in sRGB" it implies you capture JPEGs, not RAW. But I think you shoot RAW with 350D, it doesn't matter what color space you have used in camera settings - RAW images have no colorspace AFAIK. The colorspace is only applied in Camera RAW (or Lightroom or Bibble or whatever RAW converter you use).

To reiterate: the color space you set in your camera only applies to the JPEGs it saves, and is ignored for RAWs.

So when you move to PS you'll be working in sRGB, aRGB or Prophoto. sRGB is fine for vast majority of cases, but most here advocate using Prophoto just to be safe and convert to sRGB as the last output step. I'm one of them, although I'm pretty sure Prophoto offers only academic improvement in 99% of the cases. It's just that there's not much hassle for using Prophoto vs sRGB and it's a good way to future-proof my files for better output devices (monitors and printers).

Not familiar with LAB, although it has its advocates.
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« Reply #10 on: February 22, 2010, 05:46:48 AM »
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Now if you shoot in camera JPEGs most of the processing is done in camera and 94% of the information thrown away (if your camera has 12 bit ACD).
Wonderful math  
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« Reply #11 on: February 22, 2010, 08:09:36 AM »
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Quote from: ErikKaffehr
...One advantage of LAB that it separates tonality (L) from color (ab)...
Lightness and color are not fully separated in Lab. One of the biggest disadvantages of Lab as an editing space is that changes in the L channel affect color saturation. Increasing L reduces saturation and decreasing L increases saturation, in a way that looks unnatural to me.

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« Reply #12 on: February 22, 2010, 08:20:37 AM »
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Quote from: Ishmael.
I'm capturing in sRGB with a canon 350d, and processing photos for web output on the photography website I am currently developing. From the reading I've done on LL, I thought that you wanted to work as little as possible with the image in camera RAW....is that incorrect in your experience?

I’d say the opposite. You’d want to render the best possible color and tone, certainly globally before moving farther. Its going to ultimately be faster and less destructive. Its like scanning in the old days. Instead of just setting some default scan setting and “fixing” the results in Photoshop, you’d want to use the best possible scanner driver and produce the best possible quality pixels before moving on. I’d recommend you consider leaving sRGB JPEG 8-bit workflows and examining a raw workflow. This article while long, its a superb primer as to the reasons why:
http://wwwimages.adobe.com/www.adobe.com/p...renderprint.pdf
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« Reply #13 on: February 22, 2010, 08:29:27 AM »
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Quote from: ErikKaffehr
What do you mean with sRGB? That setting on the camera is just ignored (AFAIK) if you shoot "RAW" and use ACR or even better LR (Lightroom).


Ish didn’t indicate he’s shooting raw. I suspect he’s shooting JPEGs in sRGB which is OK, but not ideal as many here know.

Ish, one big issue with what may be your 8-bit JPEG sRGB workflow into Lab is the data loss in moving from such a bit depth and color space into Lab and back. With a file that has 256 levels, a conversion from RGB to Lab (in this case in Adobe RGB (1998)) will discard 22 of those levels in the conversion alone. At least doing so from a high bit (more than 8-bits per color) will start off with more data and the net data loss isn’t an issue. 22 levels on a JPEG (which is already reducing your original data every time you save it) doesn’t sound like a lot, but that’s just the color mode conversion, you haven’t taken any additional editing that would alter the numeric values into account. IOW, its a good way to toss data to the point that banding in output results. Now you seem to be saying your output is to the web. That’s a pretty low quality delivery and you may be fine with your workflow. But if you ever intent to print those images to a quality device, you’ve discarded a lot of data (both in bit depth and color gamut) you can never get back.
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Ishmael.
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« Reply #14 on: February 22, 2010, 08:32:21 AM »
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Thanks to those of you that have provided useful responses. My earlier postings were a bit vague, let me clear up where I'm coming from. I thought
that Photoshop offered more room for correction than Camera Raw because it allows you to use multiple adjustment layers, smart objects, and seems to have quite a few more ways to sharpen, clarify, & NR an image. Now, I realized that I might be wrong so I thought I would make this posting and ask people who do know......

