The right way for device input profiles and artificial light sources?

[Stephen T]

Hello,

I have been playing around with camera profiles for tungsten and fluorescent lighting.

As I understand, there are 2 things that happen when the light source changes:
1) Device RGB changes (what the camera sees).
2) XYZ changes (what the standard observer sees).

I have made simple matrix profiles. There are 3 different profiling strategies I have tried:
1) Profile with D50 reference data (ignore changes in XYZ).
2) Profile with XYZ computed for the CIE illuminant most closely matching the light source (non-standard profile white point).
3) Take XYZ from step 2 and perform a chromatic adaptation transform to D50 (standard PCS white point).
Which of these is recommended?

Profiling errors with the 3 different XYZ are similar (a good fit doesn't mean the results will be sensible in practice however). Options 2 and 3 produce exactly the same rXYZ, gXYZ and bXYZ, only the WTPT is different. Options 2 and 3 produce identical images in my workflow and I guess the non-standard white for option 2 is being accounted for?

For real photos in warm light, I have found all 3 tungsten profiles are similar. The D50 reference data produces slightly warmer reds.

There are greater differences in photos with fluorescent lighting, where the D50 reference data produces a warmer, more pleasing image. Options 2 and 3 look a bit greenish. Perhaps this is because I profiled with a light source approximating F5 daylight and the test photos were shot with inferior, lower colour temperature lights?


Compared to daylight profiles, the artificial light profiles do produce more realistic colours. I haven't done any quantitative tests yet.

It's strange that the D50 reference data seem to be useable, as the colorimetry of the target does change and the rXYZ, gXYZ, bXYZ tags can be very different. Here are some colour differences I calculated after CAT:
Mean DE94 between D50 and illuminant A after CAT to D50 = 1.5.
Mean DE94 between D50 and illuminant F5 after CAT to D50 = 2.4.

Appreciate any tips, advice.

Stephen.

[Gerhard Fürnkranz]

Shooting the target under say tungsten light while using D50-based reference data is basically supposed to result in a profile which reproduces the shot of a tungsten-lit scene as if the scene were lit with D50 instead of tungsten light (within the various limits of the camera profiling process, of course).
--
Best Regards,
Gerhard

Post by Stephen T
Hello,
I have been playing around with camera profiles for tungsten and fluorescent lighting.
1) Device RGB changes (what the camera sees).
2) XYZ changes (what the standard observer sees).
1) Profile with D50 reference data (ignore changes in XYZ).
2) Profile with XYZ computed for the CIE illuminant most closely
matching the light source (non-standard profile white point).
3) Take XYZ from step 2 and perform a chromatic adaptation transform to
D50 (standard PCS white point).
Which of these is recommended?
Profiling errors with the 3 different XYZ are similar (a good fit
doesn't mean the results will be sensible in practice however). Options
2 and 3 produce exactly the same rXYZ, gXYZ and bXYZ, only the WTPT is
different. Options 2 and 3 produce identical images in my workflow and
I guess the non-standard white for option 2 is being accounted for?
For real photos in warm light, I have found all 3 tungsten profiles are
similar. The D50 reference data produces slightly warmer reds.
There are greater differences in photos with fluorescent lighting,
where the D50 reference data produces a warmer, more pleasing image.
Options 2 and 3 look a bit greenish. Perhaps this is because I profiled
with a light source approximating F5 daylight and the test photos were
shot with inferior, lower colour temperature lights?
Compared to daylight profiles, the artificial light profiles do produce
more realistic colours. I haven't done any quantitative tests yet.
It's strange that the D50 reference data seem to be useable, as the
colorimetry of the target does change and the rXYZ, gXYZ, bXYZ tags can
be very different. Here are some colour differences I calculated after
Mean DE94 between D50 and illuminant A after CAT to D50 = 1.5.
Mean DE94 between D50 and illuminant F5 after CAT to D50 = 2.4.
Appreciate any tips, advice.
Stephen.

[Stephen T]

Hello,

Tungsten + D50 XYZ is not equal to Sunlight + D50 XYZ. The sunlight profile performs very poorly for photos taken with warm lighting.

I was wondering which of the following is most nearly "correct":
1) Tungsten + D50 XYZ?
2) Tungsten + A XYZ?
3) Tungsten + adapted A XYZ?

I think most people profile with option 1) but this doesn't seem right because the XYZ changes when the illuminant changes.


Stephen.



