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Author Topic: Your Curves  (Read 144490 times)
laughfta
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« Reply #20 on: July 25, 2007, 09:43:51 PM »
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If so, must one continue to work in ProPhotoRGB when in PS to realize this advantage? Can Adobe RGB also preserve exact hue?


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2) ProPhoto and Adobe RGB(98) are both RGB colour working spaces, the former being much wider than the latter. It stands to reason that if by converting your working space from the former to the latter you clip a channel or two in ARGB(98) that were not previously clipped in Pro Photo, the affected pixels should have a hue shift because the colour composition has changed. But to see this in a print, the affected hues would have to have been in printer gamut.


Thanks, Mark. I think I need my answers before I can clearly phrase my question:  
I am wondering if the improvements (in the ability of the curve to preserve hue) in ACR/LR are dependent on the much larger ProPhoto RGB color space? It seems as if a larger color space would mean more leeway in absorbing some of the stresses like contrast and and saturation adjustments without noticeable hue changes?
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Mark D Segal
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« Reply #21 on: July 25, 2007, 10:00:28 PM »
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Thanks, Mark. I think I need my answers before I can clearly phrase my question: 
I am wondering if the improvements (in the ability of the curve to preserve hue) in ACR/LR are dependent on the much larger ProPhoto RGB color space? It seems as if a larger color space would mean more leeway in absorbing some of the stresses like contrast and and saturation adjustments without noticeable hue changes?
[a href=\"index.php?act=findpost&pid=129927\"][{POST_SNAPBACK}][/a]

No, as far as I know preserving hue does not depend on the size of the colour space unless you move from a wider to a narrower space causing clipping of channels in the latter.

But speaking of leeway for absorbing stresses, unless you work with 16-bit files when you use a wide space such as ProPhoto, contrast and hue adjustments can trigger banding or posterization effects. This happens say with 8-bit files because you start with only 256 levels spread over a relatively huge colour space and then lose some with these adjustments, so smooth tonal transitions can give way to discrete "bumps". It is much safer to fill that huge space with 16 bit files where there is a theoretical 65536 levels, though Photoshop actually works in 15 bit reducing the levels to 32768, however the original raw files generally have a maximum 12 bit depth to start with, meaning the originating data has 4096 levels of real image data - still a huge improvement over 256 for preventing trouble in wide colour spaces.
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Mark D Segal (formerly MarkDS)
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adias
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« Reply #22 on: July 26, 2007, 12:15:26 AM »
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Mark:

I enjoyed your article and find it right on target. The reaction to using these tools comes often, in my view, from steeped habits that are hard to break.

When using Fill Light to open up shadows you mentioned also using Shadows to compensate and balance the image. I also find that the combination of Fill Light with the Curves Shadows' slider works very well. In a particular image I used Fill Light +24 and Shadows -20.
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digitaldog
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« Reply #23 on: July 26, 2007, 07:34:02 AM »
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Thanks, Mark. I think I need my answers before I can clearly phrase my question: 
I am wondering if the improvements (in the ability of the curve to preserve hue) in ACR/LR are dependent on the much larger ProPhoto RGB color space? It seems as if a larger color space would mean more leeway in absorbing some of the stresses like contrast and and saturation adjustments without noticeable hue changes?
[a href=\"index.php?act=findpost&pid=129927\"][{POST_SNAPBACK}][/a]

All the work is done under the hood in linear encoded ProPhoto RGB (primaries). If you ask for a smaller encoding color space, a conversion has to take place like all such color space conversions. How much of the image's gamut is in or out of the gamut of the encoding color space? How many out of gamut colors have to be mapped (using a RelCol itnent) from space to space? If you have colors outside of Adobe RGB (1998) and ask to encode in that space (or sRGB), there's going to be clipping. Fact of life. If that's a concern, encode in ProPhoto RGB.

Gamut mapping and resulting clipping happens just about every time we convert from color space to color space.
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Andrew Rodney
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Chris_T
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« Reply #24 on: July 26, 2007, 08:49:47 AM »
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I have yet to venture into ACR or Lightroom, but I use curves extensively in PS and consider it my tool of choice. My following comments are therfore based on PS curves only. As you have pointed out, curves do have some "side effects", namely:

- When a portion of a curve is steepened, other portions of the curve are flattened. The contrast of the steepened tonal range is increased, but at the expense of the contrast of the flattened tonal range being reduced.