Secondly, I shoot all my images and RAW and I understand that in-camera settings like sharpness, white balance, color space, ect are not applied unless you're shooting JPEG...but all these can be adjusted once you're in ACR. Now, because at this stage I'm processing for web output, I thought the best workflow for most images was this:

--convert to sRGB because most monitors cannot handle the full gamut of Adobe or Pro Photo

--make minimal adjustments, mainly exposure and fill light in ACR

--open image and convert to LAB color, where I fix up the color cast, contrast, and saturation using multiple curves adjustments layers. Add clarity and noise reduction by converting the background layer to a smart object and applying high pass, luminance NR, and median (for color noise) filters.

--convert back to RGB, resize and resharpen (using Smart Sharpen) for web, and save as JPEG


As a sidenote: I'd like to add that Deke McClelland has stated that converting betwee RGB and LAB is very marginally destructive.



OK That is my workflow and any suggestions or criticism on it is highly appreciated.
« Last Edit: February 22, 2010, 08:34:54 AM by Ishmael. » Logged
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« Reply #15 on: February 22, 2010, 08:57:48 AM »
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Quote from: Ishmael.
Thanks to those of you that have provided useful responses. My earlier postings were a bit vague, let me clear up where I'm coming from. I thought
that Photoshop offered more room for correction than Camera Raw because it allows you to use multiple adjustment layers, smart objects, and seems to have quite a few more ways to sharpen, clarify, & NR an image. Now, I realized that I might be wrong so I thought I would make this posting and ask people who do know......

Secondly, I shoot all my images and RAW and I understand that in-camera settings like sharpness, white balance, color space, ect are not applied unless you're shooting JPEG...but all these can be adjusted once you're in ACR. Now, because at this stage I'm processing for web output, I thought the best workflow for most images was this:

--convert to sRGB because most monitors cannot handle the full gamut of Adobe or Pro Photo

--make minimal adjustments, mainly exposure and fill light in ACR

--open image and convert to LAB color, where I fix up the color cast, contrast, and saturation using multiple curves adjustments layers. Add clarity and noise reduction by converting the background layer to a smart object and applying high pass, luminance NR, and median (for color noise) filters.

--convert back to RGB, resize and resharpen (using Smart Sharpen) for web, and save as JPEG


As a sidenote: I'd like to add that Deke McClelland has stated that converting betwee RGB and LAB is very marginally destructive.



OK That is my workflow and any suggestions or criticism on it is highly appreciated.

