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Author Topic: Why do raw files require tone mapping to look perceptually correct?  (Read 4176 times)

kpz

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Why do raw files opened in Lightroom seem to almost always require massaging with the highlight and shadow sliders to look perceptually correct (that is, to reproduce what my eyes see in reality)?

I notice that virtually everyone I have seen edit photos in LR (e.g. on YouTube) lowers the highlight slider and increases the shadow slider to "recover detail" in order to mitigate this effect. Why is this necessary to create a realistic image? The camera sensor sees the same photons I see, and while there's a gamma curve correction to account for the fact that the human eye doesn't perceive light in a linear fashion, that still doesn't explain the need for these strange local tone mapping adjustments.

Alternatively, am I wrong, and can one get a nearly "realistic" image without touching these sliders at all? Obviously, if overdone it creates the "bad HDR" effect. I am trying to understand why their use is so common.

edit: Actually, I suppose the same question applies also to global contrast adjustments via the tone curve, since this is just a global tone mapping. And I guess "behind the scenes" what LR does with the shadow/highlight sliders can be roughly understood as some kind of tone curve manipulation.

So, more broadly, why do raw files require significant tone curving to look "right"? In Camera Neutral, for instance, they seem flat and lacking contrast. But again -- I should be seeing raw sensor data with basically just a gamma correction, and the sensor and I are seeing the same photons, so I expect roughly the same result. What explains the discrepancy? Surely some lack of microcontrast can be attributed to Bayer interpolation and the AA filter (if present), but that's not really what I'm talking about.
« Last Edit: November 27, 2018, 01:31:20 am by kpz »
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sandymc

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #1 on: November 27, 2018, 04:34:26 am »

"General-Purpose Gamut-Mapping Algorithms: Evaluation of Contrast-Preserving Rescaling Functions for Color Gamut Mapping", Gustav J. Braun and Mark D. Fairchild
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #2 on: November 27, 2018, 12:25:09 pm »

There is no such thing as 'perceptually correct'. There is scene referred and output referred and the later is subjective and differs based on a massive number of factors:
http://www.color.org/ICC_white_paper_20_Digital_photography_color_management_basics.pdf
Take transparency film as an example unlike color neg (even farther away from output referred): which is 'perpetually accurate', Agfachrome, Kodachrome, Velvia, Ektachrome? The manufacturers produce a rendering they believe you will visually prefer and buy. Just like default raw processing conversions which may or may not be to your liking or better for a starting point in editing the image. If you don't like the defaults, change them. Simple to do in many converters including of course, Adobe's.
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Andrew Rodney
Author “Color Management for Photographers"

Bart_van_der_Wolf

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #3 on: November 27, 2018, 01:04:10 pm »

Why do raw files opened in Lightroom seem to almost always require massaging with the highlight and shadow sliders to look perceptually correct (that is, to reproduce what my eyes see in reality)?

I notice that virtually everyone I have seen edit photos in LR (e.g. on YouTube) lowers the highlight slider and increases the shadow slider to "recover detail" in order to mitigate this effect. Why is this necessary to create a realistic image?

Hi,

Because Lightroom (over)compresses the highlights (attempting to recover clipped highlights, even when they are unclipped).

Cheers,
Bart
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kpz

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #4 on: November 27, 2018, 01:50:16 pm »

Thanks for the answers. I wonder, can anyone recommend a readable but complete technical reference for color and raw conversion? I realize this a large question and a great deal of ink has been spilled, but surely there must be a more efficient way to learn this material than just picking up information online piecemeal.

Bart, if Lightroom over-compresses highlights by default, wouldn't lowering the highlight slider compress them even *more*? (Here I understand "highlight compression" to mean tone mapping the highlights to a lower, wider range of values, but perhaps I am getting the definition wrong.)
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Bart_van_der_Wolf

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #5 on: November 27, 2018, 02:36:07 pm »

Bart, if Lightroom over-compresses highlights by default, wouldn't lowering the highlight slider compress them even *more*? (Here I understand "highlight compression" to mean tone mapping the highlights to a lower, wider range of values, but perhaps I am getting the definition wrong.)

LR started compressing most tonality in highlights with then called Process Version 2012 (if I recall correctly). That indeed requires to apply a negative Highlights control setting to restore some of the original highlight tonality, e.g. white clouds to regain some of the original structure/definition. I don't know if/how the latest process version mangles tonality, since I've moved on to Capture One when Adobe introduced a subscription software model. I'm glad I did.

Cheers,
Bart
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #6 on: November 27, 2018, 02:40:32 pm »

LR started compressing most tonality in highlights with then called Process Version 2012 (if I recall correctly).
Long before that but rather poorly. PV2012 was the version they got it 'right' (not ugly coloration upon recovery). And it doesn't do this all the time per se; if one or two channels are clipped, it can rebuild them from the one unclipped channel. But really, don't clip any channels you want highlight detail within and then there's no reason to go there.
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Andrew Rodney
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Bart_van_der_Wolf

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #7 on: November 27, 2018, 02:48:42 pm »

Long before that but rather poorly. PV2012 was the version they got it 'right' (not ugly coloration upon recovery). And it doesn't do this all the time per se; if one or two channels are clipped, it can rebuild them from the one unclipped channel. But really, don't clip any channels you want highlight detail within and then there's no reason to go there.

