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### AuthorTopic: DxO PhotoLab 2 working space tests  (Read 1525 times)

#### 32BT

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##### Re: DxO PhotoLab 2 working space tests
« Reply #40 on: January 10, 2019, 07:08:32 PM »

Indeed, sRGB is using a TRC, not a gamma curve. I don't know if any other 'common' RGB working spaces do this (certainly not ColorMatch RGB, Adobe RGB (1998) or ProPhoto RGB).

Exactly, with the TRC and the flat start, they try to overcome the problem that the values would look like for example 0, 0, 0, 1, 1, 2, 3, 5 which in turn creates really funky inversions as well.

They simply shouldn't have gone there and just store the gamma value as per the directive and then let the engine create proper tables and, not unimportantly, proper inversions.
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#### Doug Gray

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##### Re: DxO PhotoLab 2 working space tests
« Reply #41 on: January 10, 2019, 07:19:53 PM »

No, you can safely assume that the gamma 2.2 space is more or less perceptually uniform, since that is the exact idea behind gamma encoding. So, the first step in 8bit perceptual space = (2/255) = .78% thus about equivalent to a deltaE76 of .78

The first step is (1/255), not (2/255) though the latter roughly corresponds to a single step in 16 bit, linear RGB space.
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No, you can safely assume that the gamma 2.2 space is more or less perceptually uniform

That's just not true. For one thing L* for RGB(1,1,1) or (2,2,2) is not anywhere near .78. Heck, it's not anywhere near .1
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Please don't go overboard on the linear part of that L curve. First of all that RGB = 0 doesn't start in pure black, second that linear part was not about dark pixels in a sea of white for example.

Let's stick with the ICC math please. RGB=0 is Y=L*=0.  That is black. That monitors can't show black unless they are turned off in a dark room is a rather different issue.
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All numbers aside: i've implemented a colormanagement engine for display purposes long ago which allowed adjustable out-of-gamut rendering. Digidog may remember, i've send him an example. I vaguely recall that too little bits gave visible posterisation up to the first  4 or 6 steps. And that's just allowing gamma 2.2, imagine allowing anything up to 3.0 or whatever photoshop allows one to save these days...

Your point about seeing posterization in low luminance situations where the background is also low luminance is a good one. L* deltaE's are meaningful only when looking at "colors" against an L* background of approximately 50, similar to a neutral gray card. I darker environments one can see posterization at low L* values. Hence things like motion pictures with verly large dynamic ranges benefit from things like 16 bit (small floats) spearheaded by Industrial Light and Magic. In movies one has time to adapt to a much larger dynamic ranges and encoding in floats, even 16 bit floats, provides the require range while still eliminating posterization.
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#### digitaldog

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##### Re: DxO PhotoLab 2 working space tests
« Reply #42 on: January 10, 2019, 07:20:12 PM »

Exactly, with the TRC and the flat start, they try to overcome the problem that the values would look like for example 0, 0, 0, 1, 1, 2, 3, 5 which in turn creates really funky inversions as well.

They simply shouldn't have gone there and just store the gamma value as per the directive and then let the engine create proper tables and, not unimportantly, proper inversions.
Kind of depends which sRGB you (and Doug) are referring to!
https://ninedegreesbelow.com/photography/srgb-profile-comparison.html
The author agrees with you:

From a programming point of view, it would be nice if the sRGB TRC just vanished, to be replaced with the much more tractable gamma=2.2 TRC. From a pragmatic point of view, that is not going to happen any time soon, if ever — there are just too many untagged sRGB images floating around that require the sRGB profile TRC, not to mention legacy software and all the cameras that produce sRGB jpegs.
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Andrew Rodney
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#### Doug Gray

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##### Re: DxO PhotoLab 2 working space tests
« Reply #43 on: January 10, 2019, 07:26:21 PM »

I don't quite understand how you get to your values. The first step in a gamma encoded space should be brighter than L since they don't have the flat bit like L has. (i.e. the derivative in 0 is infinite for gamma spaces, not so for L, therefore the deltaE should always be larger than the Y <-> L relation.)

Perhaps we can resolve this if you specific two specific values in a defined colorspace (e.g., Adobe RGB 8 bit), then calculate the Y value and L* value from that. I've done that above (except for the Y) , what are your numbers?

