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Author Topic: DCamProf - a new camera profiling tool  (Read 767323 times)

GWGill

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Re: DCamProf - a new camera profiling tool
« Reply #360 on: June 01, 2015, 10:16:42 am »

Oh, realized it's not the same thing. Fairchild's "spectral adaptation" is CAT based on full spectral data, the human adaptation is still modeled just like in an ordinary CAT. In the paper he uses a "color constant" version as reference though, which is the exact same thing as a relighting transform.
I think you mis-interpret the testing in that paper. The "color constant" version was simply one of the 3 other algorithms the spectral CAT was sanity checked against in a lightweight psychophysical experiment. The following more precise Computational Model Testing used CAT02 as the reference, not the "color constant" computation, although it fared well - but note the caveat about  metamerism, which is the principle reason I don't think it should be the default in camera profiling.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #361 on: June 01, 2015, 10:18:22 am »

I can't agree - there is no reason to assume that being able to spectrally accurately re-light a scene is what human mechanisms of color constancy are striving for.  They are evolved mechanisms that are useful to us, that bear the same resemblance to re-lighting as any 3-channel white balancing algorithms does.

I probably wasn't clear, I don't suggest relighting as an alternative to CAT, although I'm interested in it's performance which is lightly touched upon in Fairchild's spectral adaptation paper. So I don't think we are in disagreement.

I don't think it's desirable to use a CAT when relighting is what you want to do. When do I want to do relighting? Say you make a profile for tungsten light, then I need XYZ reference values for tungsten, the values for the observer under this particular light for the particular target. This is calculated from spectra when possible, ie relighting. Then we need D50 XYZ reference values for the PCS, these are then created by applying CAT to those tungsten reference values to keep the color appearance.

That is CAT is used when moving to the PCS, and relighting is used when we need values for the actual calibration illuminant.

In 90% of the use cases, perhaps 95% of ICC profile use cases, the user will make a D50 profile and have a file with D50 reference values, and in this case CAT/relighting is not used.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #362 on: June 01, 2015, 10:21:04 am »

I don't think it should be the default in camera profiling.

Color constancy will not be the default in the next release. Default will be relight to calibration illuminant, use CAT to get back to PCS.

There are three scenarios for relighting in DCamProf: 1) recalculation from spectra (ideal), 2) recalculation from simulated spectra (using DCamProf's ability to render a spectrum from an XYZ coordinate), 3) using Bradford CAT. The last is "wrong" as CAT is not relighting, but among the CATs the linear bradford seems to be the least bad at the task, it performs worse than the spectral simulation though.

For the CAT, CAT02 is used.
« Last Edit: June 01, 2015, 10:27:31 am by torger »
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AlterEgo

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Re: DCamProf - a new camera profiling tool
« Reply #363 on: June 01, 2015, 10:22:26 am »

From comparing SSF vs. direct, seems SSF algorithm may need some further improvements; however it may be also something with the measurements, too. The direct transform looks not bad at all (on UK scale, same as "near excellent" in US).

and extra data to illustrate direct profiling (w/o going through SSF) results from the prev. post

attched PatchTool compare report :

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torger

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Re: DCamProf - a new camera profiling tool
« Reply #364 on: June 01, 2015, 10:31:32 am »

For normal profiles I use grey step wedge only to check linearity and diagnose flare and glare. This is an important check, and important warning. Flare can't be effectively eliminated based on a tight group of neutral patches as it changes across the target; but border pattern on SG targets combined with the pair composed of the black and the white patches in the middle allows to have a TPS (I actually use 2 TPSs, one additive, and one multiplicative) to compensate for flare.

Practical recommendation was to use polarizing filters to eliminate the effect, but now it is abandoned as proper target design that includes the interleaved black and white patch pattern and math do a better (much better in fact) job.

Thanks for sharing the "tricks" I really appreciate it.
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #365 on: June 01, 2015, 10:39:41 am »

If we go a step back for a moment... If we shoot a target 2 times, once with a light source filtered to 5000K, and once with a light source filtered to 6500K, we will have 2 sets of device values that are not in linear proportion to each other. Same, if we take 2 shots under daylight sunny and daylight overcast. The usual point here is that the results should not look the same anyway due to human perception. However it is a general statement, applied backwards, to justify already existing math. Contrary to scanners where the light spectrum is fairly stable, and viewing prints under D50 lights in booths, photography deals with a much more diversity in light sources, and constant complaints on wrong colour is not because photographers and clients are being capricious, but because there is a problem there indeed. This problem stems mostly from the nature of CATs.