- Writing curves to adjust tones can result in saturation/hue shifts, and vice versa.

To overcome these, many will combine curves with masks, blending modes, and/or channel mixing. People like Margulis and Varis also suggest separting tonal correction from color correction. In my experience, how to deal with these side effects depends tremendously on the kind of image being corrected. With some images, these side effects are inconsequential and simple curves by themselves will work. With other images, I find myself trying out different methods to compensate for them.  I often wonder why Adobe don't make it simpler by providing separate curves tools, such as one for tone, one for hue and one for saturation. I also wonder why such side effects are not treated more thoroughly (or at all) in the 500+ page PS books.

Your article is an heroic attempt, but I think that it can be improved by stating the PS curves' side effects as baselines first, before comparing them against ACR's or Lightroom's curves.  As mentioned earlier, the kind of images being used for comparison makes a huge difference. With an agreed upon set of baselines or images for this purpose, comparisons and discussions can be made with more context and objectivity. BTW, this is a generic problem when subjective comparisons are made online, and you are by no means being singled out. But since you must have put lots of efforts into this article, I thought it's worth my $.002.

Here are a few links for all you curves enthusiasts out there:

For the novice (and not so novice):
http://ronbigelow.com/articles/curves-1/curves-1.htm

A great (and rare) description of curves' tonal contrast tradeoff:
http://ronbigelow.com/articles/shadow/shad...-highlight2.htm

A traditionalist's (brave and bound to be controversial) view on curves:
http://www.arraich.com/ps8_CurvesCommentary1.htm

At the other end of the spectrum, this guy believes in curves, and nothing but curves:
http://www.curvemeister.com/
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Mark D Segal
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« Reply #25 on: July 26, 2007, 01:29:30 PM »
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I often wonder why Adobe don't make it simpler by providing separate curves tools, such as one for tone, one for hue and one for saturation. I also wonder why such side effects are not treated more thoroughly (or at all) in the 500+ page PS books.

As mentioned earlier, the kind of images being used for comparison makes a huge difference. With an agreed upon set of baselines or images for this purpose, comparisons and discussions can be made with more context and objectivity. BTW, this is a generic problem when subjective comparisons are made online, and you are by no means being singled out. But since you must have put lots of efforts into this article, I thought it's worth my $.002.

[a href=\"index.php?act=findpost&pid=129984\"][{POST_SNAPBACK}][/a]

Hi Chris,

If you wish to separate luminosity from colour using curves you can create two curves adjustment layers, one in Luminosity mode and one in Color mode, then vary their opacities to taste. Or you can do what I demonstrated in the article working with a combination of Curves and HSB adjustment layers. Adobe could always add more tools, but with Photoshop having gone through 10 versions by now and given the tools it already has for achieving this objective, perhaps simplifying this separation is either not high on their list of priorities, or customer demand has not made it so, or both.

And that of course is not unrelated to what you say about the 500+page books not exploring this issue very much; you are right - with the exceptions of Dan Margulis and Michael Kieran, most of the authors devote most of their space for Curves on showing how to adjust contrast with the composite curve and colour balance with the individual channel curves. It seems as if the saturation boost from Curves just hasn't had much traction as a big-ticket issue.

You are right about the hazards of selecting images for making demonstrations. I mentioned that twice-over and I explained the criteria for the images I used. I don't know with whom I was supposed to obtain an authoritative agreement on "baseline" images, or whether such a thing would even be an operationally fesible approach.

That said, there are some kinds of images which we know in advance would fare poorly without complex correction procedures that go beyond what we can do within a raw converter or relatively straightforward curves moves in Photoshop. I mentioned some of the more obvious ones just to make the point that no one piece of software or one approach is the be-all and end-all for every conceivable situation. The article is long enough just covering its basic intent. I could probably generate another ten pages or so demonstrating both how to select very breakable images and then proceed to break them - but who really needs that?
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Mark D Segal (formerly MarkDS)
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« Reply #26 on: July 26, 2007, 01:45:48 PM »
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Adobe could always add more tools, but with Photoshop having gone through 10 versions by now and given the tools it already has for achieving this objective, perhaps simplifying this separation is either not high on their list of priorities, or customer demand has not made it so, or both.

with the exceptions of Dan Margulis and Michael Kieran, most of the authors devote most of their space for Curves on showing how to adjust contrast with the composite curve and colour balance with the individual channel curves. It seems as if the saturation boost from Curves just hasn't had much traction as a big-ticket issue.