I would suggest that you invest in Mr. Schewe's book "Real World Adobe Camera Raw" to learn just how powerful a raw workflow can be. Your statement "where I fix up the color cast, contrast, and saturation....." in LAB can be done(along with capture sharpening and all sorts of color corrections) in ACR and be TOTALLY NON-DESTRUCTIVE before you render it into a colorspace. Once you come into PS, you are now working on pixels(in ACR you're not) and any edits you do are destructive. If I have to output to an sRGB space, I do as much of what I can(which is a lot) and then output through the image processor, which I believe is the only place that you can specify sRGB(plain RGB is not the same). As for going into LAB and than back again is destructive, marginal(a subjective term) or not is still destructive.
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« Reply #16 on: February 22, 2010, 09:01:36 AM »
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Quote from: Ishmael.
--convert to sRGB because most monitors cannot handle the full gamut of Adobe or Pro Photo
There are displays that can fully handle Adobe RGB (1998) gamut. But that’s somewhat moot because we are always working with color spaces that have gamut disconnects between what we edit and what we output. And if you think Adobe RGB (1998) is a wide gamut space on your sRGB like display, rethink the gamut of Lab which is HUGE. Assuming you are working with wide gamut capture to wide gamut output devices, displayed on an sRGB display, sure, there are colors you can’t see but you can print! Would you rather throw away colors you can see on the final output device just because extreme colors at the edge of the working space gamut cannot be seen on an intermediate device (the display)? There are few options here. Personally, the print is my final. I’d rather see the colors there and archive them so that in the future, as technology improves, maybe I’ll see them on a display. There are all kinds of other display versus output discontent like the huge differences in dynamic range. The display compared to the final print is simply an imperfect device. We have to live with that.
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--make minimal adjustments, mainly exposure and fill light in ACR
You haven’t mentioned Dan M but others have so I’ll simply say that his suggestions are to zero out all ACR settings and render the raw data, then fix the resulting turd in Photoshop is blatantly stupid.
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--open image and convert to LAB color, where I fix up the color cast, contrast, and saturation using multiple curves adjustments layers. Add clarity and noise reduction by converting the background layer to a smart object and applying high pass, luminance NR, and median (for color noise) filters.
You could do this in any color model (CMYK, RGB etc). The point is, why fix something that doesn’t have to be broken in the first place. Its like a photographer being totally sloppy on film exposure and then having the lab push his film 2 stops. It will work. Someone could probably make a print from it. But is it ideal and good working practice? No. As photographers, we would look down on an instructor who suggest we be sloppy with exposure and fix the issue later in the lab. This is Dan’s take on image processing. Sure, if you start with a turd (or in your case, an image with a color, contrast and saturation issue), you can make it look better after applying some of Dan’s techniques. Just like you can fix the exposure in the lab. But you could render (not fix, but actually create) idealized pixels from the raw to render stage in the first place. Its faster. Its fully non destructive. It provides a history that lives with the original raw data forever. It doesn’t make your files balloon to huge sizes because it simply metadata instructions (tiny text files). Prior to those capturing in Raw and using good raw processors, the ideas Dan proposes were the only option (or as I said above, make a good scan, not a crap scan and fix it in Photoshop). Dan’s got a workflow to sell and if you are caught with crappy, rendered data and no original (raw or film for a scan), his techniques are very, very useful. But short of that, they are simply idiotic. Its like the lab tech that will teach you the intricacies of push processing film because that’s all he knows. Proper exposure simply isn’t on his radar. Look at the god awful originals Dan shows in the before examples and ask yourself, “Do I capture this kind of rubbish”? If so, stick with his techniques. If not, if you believe that GIGO:Garbage In Garbage Out is something to avoid, move on.
Quote
As a sidenote: I'd like to add that Deke McClelland has stated that converting betwee RGB and LAB is very marginally destructive.
Given just that sentence, I could say Deke is wrong. But since he hasn’t defined anything like the original color space, bit depth and the problem that needs to be fixed (and why), I’ll cut him some slack unless you can find the exact quote.
ALL image processing in Photoshop which alters numeric values is destructive. That’s why we work in high bit, use adjustment layers (which will introduce the numeric rounding errors at some point). A good workflow is one that gets you the desired goals as quickly as possible with the best quality data. I simply don’t see why anyone with the intelligence of Dan or those who think he’s a bloody genius would want to start any image processing workflow with anything but ideal data.
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« Reply #17 on: February 22, 2010, 09:48:19 AM »
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Quote from: KeithR
Once you come into PS, you are now working on pixels(in ACR you're not) and any edits you do are destructive.

If you are not working on pixels in RAW, what are you working on? Surely the sensor in the camera must ouput a file which is composed of pixels (one for each sensel) each one representing a location and a colour/lightness value? Or if not, what are they?

John
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« Reply #18 on: February 22, 2010, 10:26:16 AM »
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Quote from: crames
Lightness and color are not fully separated in Lab. One of the biggest disadvantages of Lab as an editing space is that changes in the L channel affect color saturation. Increasing L reduces saturation and decreasing L increases saturation, in a way that looks unnatural to me.

I think if they had taken an approach such as Gram-Schmidt orthogonalization instead of simple vector differences, then the components of the color space would have been better separated. In image/video compression the DCT works along a similar line as it is very close to the orthogonal components of principal components of natural image data.
« Last Edit: February 22, 2010, 10:35:45 AM by joofa » Logged

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« Reply #19 on: February 22, 2010, 11:07:35 AM »
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As many who knew him would agree, Bruce Fraser was one of the best writers on the subject of imaging and this post, dating back well over a decade is a good read on the subject of using Lab:
Quote
Let me make it clear that I'm not adamantly opposed to Lab workflows. If
they work for you, that's great, and you should continue to use them.

My concern is that Lab has been oversold, and that naive users attribute to
it an objective correctness that it does not deserve.


Even if we discount the issue of quantization errors going from device space
to Lab and vice versa, which could be solved by capturing some larger number
of bits than we commonly do now, (though probably more than 48 bits would be
required), it's important to realise that CIE colorimetry in general, and
Lab in particular, have significant limitations as tools for managing color
appearance, particularly in complex situations like photographic imagery.

CIE colorimetry is a reliable tool for predicting whether two given solid
colors will match when viewed in very precisely defined conditions. It is
not, and was never intended to be, a tool for predicting how those two
colors will actually appear to the observer. Rather, the express design goal
for CIELab was to provide a color space for the specification of color
differences. Anyone who has really compared color appearances under
controlled viewing conditions with delta-e values will tell you that it
works better in some areas of hue space than others.