I get my values from two places which I've crosschecked. MATLAB and http://www.brucelindbloom.com/  The formulas are also in the specs at www.color.org.
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#### Doug Gray

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##### Re: DxO PhotoLab 2 working space tests
« Reply #44 on: January 10, 2019, 07:54:38 PM »

Kind of depends which sRGB you (and Doug) are referring to!
https://ninedegreesbelow.com/photography/srgb-profile-comparison.html
The author agrees with you:

From a programming point of view, it would be nice if the sRGB TRC just vanished, to be replaced with the much more tractable gamma=2.2 TRC. From a pragmatic point of view, that is not going to happen any time soon, if ever — there are just too many untagged sRGB images floating around that require the sRGB profile TRC, not to mention legacy software and all the cameras that produce sRGB jpegs.

I quite agree. The changeover from a linear ramp to gamma 2.4 on the standard IEC sRGB (the one Adobe uses) is only of minimal value in 8 bits and provides nothing of value in larger bit spaces. Also, sRGB suffers from shifts in chromaticity when scaled. For instance RGB (10,75,100) will have a different chromaticity than RGB (20,150,200) while Adobe RGB and ProPhoto RGB will not see any shifts in chromaticity.

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#### GWGill

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##### Re: DxO PhotoLab 2 working space tests
« Reply #45 on: January 11, 2019, 12:01:36 AM »

Indeed, sRGB is using a TRC, not a gamma curve. I don't know if any other 'common' RGB working spaces do this (certainly not ColorMatch RGB, Adobe RGB (1998) or ProPhoto RGB).

at least:

ProPhoto
ROMMRGB
RIMMRGB
scRGB
EBU3213
SMPTERP145
Rec709
Rec2020
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#### GWGill

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##### Re: DxO PhotoLab 2 working space tests
« Reply #46 on: January 11, 2019, 12:05:43 AM »

No, you can safely assume that the gamma 2.2 space is more or less perceptually uniform, since that is the exact idea behind gamma encoding. So, the first step in 8bit perceptual space = (2/255) = .78% thus about equivalent to a deltaE76 of .78
"More or less" is right. In practice intraocular glare and adaptation makes it not such a neat mathematical property.
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#### Jack Hogan

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##### Re: DxO PhotoLab 2 working space tests
« Reply #47 on: January 11, 2019, 03:40:16 AM »

The problem with a simple gamma function is that it is 'not invertible at the origin'.  This causes a whole lot of practical issues including the massive amplification of small intensity values, aka noise, exactly where one does not want it, in the darkest portions of an image (take the derivative of the function to understand why).  Hence the practical need for a linear toe in sRGB and any other non-noise-adding color space (Melissa anyone?).

Thankfully for those of us who do not like color spaces that add noise, PS does introduce linear toes in Color Space gammas whether they specify them or not, through ACE (the previously mentioned Adobe Color Engine).  Yes, including in Adobe RGB.

Jack
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#### digitaldog

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##### Re: DxO PhotoLab 2 working space tests
« Reply #48 on: January 11, 2019, 09:11:25 AM »

at least:

ProPhoto
ROMMRGB
RIMMRGB
scRGB
EBU3213
SMPTERP145
Rec709
Rec2020
TRC or true gamma?
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#### digitaldog

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##### Re: DxO PhotoLab 2 working space tests
« Reply #49 on: January 11, 2019, 09:20:58 AM »

The problem with a simple gamma function is that it is 'not invertible at the origin'.  This causes a whole lot of practical issues including the massive amplification of small intensity values, aka noise, exactly where one does not want it, in the darkest portions of an image (take the derivative of the function to understand why).  Hence the practical need for a linear toe in sRGB and any other non-noise-adding color space (Melissa anyone?).

Thankfully for those of us who do not like color spaces that add noise, PS does introduce linear toes in Color Space gammas whether they specify them or not, through ACE (the previously mentioned Adobe Color Engine).  Yes, including in Adobe RGB.