________________________________
From: Gerhard Fürnkranz <nospam456-***@public.gmane.org>
To: argyllcms-***@public.gmane.org
Sent: Wednesday, 4 December 2013 4:31 AM
Subject: [argyllcms] Re: The right way for device input profiles and artificial light sources?



Shooting the target under say tungsten light while using D50-based reference data is basically supposed to result in a profile which reproduces the shot of a tungsten-lit scene as if the scene were lit with D50 instead of tungsten light (within the various limits of the camera profiling process, of course).
--
Best Regards,
Gerhard




Stephen T <stwebvanuatu-/***@public.gmane.org> schrieb:
Hello,

Post by Stephen T
I have been playing around with camera profiles for tungsten and fluorescent lighting.
1) Device RGB changes (what the camera sees).
2) XYZ changes (what the standard observer sees).
1) Profile with D50 reference data (ignore changes in XYZ).
2) Profile with XYZ computed for the CIE illuminant most closely matching the light source (non-standard profile white point).
3) Take XYZ from step 2 and perform a chromatic adaptation transform to D50 (standard PCS white point).
Which of these is recommended?
Profiling errors with the 3 different XYZ are similar (a good fit doesn't mean the results will be sensible in practice however). Options 2 and 3 produce exactly the same rXYZ, gXYZ and bXYZ, only the WTPT is different. Options 2 and 3 produce identical images in my workflow and I guess the non-standard white for option 2 is being accounted for?
For real photos in warm light, I have found all 3 tungsten profiles are similar. The D50 reference data produces slightly warmer reds.
There are greater differences in photos with fluorescent lighting, where the D50 reference data produces a warmer, more pleasing image. Options 2 and 3 look a bit greenish. Perhaps this is because I profiled with a light source approximating F5 daylight and the test photos were shot with inferior, lower colour temperature lights?
Compared to daylight profiles, the artificial light profiles do produce more realistic colours. I haven't done any quantitative tests yet.
It's strange that the D50 reference data seem to be useable, as the colorimetry of the target
Mean DE94 between D50 and illuminant A after CAT to D50 = 1.5.
Mean DE94 between D50 and illuminant F5 after CAT to D50 = 2.4.
Appreciate any tips, advice.
Stephen.

[Gerhard Fuernkranz]

Well, the basic question is: What kind of reproduction is intended? For the standard use case "reproduce the colorimetry of a scene illuminated by a custom illumination source", the _actual_ illumination source spectrum should be used for

1. capturing the target,
2. capturing the scene,
3. and also for computing the XYZ reference data of target's patches from their reflectance spectra

if color constancy of the colors in the scene under changing illumination source is _not_ granted.

If color constancy were granted, then using D50-based reference data instead should not make a difference (as long as the resulting profile is applied with a whitepoint-relative intent), since the color change under different illumination sources would follow a chromatic adaptation then.

Also keep mind that even a perfect colorimetrically correct reproduction does not necessarily preserve the _color appearance_ as well, since the latter depends vastly on the viewing conditions/environment. Note for instance that the _appearance_ of the same scene under rather dim tungsten light is per se warmer than under bright daylight, even if color constancy were granted and thus the relative colorimetry were the same. So if the reproduction (e.g. on a monitor) does not "look" like the original scene, the reason could also be a color appearance issue...

[ And still all limitations of input device profiling do apply, e.g. cameras are partially color bind wrt. the human vision, so it is generally not possible to build a profile which is correct for _all_ possible colors; the target may not be sufficiently representative for the captured scene, etc. ]