Exactly! With the exception of Dan, and to a far, less aggressive stance Kieran, I suspect millions of images have been adjusted using curves since 1990 in Photoshop, maybe hundreds of thousands in CR since it shipped and we've hardly heard anyone complain. Like Jeff said in the other Curves posts, if you want Adobe engineers to provide a new feature, you have to illustrate why such a feature is absolutely necessary. Some images may very well suffer from arbitrary yanking of curves but by and large, only one person has made this a political cause when very few end users have seen any such issues. To suggest that building a tool a certain way based on what people expect to see in most images is damaging to all images and thus makes the tool unsuitable for professional use is a huge stretch to say the least. But I'd agree that it might be useful to have additional controls. I'd really like to see a separate saturation curve like we had in LinoColor years ago. Perhaps the curves dialog in Photoshop/CR/LR could have a fourth (in RGB mode) curve that only affects saturation. You could use it alone as was the case in LinoColor or you could lock it into place for certain images when pulling the so called 'master curve". Or you could just leave it alone, allowing the curves to operate as they have for 17 years.
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Andrew Rodney
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Guillermo Luijk
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« Reply #27 on: July 26, 2007, 05:00:08 PM »
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This is a very interesting topic, and I usually find in Photograph forums some people showing others how to adjust contrast in their pictures using the Curves tool (or the Levels tool), usually without caring at all about the fusion mode used to preserve tone. Glad to see some people really care about this.

I have done the following experiment with a photograph to find out how well PS keeps tones depending on the colour profile in which the picture is. I am not sure if what I did provides any conclusion, but I will explain.

One of my programs (Tone Hacker) plots a Hue histogram (it's rare to find them, although I consider them really useful that's why I put it there) based on the RGB->HSV conversion formulas found here: RGB to HSV formulas.

- I have opened a photograph in Photoshop in sRGB, saved it (JPG) and calculated its Hue histogram
- Then I have added a strong contrast 'S' shaped curve in Normal fusion mode and saved again (JPG) and calculated its Hue histogram
- Then I have switched the curve from Normal to Luminance fusion mode and saved again (JPG) and calculated its Hue histogram

- And I have repetead the 3 steps for the image converted first to AdobeRGB, obtaining the histograms and...
- Repeated again the process converting from the original sRGB to ProPhoto obtaining the histograms too.

Finally I show you here the original image and the curve applied:

 .  

And here I compare (the upper is the Hue histogram resulting when having applied the curve, and the lower histogram is always the original in each colour profile, just upside down for easy difference detection):

[span style=\'font-size:14pt;line-height:100%\']sRGB[/span]

Normal fusion mode (left) vs Luminance fusion mode (right)
 .  


[span style=\'font-size:14pt;line-height:100%\']AdobeRGB[/span]

Normal fusion mode (left) vs Luminance fusion mode (right)
 .  


[span style=\'font-size:14pt;line-height:100%\']ProPhoto[/span]

Normal fusion mode (left) vs Luminance fusion mode (right)
 .  



The curve is very severe. It's easy to see that when the Normal fusion mode in the curve clearly modifies the Hue in the whole scene, however the Luminance fusion mode preserves the Hue and does it independently of which colour profile is being used.

Of course the effect of the curve in each colour profile is different, as histograms are, but the important conclusion is that HUE IS ALWAYS PRESERVED IF LUMINANCE FUSION MODE IS USED.

Am I right or I am talking bullshit?

Regards.