Jack
Color spaces that add noise? I'm lost. Got visually image examples?
If you are referring to Melissa RGB, it is only used for the Histogram and RGB readouts.
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#### Doug Gray

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##### Re: DxO PhotoLab 2 working space tests
« Reply #50 on: January 11, 2019, 11:46:52 AM »

The problem with a simple gamma function is that it is 'not invertible at the origin'.  This causes a whole lot of practical issues including the massive amplification of small intensity values, aka noise, exactly where one does not want it, in the darkest portions of an image (take the derivative of the function to understand why).  Hence the practical need for a linear toe in sRGB and any other non-noise-adding color space (Melissa anyone?).

A pure gamma function isn't technically invertible at 0 since the function x^(-2.2) isn't defined for negative or zero values. Thus, (x^2.2)^(-2.2) isn't defined. However, it is defined if one takes the limit and that value, for x=0, is 0. This is how Adobe converts from a 2.2 gamma to 1.0 gamma.
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Thankfully for those of us who do not like color spaces that add noise, PS does introduce linear toes in Color Space gammas whether they specify them or not, through ACE (the previously mentioned Adobe Color Engine).  Yes, including in Adobe RGB.

Adobe ACE does not introduce a toe in pure gamma colorspaces. Easily tested in Photoshop. Zero and low values convert as expected (zero converts to zero) from one colorspace to another with no evidence of a toe outside of the sRGB.
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#### Doug Gray

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##### DeltaE's for 8 bit RGB Steps in Various Colorspaces
« Reply #51 on: January 11, 2019, 02:01:54 PM »

Here's a graph of delta E 2000's for 8 bit colorspaces. Included are sRGB, Adobe RGB, ProPhoto RGB, and a linear RGB space.

Gamma of the colorspaces:
sRGB: linear with gamma=2.4
ProPhoto RGB 1.8
Linear RGB: 1.0

The graph shows the Delta E 2000 for adjacent steps in the 8 bit colorspaces from (0,0,0),(1,1,1),...(255,255,255). For 16 bit images just divide the Delta E's by 256, or 128 in the case of Adobe Photoshop which scales 16 bit images to 15 bits.

There simply is no issue where accuracy at low luminance needs more than 15/16 bits and 8 bits is right at the threshold of observability except for the linear gamma which gets pretty bad at low luminance.

sRGB does a decent job in 8 bits. At least for the tone curve. sRGB's big weakness is it's really narrow gamut. Something that's well explored by many. In particular, Andrew Rodney has extensively gone into sRGB's printing.

All of these, even with Adobe's 15 bit limit, work fine in "16" bit RGB spaces. Even linear, gamma=1.0 spaces which have a max dE00 of .023.
« Last Edit: January 11, 2019, 02:05:10 PM by Doug Gray »
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#### Jack Hogan

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##### Re: DxO PhotoLab 2 working space tests
« Reply #52 on: January 11, 2019, 05:40:49 PM »

A pure gamma function isn't technically invertible at 0 since the function x^(-2.2) isn't defined for negative or zero values. Thus, (x^2.2)^(-2.2) isn't defined. However, it is defined if one takes the limit and that value, for x=0, is 0. This is how Adobe converts from a 2.2 gamma to 1.0 gamma.
Adobe ACE does not introduce a toe in pure gamma colorspaces. Easily tested in Photoshop. Zero and low values convert as expected (zero converts to zero) from one colorspace to another with no evidence of a toe outside of the sRGB.

This info came from a discussion with Adobe cognoscenti quite a while ago.  I am currently on the road, I'll see if I find it once I get back in a few days.  In the meantime one could try switching a dark noisy image patch back and forth from sRGB to Adobe RGB to linear gamma a number of times in Matlab vs PS ACE and see if one gets the same result.

Jack
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#### Doug Gray

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##### Re: DxO PhotoLab 2 working space tests
« Reply #53 on: January 11, 2019, 06:22:53 PM »

This info came from a discussion with Adobe cognoscenti quite a while ago.  I am currently on the road, I'll see if I find it once I get back in a few days.  In the meantime one could try switching a dark noisy image patch back and forth from sRGB to Adobe RGB to linear gamma a number of times in Matlab vs PS ACE and see if one gets the same result.