When we deal with imagery, rather than matching plastics or paint swatches,
a whole host of perceptual phenomena come into play that Lab simply ignores.

Simultaneous contrast, for example, is a cluster of phenomena that cause the
same color under the same illuminant to appear differently depending on the
background color against which it is viewed. When we're working with
color-critical imagery like fashion or cosmetics, we have to address this
phenomenon if we want the image to produce the desired result -- a sale --
and Lab can't help us with that.

Lab assumes that hue and luminance can be treated separately -- it assumes
that hue can be specified by a wavelength of monochromatic light -- but
numerous experimental results indicate that this is not the case.
For
example, Purdy's 1931 experiments indicate that to match the hue of 650nm
monochromatic light at a given luminance would require a 620nm light at
one-tenth of that luminance. Lab can't help us with that. (This phenomenon
is known as the Bezold-Brucke effect.)

Lab assumes that hue and chroma can be treated separately, but again,
numerous experimental results indicate that our perception of hue varies
with color purity.
Mixing white light with a monochromatic light does not
produce a constant hue, but Lab assumes it does -- this is particularly
noticable in Lab modelling of blues, and is the source of the blue-purple
shift.

There are a whole slew of other perceptual effects that Lab ignores, but
that those of us who work with imagery have to grapple with every day if our
work is to produce the desired results.

So while Lab is useful for predicting the degree to which two sets of
tristimulus values will match under very precisely defined conditions that
never occur in natural images, it is not anywhere close to being an adequate
model of human color perception. It works reasonably well as a reference
space for colorimetrically defining device spaces, but as a space for image
editing, it has some important shortcomings.

One of the properties of LCH that you tout as an advantage -- that it avoids
hue shifts when changing lightness -- is actually at odds with the way our
eyes really work. Hues shift with both lightness and chroma in our
perception, but not in LCH**.


None of this is to say that working in Lab or editing in LCH is inherently
bad. But given the many shortcomings of Lab, and given the limited bit depth
we generally have available, Lab is no better than, and in many cases can be
worse than, a colorimetrically-specified device space, or a colorimetrically
defined abstract space based on real or imaginary primaries.

For archival work, you will always want to preserve the original capture
data, along with the best definition you can muster of the space of the
device that did the capturing. Saving the data as Lab will inevitably
degrade it with any capture device that is currently available. For some
applications, the advantages of working in Lab, with or without an LCH
interface, will outweigh the disadvantages, but for a great many
applications, they will not. Any time you attempt to render image data on a
device, you need to perform a conversion, whether you're displaying Lab on
an RGB monitor, printing Lab to a CMYK press, displaying scanner RGB on an
RGB monitor, displaying CMYK on an RGB monitor, printing scanner RGB to a
CMYK press, etc.

Generally speaking, you'll need to do at least one conversion, from input
space to output space. If you use Lab, you need to do at least two
conversions, one from input space to Lab, one from Lab to output space. In
practice, we often end up doing two conversions anyway, because device
spaces have their own shortcomings as editing spaces since they're generally
non-linear.

The only real advantage Lab offers over tagged RGB is that you don't need to
send a profile with the image. (You do, however, need to know whether it's
D50 or D65 or some other illuminant, and you need to realise that Lab (LH)
isn't the same thing as Lab.) In some workflows, that may be a key
advantage. In many, though, it's a wash.

One thing is certain. When you work in tagged high-bit RGB, you know that
you're working with all the data your capture device could produce. When you
work in Lab, you know that you've already discarded some of that data.

Bruce

** this is one of Dan’s reasons that RGB workflows in raw converters or using the so called “Master Curve” (another of his made up terms) is off base. Adobe could easily have made the curves work as Dan demands of them, but much smarter people like Thomas Knoll have discussed that most users find this effect counter to what they expect and desire and now you know why. One can easily counteract this increase in saturation using the appropriate layer blend in Photoshop. Also, there is an article on this site by Mark Segal that goes into this debate in some detail: http://www.luminous-landscape.com/essays/Curves.shtml. You’ll note that as usual, Dan will not venture out of his private list to address this piece. Peer review isn’t something he wishes to address.
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Andrew Rodney
Author “Color Management for Photographers”
http://digitaldog.net/
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