Hi Andrew,

I'm not talking about highlight 'recovery', which PV2012 indeed did a good job of even if one or two channels were clipped. I'm instead referring to the loss of tonality in unclipped bright to highlight regions. They look lifeless until the Highlight control gets a large negative setting.

Cheers,
Bart
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kpz

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #8 on: November 27, 2018, 02:49:52 pm »

LR started compressing most tonality in highlights with then called Process Version 2012 (if I recall correctly). That indeed requires to apply a negative Highlights control setting to restore some of the original highlight tonality, e.g. white clouds to regain some of the original structure/definition. I don't know if/how the latest process version mangles tonality, since I've moved on to Capture One when Adobe introduced a subscription software model. I'm glad I did.

So, by "highlight compression" you mean mapping the highlights to a narrower range than indicated by the raw file, and perhaps lowering the white point to compensate? And then the negative Highlights setting undos this "narrowing"?
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #9 on: November 27, 2018, 02:52:29 pm »

Hi Andrew,

I'm not talking about highlight 'recovery', which PV2012 indeed did a good job of even if one or two channels were clipped. I'm instead referring to the loss of tonality in unclipped bright to highlight regions. They look lifeless until the Highlight control gets a large negative setting.

Cheers,
Bart
Then I honestly don't know what you're referring to then. Perhaps with some default settings and profiles, with some cameras, highlights are getting 'compressed' but it certainly isn't anything forced upon the user.
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Andrew Rodney
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #10 on: November 27, 2018, 03:50:26 pm »

Then I honestly don't know what you're referring to then. Perhaps with some default settings and profiles, with some cameras, highlights are getting 'compressed' but it certainly isn't anything forced upon the user.
From what I see, PV 1 is buggy. PV2 onward isn't. PV1 shows that the white tile (BableColor target) is Lstar 100. It's not. And RawDigger confirms that this raw isn't over exposed what so ever (fig 1.).
Fig 2 shows PV1 where the tile (with default settings) shows Lstar 100. Yet PV2 onward with no additional settings in LR show it as Lstar 98 which is pretty much exactly what it should be according to actual measurements of the target.
So I don't see PV 2 onward compressing anything and in fact it appears to be reporting what it should while PV1 indicates the raw is over exposed but RawDigger claims otherwise.
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Andrew Rodney
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kpz

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #11 on: November 27, 2018, 05:25:23 pm »

Take transparency film as an example unlike color neg (even farther away from output referred): which is 'perpetually accurate', Agfachrome, Kodachrome, Velvia, Ektachrome? The manufacturers produce a rendering they believe you will visually prefer and buy. Just like default raw processing conversions which may or may not be to your liking or better for a starting point in editing the image. If you don't like the defaults, change them. Simple to do in many converters including of course, Adobe's.

Is your claim then that Lightroom is by default tone mapping the raw data to produce an image they think is more pleasing to me by messing with the highlights and shadows? I realize there are some adjustments baked in, which can be removed by going to an earlier process version and zeroing the sliders, but those are for things like the black point and global contrast (if I recall correctly), not the kind of local shadows/highlights manipulations I was thinking of.

What then explains why the "scene referred" image in that link, and other raw files, are noticeably different from what people remember seeing? Is it somehow related to the fact that the human eye can see vastly more colors than a typical sRGB monitor is capable of producing, and so some tone mapping is required to fit everything in (a sort of "HDR for color")?
« Last Edit: November 27, 2018, 05:42:16 pm by kpz »
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #12 on: November 27, 2018, 05:28:42 pm »

Is your claim then that Lightroom is by default tone-mapping the raw data to produce an image they think is more pleasing to me by messing with the highlights and shadows?
That and more. Again depending on a massive number of factors.
The URL shows two output referred renderings that differ and a scene referred one that's far from pleasing. As one should expect from a scene referred rendering (it's not output referred as explained).
Number of colors; want to really go there?  ;)


http://digitaldog.net/files/ColorNumbersColorGamut.pdf
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Andrew Rodney
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kpz

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #13 on: November 27, 2018, 05:39:12 pm »

Okay, thank you. I think I am slowly beginning to understand.

I am still wondering, what exactly is going on perceptually that makes the scene referred renderings so "flat"? Obviously it is the case that they are much less pleasing than what we perceive with our eyes. But *why*?

Does the need for this tone mapping come (at least partially) from the need to deal with out of gamut colors? In other words, is it that monitors just can't display all the colors necessary for something perceptually correct, so some sort of HDR-type procedure is necessary as a substitute?

Also, if the scene referred renderings are colorimetrically accurate, and the "artistically correct" output referred ones usually aren't, then what is the purpose of a colorchecker? If accurate color is displeasing, why attempt to get perfectly accurate color anyway? And -- if LR messes with highlights and shadows, how come your test image gave the right LAB value? Wouldn't we expect the LR defaults to change it slightly?
« Last Edit: November 27, 2018, 05:42:50 pm by kpz »
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #14 on: November 27, 2018, 06:41:29 pm »

Okay, thank you. I think I am slowly beginning to understand.