PS: I have to write one day the saturation histogram routine and repeat the tests.
« Last Edit: July 26, 2007, 05:01:38 PM by GLuijk » Logged

Mark D Segal
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« Reply #28 on: July 26, 2007, 07:09:50 PM »
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Hi Guillermo,

Glad you found the article stimulating enough to spend time testing images with your software. Your finding that hue is preserved in Luminosity Blend Mode is expected. It should also be expected that when you progress from a very wide space such as Pro Photo to a narrow one such as sRGB, colours will be clipped, and the affected pixels will therefore change hue.

But there is another variable in your workflow - conversion to JPG, which is a lossy compression format that throws away much data. It would be interesting to see what happens to hues if you had saved the files as PSD or TIFF rather than JPG, just in order to eliminate any possible influence of the JPG process on the results.

For those of us who like to see comparative results in actual images rather than (somewhat unfamiliar) histogram formulations, it would also be of interest to see the images in their "before" and "after" state, with a sampling of Hue measurements somwhat like what I attempted in the essay.

I'd be interested in your opinion about whether the measurement of hue coming from your program is any more accurate than we would get taking hue readings using the HSB read-outs in Photoshop's info palette - because as I mentioned in the article, concerns do exist about the accuracy of hue measurements in the HSB or HSV model.
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laughfta
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« Reply #29 on: July 26, 2007, 11:44:53 PM »
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Hi Guillermo,

This is  fascinating piece of work. Thanks for bringing it.  It sure is satisfying to see an exact match in data where the luminosity blend mode was used!  And really interesting to see what takes place in the Normal blend mode, as well.

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S: I have to write one day the saturation histogram routine and repeat the tests.


I sure hope you post it when you do!
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Guillermo Luijk
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« Reply #30 on: July 27, 2007, 06:00:47 AM »
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OK I will do so, but please tell me how to take indidivual HSB samples in Photoshop, I am not a PS expert and try to use it as less as possible (did you know PS makes 15-bit edition? half of the levels in the 0..65535 range are automatically set to 0 as soon as you load a 16-bit TIFF in Photoshop).

In the last times I have become a fan of linear images and linear edition, where curves have a very straight forward interpretation.

One of the things than can easily and perfectly be done in linear using curves is exposure correction. Surely ACR performs exposure correction in linear mode.
With a line curve in Normal blending mode, exposure adjustment can be done like this:



This is consistent both with exposure theory (amount of light impacting the sensor is proportional to captured level), so as with the Hue formulas described here, where both the upper and lower parts of Hue~(G-B )/(MAX-MIN)... are scaled with the same factor keeping thus the Hue.
In fact it has to be that way as a Hue model depending on exposure would be nonsense.

Another thing that can be applied in linear mode with line-curves is White Balance, since WB is nothing but a linear scaling of three RGB channels.



These two curves for instance provide a tungsten WB (Rojo=Red, Azul=Blue, Green remains unchanged) suitable for shots under indoor lighting.
Unfortunately Bayer demosaicing algorithms are optimised to be applied on already balanced RAW data, and performing WB after Bayer interpolation leads to artifacts in the borders of differently coloured areas. So this is rather a nice didactical concept than a useful practical technique.

If you would like to practice with linear RAW developing and editing, I recommend you a fantastic freeware program called DCRAW.
« Last Edit: July 27, 2007, 06:26:08 AM by GLuijk » Logged

laughfta
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« Reply #31 on: July 27, 2007, 07:05:02 AM »
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Another interesting post, Guillermo!

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OK I will do so, but please tell me how to take indidivual HSB samples in Photoshop


If you open the palette options, which can be reached through the arrow in the upper right corner of the info palette, you can change the mode to HSB. The eyedropper tool will show you the HSB info in the image.

The Color Sampler tool, found in the dropdown next to the eyedropper tool, will set points on your images if you alt click on the image.
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Chris_T
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« Reply #32 on: July 27, 2007, 08:33:01 AM »
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If you wish to separate luminosity from colour using curves you can create two curves adjustment layers, one in Luminosity mode and one in Color mode, then vary their opacities to taste. Or you can do what I demonstrated in the article working with a combination of Curves and HSB adjustment layers. Adobe could always add more tools, but with Photoshop having gone through 10 versions by now and given the tools it already has for achieving this objective, perhaps simplifying this separation is either not high on their list of priorities, or customer demand has not made it so, or both.