For now, the only publicly available way to tame CATs in wrong places is to create ad hoc profiles, that is to profile for the light is the scene (easy in studio, very difficult anywhere on location). What happens when an ad hoc profile is created usually is that the CAT is an organic result of computing a transform between a shot taken under studio lights and target reference being D50/2°. Effectively it is what we here refer to as "re-lightning". But in this case we try to use not "some" lights, but lights as close to D50 as possible - and that helps to keep adaptation to minimum. Try the same with an arbitrary light - and the results are not usable. This alone demonstrates the problem we are having with CATs, "re-lightning", and profiling.
« Last Edit: June 01, 2015, 10:48:45 am by Iliah »
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #366 on: June 01, 2015, 10:47:17 am »

but are we not estimating/correcting flare for the actual scene of shooting target for profile building - so that it will help with creating a profile too, no ?


Sorry, I lost you here... The target is the subject in the shot for profiling, and the flare/glare needs to be estimated from the target and eliminated as much as possible from the target (smoothness here is the paramount, so a spline is a good way to do it; better to have a larger error than to sacrifice smoothness), not from some other object or other scene, as flare and glare are scene-dependent and position-dependent (varies with the scene and depends on the coordinates in the scene), right?
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #367 on: June 01, 2015, 10:52:22 am »

Interesting indeed, it seems like I may need to revisit the use of CATs once more... I'll let it stay in the way described in the upcoming release though, at least for a few days ;)

How well CATs really work is indeed a bit of a mystery, hard to figure out from just reading papers. From reading, guessing and some limited numerical testing I've assumed that they are quite okay. We'll need to do some practical testing and judgements by eye too I guess. If they mostly produce useless results, I will have to return to color constancy as the default mode.

However if I understand Iliah correctly, you really need to make profiles with D50 lights to get good results, that is it's better to use a D50-designed profile even if it's used under tungsten light?
« Last Edit: June 01, 2015, 10:54:41 am by torger »
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #368 on: June 01, 2015, 10:57:38 am »

> In 90% of the use cases, perhaps 95% of ICC profile use cases, the user will make a D50 profile and have a file with D50 reference values, and in this case CAT/relighting is not used.

That's exactly what happens, and that's the wrong way of doing things. If the light in the scene is not accounted for, the transform is going to be wrong.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #369 on: June 01, 2015, 10:59:36 am »

Sorry, I lost you here... The target is the subject in the shot for profiling, and the flare/glare needs to be estimated from the target and eliminated as much as possible from the target (smoothness here is the paramount, so a spline is a good way to do it; better to have a larger error than to sacrifice smoothness), not from some other object or other scene, as flare and glare are scene-dependent and position-dependent (varies with the scene and depends on the coordinates in the scene), right?

Iliah, do you know how much of a problem glare is in a proper copy setup in a dark room? The camera outside "the family of angles" etc? Currently I assume that glare is negligible in this situation, but I'm not sure if that really is the case.
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AlterEgo

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Re: DCamProf - a new camera profiling tool
« Reply #370 on: June 01, 2015, 11:02:33 am »

Sorry, I lost you here... The target is the subject in the shot for profiling, and the flare/glare needs to be estimated from the target and eliminated as much as possible from the target (smoothness here is the paramount, so a spline is a good way to do it; better to have a larger error than to sacrifice smoothness), not from some other object or other scene, as flare and glare are scene-dependent and position-dependent (varies with the scene and depends on the coordinates in the scene), right?
yes and what is wrong with replacing b/w patches in the target (for example the one that does not have them in the first place) with b/w patches for a 2nd shot under the same light, camera position, etc... just like you do flatfielding... remove the target, place checkerboard target... advantage of checkerboard target is that you can make smaller patches (for example) and they will cover the whole area of the original target - unlike SG where you have those patches along the edge and in the center
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #371 on: June 01, 2015, 11:04:49 am »

>you really need to make profiles with D50 lights to get good results, that is it's better to use a D50-designed profile even if it's used under tungsten light?

Depends on what we consider acceptable. To create an Illum.A or Studio Tungsten (3200K) profile, I shoot under that light, use target spectral reference measured (or re-calculated) to the light spectrum in the scene, and after that use some (s)CAT to make a profile with D50 WP. Incidentally, Spectrolino measurements are taken with Illum.A light source, and D50 reference is the result of spectral re-calculation. For creating D65 reference, they have a filter attachment.
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #372 on: June 01, 2015, 11:08:04 am »

yes and what is wrong with replacing b/w patches in the target (for example the one that does not have them in the first place) with b/w patches for a 2nd shot under the same light, camera position, etc... just like you do flatfielding... remove the target, place checkerboard target... advantage of checkerboard target is that you can make smaller patches (for example) and they will cover the whole area of the original target - unlike SG where you have those patches along the edge and in the center

I see. The substitution patches need to be of the same size as patches on the target, made on the same substrate, and using the same pigments. There are minor geometry changes involved, like different number and locations of white patches will cause different flare.
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #373 on: June 01, 2015, 11:13:06 am »

Iliah, do you know how much of a problem glare is in a proper copy setup in a dark room? The camera outside "the family of angles" etc? Currently I assume that glare is negligible in this situation, but I'm not sure if that really is the case.