Best Regards,
Gerhard

Post by Stephen T
Hello,
Tungsten + D50 XYZ is not equal to Sunlight + D50 XYZ. The sunlight profile performs very poorly for photos taken with warm lighting.
1) Tungsten + D50 XYZ?
2) Tungsten + A XYZ?
3) Tungsten + adapted A XYZ?
I think most people profile with option 1) but this doesn't seem right because the XYZ changes when the illuminant changes.
Stephen.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
*Sent:* Wednesday, 4 December 2013 4:31 AM
*Subject:* [argyllcms] Re: The right way for device input profiles and artificial light sources?
Shooting the target under say tungsten light while using D50-based reference data is basically supposed to result in a profile which reproduces the shot of a tungsten-lit scene as if the scene were lit with D50 instead of tungsten light (within the various limits of the camera profiling process, of course).
--
Best Regards,
Gerhard
Hello,
I have been playing around with camera profiles for tungsten and fluorescent lighting.
1) Device RGB changes (what the camera sees).
2) XYZ changes (what the standard observer sees).
1) Profile with D50 reference data (ignore changes in XYZ).
2) Profile with XYZ computed for the CIE illuminant most closely matching the light source (non-standard profile white point).
3) Take XYZ from step 2 and perform a chromatic adaptation transform to D50 (standard PCS white point).
Which of these is recommended?
Profiling errors with the 3 different XYZ are similar (a good fit doesn't mean the results will be sensible in practice however). Options 2 and 3 produce exactly the same rXYZ, gXYZ and bXYZ, only the WTPT is different. Options 2 and 3 produce identical images in my workflow and I guess the non-standard white for option 2 is being accounted for?
For real photos in warm light, I have found all 3 tungsten profiles are similar. The D50 reference data produces slightly warmer reds.
There are greater differences in photos with fluorescent lighting, where the D50 reference data produces a warmer, more pleasing image. Options 2 and 3 look a bit greenish. Perhaps this is because I profiled with a light source approximating F5 daylight and the test photos were shot with inferior, lower colour temperature lights?
Compared to daylight profiles, the artificial light profiles do produce more realistic colours. I haven't done any quantitative tests yet.
Mean DE94 between D50 and illuminant A after CAT to D50 = 1.5.
Mean DE94 between D50 and illuminant F5 after CAT to D50 = 2.4.
Appreciate any tips, advice.
Stephen.

[Stephen T]

Thanks Gerhard,

Firstly, performing CAT to convert XYZ for illuminant A to D50 was a mistake. The adapted A and D50 XYZ are essentially the same. Ignore the adapted A.

This leaves two options:
A) Profile with XYZ for illuminant A. The raw photo application would then have to recognise the non-standard profile white point and apply a chromatic adaptation transform from XYZ_A to XYZ_D50 or whatever. I'm not sure mine does!
B) Profile with XYZ for illuminant D50.

There is more to this than colour management. Like dcraw and most others, there are two steps before the "device RGB" is output from the raw photo application:
1) White balance by scaling raw photo RGB ("native RGB").
2) Interpolation (giving "device RGB").
I remember reading somewhere that demosaicing is more succesful for white balanced images.

Because the device RGB are approximately white-balanced, I think profiling option B is appropriate (i.e. the illuminant A effect has been corrected by reducing the R channel gain and increasing the B channel gain, I can see this in the dcraw multipliers). Do you agree???

Now, if the RGB white-balance was perfect then:

RGB_n * scaling = RGB_s

where RGB_n is native RGB, which is then multiplied by white-balance scaling coefficients, and RGB_s is the RGB response for sunlight for example (a "standard" light source for camera profiling and fairly close to D50). Then only one profile is required, since:

XYZ_D50 = RGB_s * M_s =  RGB_n * scaling * M_s

where the matrix profile M_s has been computed with D50 reference data, it converts RGB_s to XYZ_D50.

In reality, the white-balance scaling is imperfect. For example, I compared white-balanced RGB_A versus RGB_s from .ti3 files and found the red and orange patches in RGB_A relatively weak in the R channel. The white and neutral grey patches agreed within 1 or 2%, so the white balance was OK in both images. When the M_s profile is applied to the scaled RGB_A data, red and orange colours are weak, as I had noticed in some of my photos. I have tried playing around with white balance and found no satisfactory solution. It's much easier to use a custom profile for illuminant A:

XYZ_D50 = RGB_n * scaling * M_A

where M_A converts scaled RGB_n to XYZ_D50 and corrects the white balance problems.

I expect the "standard" sunlight profile is satisfactory only for light sources near D50. For very warm light, very cool light, fluorescents etc., with spectra very different to sunlight, white-balance scaling of native RGB will be less successful and custom profiles will help to correct the errors.

Stephen.


________________________________
From: Gerhard Fuernkranz <nospam456-***@public.gmane.org>
To: ***@freelists.org
Sent: Sunday, 8 December 2013 3:08 AM
Subject: [argyllcms] Re: The right way for device input profiles and artificial light sources?




Well, the basic question is: What kind of reproduction is
intended? For the standard use case "reproduce the colorimetry of
a scene illuminated by a custom illumination source", the _actual_
illumination source spectrum should be used for

1. capturing the target,
2. capturing the scene,

3. and also for computing the XYZ reference data of target's patches from their reflectance spectra

if color constancy of the colors in the scene under changing illumination source is _not_ granted.