I'm doing exactly what you suggested. Create and close an unmodified curves adjustment layer, change the layer's blending mode, activate and modify the layer. Even with an action, I think this can be "simpler". In addition to curves, it would be nice to have tools that are specific for luminosity and color corrections separately.

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And that of course is not unrelated to what you say about the 500+page books not exploring this issue very much; you are right - with the exceptions of Dan Margulis and Michael Kieran, most of the authors devote most of their space for Curves on showing how to adjust contrast with the composite curve and colour balance with the individual channel curves. It seems as if the saturation boost from Curves just hasn't had much traction as a big-ticket issue.

Most readers, including myself, were thrilled by the initial application of an S curve. Many look no further and think of these authors as godsends. But some, like myself, either by closer scrutiny of their work or by reading, will learn to notice the side effects. When it comes to subjective evalutaion, nothing beats having someone pointing out a problem or difference, and then comparing two edits side by side. Once I'm aware of a problem that was unnoticed before, it can stand out like a sore thumb. If more users are made aware of the same, a traction will build up. I value writers who are able and willing to disclose and educate, much like I value MDs who alert me of a prescription's side effects.

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You are right about the hazards of selecting images for making demonstrations. I mentioned that twice-over and I explained the criteria for the images I used. I don't know with whom I was supposed to obtain an authoritative agreement on "baseline" images, or whether such a thing would even be an operationally fesible approach.

That said, there are some kinds of images which we know in advance would fare poorly without complex correction procedures that go beyond what we can do within a raw converter or relatively straightforward curves moves in Photoshop. I mentioned some of the more obvious ones just to make the point that no one piece of software or one approach is the be-all and end-all for every conceivable situation. The article is long enough just covering its basic intent. I could probably generate another ten pages or so demonstrating both how to select very breakable images and then proceed to break them - but who really needs that?
[a href=\"index.php?act=findpost&pid=130021\"][{POST_SNAPBACK}][/a]

My suggestion for you to include a set of images for your article is perhaps unrealistic. But I do believe for digital imaging tools evaluations and comparisons, a set of images can be really helpful. Without them, the reviews and threads on tools such as noise reduction and sharpening are without context, meaningless, misleading, and can easily lead to the next world war. I remain hopeful that someone somewhere will take the dive. It will make perfect sense for the vendor of the next tool (e.g. noise reduction or sharpening) to do so to demonstrate how his product is superior.

Digital imaging is still at its infancy, and unfortunately being dominated by a single editing product. Millions of images having been edited by a certain tool in a certain way is only a reflection of the current state of acceptance. It does not mean it is cast in stone. There was a time when the world was thought to be flat.

Off my soap box.
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« Reply #33 on: July 27, 2007, 08:55:49 AM »
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When it comes to subjective evalutaion, nothing beats having someone pointing out a problem or difference, and then comparing two edits side by side. Once I'm aware of a problem that was unnoticed before, it can stand out like a sore thumb. If more users are made aware of the same, a traction will build up. I value writers who are able and willing to disclose and educate, much like I value MDs who alert me of a prescription's side effects.

Dan Margulis, as recently as yesterday continues to say his ideas about the curves in and out of Photoshop are valid, that Mark's images fail to disprove his points. Problem is, Dan refuses to supply images (raws) to prove HIS point, unlike Mark. Dan states certain images exhibit issues (damage) due to the design of CR/LR.Images with saturated reds and so forth.  In one post he called it 'sloppy math". Yet when one of us submits such images to illustrate the rendering controls are available to produce good results, such images are dismissed by Dan. So, the only alternative to get to the bottom of this is having him submit images he says are damaged by the rendering options in CR/LR.