I am still wondering, what exactly is going on perceptually that makes the scene referred renderings so "flat"? Obviously it is the case that they are much less pleasing than what we perceive with our eyes. But *why*?

Does the need for this tone mapping come (at least partially) from the need to deal with out of gamut colors? In other words, is it that monitors just can't display all the colors necessary for something perceptually correct, so some sort of HDR-type procedure is necessary as a substitute?

Also, if the scene referred renderings are colorimetrically accurate, and the "artistically correct" output referred ones usually aren't, then what is the purpose of a colorchecker? If accurate color is displeasing, why attempt to get perfectly accurate color anyway? And -- if LR messes with highlights and shadows, how come your test image gave the right LAB value? Wouldn't we expect the LR defaults to change it slightly?
Answers:
1. Flat? Just not output referred (rendered for some output). Explained in the article.
2. Nothing per se about out of gamut colors. There are 'colors' a camera can capture we cannot see and thus not colors. There are colors we can see it cannot capture.
3. The ColorChecker is nothing more than a color reference, designed way back in 1976, long before any of us were using digital cameras: http://www.babelcolor.com/colorchecker.htm
4. I have no idea what some are speaking of here about LR 'messing' with highlights and shadows. It is producing output referred color based on all kinds of sliders and other 'edits' affecting the raw to rendered previews under the hood. My test image gave the correct Lstar values but that only tells us about the Lstar values of that highlight. And further, by moving a few sliders, I could alter those Lstar values a lot.
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Andrew Rodney
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Bart_van_der_Wolf

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #15 on: November 27, 2018, 06:43:43 pm »

Then I honestly don't know what you're referring to then. Perhaps with some default settings and profiles, with some cameras, highlights are getting 'compressed' but it certainly isn't anything forced upon the user.


I'm specifically referring to:
https://luminous-landscape.com/tonal-adjustments-in-the-age-of-lightroom-4/, and especially the part about Highlight Detail.

Quote
Highlight Detail

The Highlights slider, moved to the left, increases the highlight recovery as seen above.  It works especially well when just one or two of the channels are clipped, as with saturated colors.  But the Highlights slider also increases separation and detail in washed-out highlights.

Followed by two examples of a cloud, not clipped (!):
 
The image on the left with Highlights at 0, and the one on the right at -100.

Cheers,
Bart
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #16 on: November 27, 2018, 06:46:24 pm »


I'm specifically referring to:
https://luminous-landscape.com/tonal-adjustments-in-the-age-of-lightroom-4/, and especially the part about Highlight Detail.

Followed by two examples of a cloud, not clipped (!):
 
The image on the left with Highlights at 0, and the one on the right at -100.

Cheers,
Bart
I still don't see your point other than sliders for affecting highlights can affect highlights. I only see that as a plus in how we might wish to render our highlights (and images).
Pull the sider to plus 100, I suspect you'll blow out the highlights. Highlights which of course are not blown out in the actual raw.
The same is true for other sliders or curves that allow us to affect anything on the tone curve.
What am I missing?  ;)
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Andrew Rodney
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kpz

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #17 on: November 27, 2018, 06:51:29 pm »

1. Flat? Just not output referred (rendered for some output). Explained in the article.

Could you suggest a reference for the details of what must happen to take a raw file and output refer it to make it visually pleasing? I think, after reading your replies, what this is what I really want to know (the approximate steps one takes to go from "scene referred" to output and why). I'm interested in both the technical side and the science of human perception. Any suggestions in this direction would be greatly appreciated...
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digitaldog

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #18 on: November 27, 2018, 06:55:01 pm »

Could you suggest a reference for the details of what must happen to take a raw file and output refer it to make it visually pleasing?
No. It's subjective. What I can recommend with the ACR/LR products is to consider working top down, left to right (for ACR) as the controls have been provided in an 'order' the designers believe should be used first. But you can break the rules. All edits are applied when rendering the raw in the best order, not the user order.
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Andrew Rodney
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Bart_van_der_Wolf

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Re: Why do raw files require tone mapping to look perceptually correct?
« Reply #19 on: November 27, 2018, 06:59:43 pm »

I still don't see your point other than sliders for affecting highlights can affect highlights. I only see that as a plus in how we might wish to render our highlights (and images).
Pull the sider to plus 100, I suspect you'll blow out the highlights. Highlights which of course are not blown out in the actual raw.
The same is true for other sliders or curves that allow us to affect anything on the tone curve.
What am I missing?  ;)

Read the rest of the above-linked article, it has more examples of how lifeless the highlights are rendered, UNLESS one uses a -100 correction.

   
Increasing 'Exposure' even more heavily compresses the highlight tonality to avoid clipping or washing out, which can only be corrected by a negative Highlights correction (-100 in the example).

EDIT: Also read the section about "Brightening Highlights". Even the unadjusted histogram of the stepwedge shows tonal compression in the highlights, and it only gets worse with brightening the image.



Cheers,
Bart

« Last Edit: November 27, 2018, 07:04:58 pm by BartvanderWolf »
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