If color constancy were granted, then using D50-based reference
data instead should not make a difference (as long as the
resulting profile is applied with a whitepoint-relative intent),
since the color change under different illumination sources would
follow a chromatic adaptation then.

Also keep mind that even a perfect colorimetrically correct
reproduction does not necessarily preserve the _color appearance_
as well, since the latter depends vastly on the viewing
conditions/environment. Note for instance that the _appearance_ of
the same scene under rather dim tungsten light is per se warmer
than under bright daylight, even if color constancy were granted
and thus the relative colorimetry were the same. So if the
reproduction (e.g. on a monitor) does not "look" like the original
scene, the reason could also be a color appearance issue...

[ And still all limitations of input device profiling do apply,
e.g. cameras are partially color bind wrt. the human vision, so it
is generally not possible to build a profile which is correct for
_all_ possible colors; the target may not be sufficiently
representative for the captured scene, etc. ]

Best Regards,
Gerhard


Am 07.12.2013 11:26, schrieb Stephen T:

Hello,

Post by Stephen T
Tungsten + D50 XYZ is not equal to Sunlight + D50 XYZ. The

sunlight profile performs very poorly for photos taken with warm
lighting.

Post by Stephen T
1) Tungsten + D50 XYZ?
2) Tungsten + A XYZ?
3) Tungsten + adapted A XYZ?
I think most people profile with option 1) but this doesn't seem

right because the XYZ changes when the illuminant changes.

Post by Stephen T
Stephen.
________________________________
Sent: Wednesday, 4 December 2013 4:31 AM
Subject: [argyllcms] Re: The right way for device input profiles and artificial light sources?
Shooting the target under say tungsten light while using D50-based reference data is basically supposed to result in a profile which reproduces the shot of a tungsten-lit scene as if the scene were lit with D50 instead of tungsten light (within the various limits of the camera profiling process, of course).
--
Best Regards,
Gerhard
Hello,

Post by Stephen T
I have been playing around with camera

profiles for tungsten and fluorescent
lighting.

Post by Stephen T

Post by Stephen T
As I understand, there are 2 things that
1) Device RGB changes (what the camera

sees).

Post by Stephen T

Post by Stephen T
2) XYZ changes (what the standard observer

sees).

Post by Stephen T

Post by Stephen T
I have made simple matrix profiles. There

are 3 different profiling strategies I have

Post by Stephen T

Post by Stephen T
1) Profile with D50 reference data (ignore

changes in XYZ).

Post by Stephen T

Post by Stephen T
2) Profile with XYZ computed for the CIE

illuminant most closely matching the light
source (non-standard profile white point).

Post by Stephen T

Post by Stephen T
3) Take XYZ from step 2 and perform a

chromatic adaptation transform to D50
(standard PCS white point).

Post by Stephen T

Post by Stephen T
Which of these is recommended?
Profiling errors with the 3 different XYZ are similar (a good fit doesn't mean the results will be sensible in practice however). Options 2 and 3 produce exactly the same rXYZ, gXYZ and bXYZ, only the WTPT is different. Options 2 and 3 produce identical images in my workflow and I guess the non-standard white for option 2 is being accounted for?
For real photos in warm light, I have found

all 3 tungsten profiles are similar. The D50
reference data produces slightly warmer
reds.

Post by Stephen T

Post by Stephen T
There are greater differences in photos with fluorescent lighting, where the D50 reference data produces a warmer, more pleasing image. Options 2 and 3 look a bit greenish. Perhaps this is because I profiled with a light source approximating F5 daylight and the test photos were shot with inferior, lower colour temperature lights?
Compared to daylight profiles, the artificial light profiles do produce more realistic colours. I haven't done any quantitative tests yet.
It's strange that the D50 reference data

seem to be useable, as the colorimetry of
the target does change and the rXYZ, gXYZ,
bXYZ tags can be very different. Here are
some colour differences I calculated after

Post by Stephen T

Post by Stephen T
Mean DE94 between D50 and illuminant A after

CAT to D50 = 1.5.

Post by Stephen T

Post by Stephen T
Mean DE94 between D50 and illuminant F5

after CAT to D50 = 2.4.

Post by Stephen T

Post by Stephen T
Appreciate any tips, advice.
Stephen.