He's not been shy in the past in providing JPEGs of supposed damage done by this or that editing move. JPEGs in this discussion are useless for obvious reasons. Despite numerous requests from many different posters, neither the spreadsheet that proves his point using 'exact math' nor a single Raw file has been submitted for what I would call fair peer review. What's he afraid of? Wouldn't showing us raws, providing the exact rendering moves in LR or CR be educational and provide proof of concept? As I said in the original post about this subject, it isn't necessary for those who may disagree with Dan to disprove his point, its up to him to prove it. That's good science. For months, many have tried to get such proof. IF it doesn't exist it makes much of what he writes highly suspicious. Show me the Raw.
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laughfta
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« Reply #34 on: July 27, 2007, 09:22:36 AM »
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I am wondering about the  difference in the histograms that Guillermo provided illustrating the difference between "normal" and "luminence" blending mode in ProPhoto RGB. These were done in PS.  
If they were produced in ACR/LR, would the hues show a shift in response to a curve adjustment?
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« Reply #35 on: July 27, 2007, 10:19:11 AM »
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(did you know PS makes 15-bit edition? half of the levels in the 0..65535 range are automatically set to 0 as soon as you load a 16-bit TIFF in Photoshop).

In the last times I have become a fan of linear images and linear edition, where curves have a very straight forward interpretation.

One of the things than can easily and perfectly be done in linear using curves is exposure correction. Surely ACR performs exposure correction in linear mode.
With a line curve in Normal blending mode, exposure adjustment can be done like this:

[a href=\"index.php?act=findpost&pid=130098\"][{POST_SNAPBACK}][/a]

Hi Guillermo,

Actually the bit depth and number of resulting levels is even a bit more complicated than what you describe - as you most likely know. Our best Canon DSLRs, for example, capture in 12-bit depth. That gives us 2^12 or 4096 layers of "real" information. Then somehow - and I'm not sure how the "somehow" happens - this gets translated by Camera Raw into 15-bit depth, giving us 32,768 levels, but in Photoshop this is called 16 bit depth which in theory is 65536 levels - and again I'm not sure exactly how those additional 32768 levels get "filled". Whatever, 4096 levels of originating data is surely much better than the 256 to which we would be limited with 8 bit files.

I agree with you about the use of linear curves. I use them very often - I find they provide a very controlled and gentle way to edit contrast in an image - but of course they are not always sufficient - when you don't want to flatten the image too much, and you don't want to sacrifice highlight or shadow detail but you still need more contrast, that is where the various styles of S-curves, etc. become most helpful.
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Mark D Segal (formerly MarkDS)
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« Reply #36 on: July 27, 2007, 10:38:06 AM »
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From a top Photoshop engineer:

The high-bit representation in Photoshop has always been "15   1" bits
(32767 (which is the total number of values that can be represented by 15
bits of precision)   1).  This requires 16 bits of data to represent is
called "16 bit".  It is not an arbitrary decision on how to display this
data, it is displaying an exact representation of the exact data Photoshop
is using, just as 0-255 is displayed for 8 bit files.
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« Reply #37 on: July 27, 2007, 10:41:26 AM »
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I'm doing exactly what you suggested. Create and close an unmodified curves adjustment layer, change the layer's blending mode, activate and modify the layer. Even with an action, I think this can be "simpler". In addition to curves, it would be nice to have tools that are specific for luminosity and color corrections separately.


My suggestion for you to include a set of images for your article is perhaps unrealistic. But I do believe for digital imaging tools evaluations and comparisons, a set of images can be really helpful.


[a href=\"index.php?act=findpost&pid=130122\"][{POST_SNAPBACK}][/a]

Chris, what you are suggesting about a specific tool for separately editing Luminosity and Color in Curves may be said to exist already in Lab space, but we have been very reliably informed that adjusting L while holding a* and b* constant still produces "chroma effects". So perhaps something else that does this with surgical precision and doesn't need several layers could be helpful to some people. As Jeff Schewe once informed us somewhere on this website, the only way to get that from Adobe would be to put in a feature request, and you need to make the case for the feature - they won't listen to an argument confined to sentiments like "it would be nice to have......", because all kinds of things would be nice to have, but the requestor needs to make a case to them about the value-added it would convey to the program relative to what it does now. This may be a challenge, but hey - one doesn't know until one trys.