Jack

No problem. For anyone interested in experimenting, I've attached an untagged 16 bit tiff image that contains all 65536 neutral values (R=G=B). When loaded into Photoshop, half the values are tossed as it rounds to 32768 values when converting to 15 bits. If you save it, it will save rounded with 32768 values.

Not all Adobe cognoscenti are equal.

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#### 32BT

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##### Re: DxO PhotoLab 2 working space tests
« Reply #54 on: January 11, 2019, 06:54:01 PM »

Back in the day (2006-ish) Photoshop allowed asymmetric CMM conversions from which I could derive the following:

ACE was well behaved (high precision)

The Apple CMM was not (low precision)

ACE properly inverted high precision conversions from another CMM, and certainly didn't resort to flatlining the luts.

I have no idea what's state of the art in today's world with everything native 64bit and some of it handed to the graphics card at 128bit, although handing this stuff down to the graphics card black box is prone to even more variations. The whole thing started all over with the introduction of Tablets, and now Apple introducing yet another colorspace while they don't exactly dictate standards. I pretty much stopped caring to be honest, which, I have to admit, is a bit of a defeat considering I spend longer than I care to remember in digital premedia and colorimaging. I guess it all turned out to be merely a long winded path of realisation toward the idea that the artistic content of your images is just infinitely more important than the technical merits of its representation. You can be ever so meticulous in preparing your images for future consumption, only to find that your superseded by an iphone dominated world with continuously deteriorating standards of quality.

(No, I have nothing against the next generation, I have something against the planned obsolescence throw away society and its corresponding reduction to mediocrity.)

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#### Doug Gray

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##### Re: DxO PhotoLab 2 working space tests
« Reply #55 on: January 11, 2019, 07:36:19 PM »

Back in the day (2006-ish) Photoshop allowed asymmetric CMM conversions from which I could derive the following:

ACE was well behaved (high precision)

The Apple CMM was not (low precision)

I agree ACE has been and remains well behaved on my Win 10 machine. Microsoft's ICM is not. It can significantly shift colors just doing a sRGB->Adobe RGB and back. It also has an old school view of Absolute Col. conversions using matrix profiles and doesn't adapt to D50. The ICC was ambiguous at the time ICM was created and clarified their intent over 15 years ago but Microsoft hasn't adapted.
Quote

ACE properly inverted high precision conversions from another CMM, and certainly didn't resort to flatlining the luts.

I have no idea what's state of the art in today's world with everything native 64bit and some of it handed to the graphics card at 128bit, although handing this stuff down to the graphics card black box is prone to even more variations. The whole thing started all over with the introduction of Tablets, and now Apple introducing yet another colorspace while they don't exactly dictate standards. I pretty much stopped caring to be honest, which, I have to admit, is a bit of a defeat considering I spend longer than I care to remember in digital premedia and colorimaging. I guess it all turned out to be merely a long winded path of realisation toward the idea that the artistic content of your images is just infinitely more important than the technical merits of its representation. You can be ever so meticulous in preparing your images for future consumption, only to find that your superseded by an iphone dominated world with continuously deteriorating standards of quality.

(No, I have nothing against the next generation, I have something against the planned obsolescence throw away society and its corresponding reduction to mediocrity.)

It has been always so. And it's a good thing.
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#### FranciscoDisilvestro

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##### Re: DxO PhotoLab 2 working space tests
« Reply #56 on: January 11, 2019, 08:50:13 PM »

I agree ACE has been and remains well behaved on my Win 10 machine. Microsoft's ICM is not.

Microsoft ICM is rubbish.

#### Doug Gray

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##### Re: DxO PhotoLab 2 working space tests
« Reply #57 on: January 11, 2019, 09:13:39 PM »

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#### GWGill

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##### Re: DxO PhotoLab 2 working space tests
« Reply #58 on: January 13, 2019, 01:18:23 AM »

TRC or true gamma?
The specs say straight line + power curve for all those spaces.
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#### digitaldog

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##### Re: DxO PhotoLab 2 working space tests
« Reply #59 on: January 13, 2019, 09:31:00 AM »

The specs say straight line + power curve for all those spaces.
IOW a simple gamma curve, as such, is sRGB the only WS without?
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Andrew Rodney
Author “Color Management for Photographers"
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