Now about the images - I infer from your comment that you are thinking of some kind of standard or normative set of test images that the whole industry would adopt as a common platform for testing everything and anything - again not a bad idea in principle (something like the input to an ISO standard I suppose), but I'm glad you realize that would have been "a bit much" for Your's Truly to take-on. You know how these things happen: committees of interested parties or existing associations get appointed to develop the material.  Years of in-committee discussion and debate plus alpha and and beta testing with a host of outside experts would take place - then IF EVER a consensus were to emerge, that organization would produce the set of images for the whole world to use - voluntarily of course.  But not a bad idea in principle. As usual, the devil would be in the details.......................
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Mark D Segal (formerly MarkDS)
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« Reply #38 on: July 27, 2007, 10:52:41 AM »
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I am wondering about the  difference in the histograms that Guillermo provided illustrating the difference between "normal" and "luminence" blending mode in ProPhoto RGB. These were done in PS. 
If they were produced in ACR/LR, would the hues show a shift in response to a curve adjustment?
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Hi Gloria, I have taken the RAW file, and developed it using ACR and ProPhoto profile as destination, using TIFF as output file format:

1. Developed with no curve applied at all (original)
2. Developed applying curve in ACR
3. Developed applying curve in PS (Normal blending mode)
4. Developed applying curve in PS (Luminance blending mode)

And I have compared the Hue histogram in 1 (original) with 2, 3 and 4.

The curve in 2 (ACR) and the one in 3,4 (PS) are very similar, and applied over equal histograms (the background histogram shown by ACR curve editor perfectly matches that one shown by PS prior to curve application).
With this I mean that any difference in the result can be assumed to be derived from differences in the internal math processing to apply the curve.

Hue comparision original (down in all three pics) vs...
ACR curve (left) / PS Normal curve (middle) / PS Luminance curve (right)
 .    .  


This is the original image appearance with no curve or any other control applied (absolutely all ACR values set to 0, WB as in shot):

Original



And these are the 3 curve edited versions (ACR curve, PS Normal, PS Luminance):

ACR curve


PS Normal


PS Luminance


And these are the 2 curves used:


I want to point that even if it seems so, the Prophoto ACR curve version is not blown. This would have surely affected to Hue.
The displayed pictures were in the last moment converted to sRGB so that you can see them as I saw them in PS ProPhoto.


Apparent conclusion: "ACR curves don't preserve Hue, and in addition to this don't work exactly as a PS Normal blending mode RGB curve. Only PS Luminance blending mode seems to preserve Hue, producing however an apparent saturation loss (or at least a feeling that this happens)." (the saturation issue is well commented in MarkDS' document).
« Last Edit: July 27, 2007, 11:12:59 AM by GLuijk » Logged

Guillermo Luijk
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« Reply #39 on: July 27, 2007, 11:00:32 AM »
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Actually the bit depth and number of resulting levels is even a bit more complicated than what you describe - as you most likely know. Our best Canon DSLRs, for example, capture in 12-bit depth. That gives us 2^12 or 4096 layers of "real" information. Then somehow - and I'm not sure how the "somehow" happens - this gets translated by Camera Raw into 15-bit depth, giving us 32,768 levels, but in Photoshop this is called 16 bit depth which in theory is 65536 levels - and again I'm not sure exactly how those additional 32768 levels get "filled". Whatever, 4096 levels of originating data is surely much better than the 256 to which we would be limited with 8 bit files.[a href=\"index.php?act=findpost&pid=130135\"][{POST_SNAPBACK}][/a]

well, I don't think ACR is actually 15 bit, I think is the PS engine who converts from true 16-bit developed RAW to 15. Not really a conversion, just a bit drop.
That is why I like to use DCRAW, as I get true 16-bit developed RAW files in their complete 0..65535 range.
The 12 bit RAW file is converted to 16 bit into any RAW developer prior to any other process stage. Then true 16-bit white balance is applied, and true 16-bit Bayer demosaicing is done. That is why a developed RAW in linear state is completely full of levels in the lower end of its histogram (no holes at all).
It's in the next step, when the colour profile conversion and gamma compensation are applied, that the lower part if the histogram gets full of holes as an Emmental cheese.

With "linear curves" I didn't mean only that they are straight lines, but also making reference to the type of image they are applied to: a LINEAR IMAGE; i.e. with no gamma compensation applied yet.
This is something that DCRAW provides and has some advantages that gamma corrected images as those coming from ACR lack, for instance the showed exposure control.
« Last Edit: July 27, 2007, 11:14:10 AM by GLuijk » Logged

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