From measurements I took from the shots made in well-equipped studios by seasoned photographers, typically flare is robbing 1/2 of a stop of dynamic range from and SG target, and the toe portion (non-linear shadows, non-linearity being more than 3% allowed by some museum instructions) is about 1 stop long. That poses a problem, as SG target is only 7 stops DR.
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AlterEgo

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Re: DCamProf - a new camera profiling tool
« Reply #374 on: June 01, 2015, 11:24:27 am »

I see. The substitution patches need to be of the same size as patches on the target, made on the same substrate, and using the same pigments. There are minor geometry changes involved, like different number and locations of white patches will cause different flare.

I am sorry - why same pigments for example, you calculate (in your setup) flare corrections using pigments from B/W patches and applying to patches of different pigment composure, no ? so why demand different from checkerboard flarefield ?
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AlterEgo

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Re: DCamProf - a new camera profiling tool
« Reply #375 on: June 01, 2015, 11:26:42 am »

typically flare is robbing 1/2 of a stop of dynamic range from and SG target... That poses a problem, as SG target is only 7 stops DR.

you mean that when you measure SG with a really good instrument (no issues with dark patches) it is 7 stops, you do a shot and from raw rgb is 6.5 stops on the same targer (between black and white patches), right ?
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #376 on: June 01, 2015, 11:30:54 am »

you mean that when you measure SG with a really good instrument (no issues with dark patches) it is 7 stops, you do a shot and from raw rgb is 6.5 stops on the same targer (between black and white patches), right ?
Exactly.
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Iliah

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Re: DCamProf - a new camera profiling tool
« Reply #377 on: June 01, 2015, 11:33:06 am »

I am sorry - why same pigments for example, you calculate (in your setup) flare corrections using pigments from B/W patches and applying to patches of different pigment composure, no ? so why demand different from checkerboard flarefield ?
Same pigments - same reflection. It is indeed a "field" problem, if one wants to recreate a magnetic field on a flat surface, it is simpler to use the same magnet sizes and same sizes and spacing than to change the distances.
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AlterEgo

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Re: DCamProf - a new camera profiling tool
« Reply #378 on: June 01, 2015, 11:40:11 am »

Same pigments - same reflection.
I see what I am missing - you mean that reflections from other patches shall be present exactly, including color patches that we in my suggestion remove to replace with flarefield, right ? but are those reflections a bigger problem vs everything else - walls, ceiling, floor, etc...  but then those are again reflecting from patches... catch22  >:( !
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Bart_van_der_Wolf

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Re: DCamProf - a new camera profiling tool
« Reply #379 on: June 01, 2015, 12:10:00 pm »

From measurements I took from the shots made in well-equipped studios by seasoned photographers, typically flare is robbing 1/2 of a stop of dynamic range from and SG target, and the toe portion (non-linear shadows, non-linearity being more than 3% allowed by some museum instructions) is about 1 stop long. That poses a problem, as SG target is only 7 stops DR.

Maybe it's clearer to make a distinction.

Semi-specular surface reflection of of a target is just reflection. This is an issue that has to do with surface smoothness and angle of illumination. Glossy targets have more specular reflecting surfaces, and semi-matte targets have a more Lambertian diffuse reflection. So observing the angle of illumination versus the position of the camera/optical axis can eliminate the reflection of the lightsource better on a glossy target, but it requires a controlled environment (no other bright reflecting or light emitting surfaces that can reflect their presence to the camera lens). Diffuse reflecting surfaces will always add more environment and direct lightsource reflection (desaturation) to the patch reflection.

Reflection inside an optical system can be veiling glare (lack of perfect anti-reflection coating due to angle and wavelength and internal reflection e.g. due to non-blackened edges of lenses or barrel surfaces) which adds a uniform amount of non-image forming light mostly noticeable in the darker tones. Flare is a pattern, a (multicolored) image of the light source as formed by reflections between lens surfaces, and is more local.  This is an optical issue that can also get worse near brighter areas in the scene or image, which suggests that white and black patches of targets are best not positioned directly next to each other.

So surface reflection is target and lighting related, internal lens / instrument reflection or Glare (maybe even flare patterns) is an optical phenomenon.

And then there is electronic noise added to the signal that the sensor (camera or spectrometer) records. The signal itself is already noisy (photon shot noise) which is worse when fewer photons are available (e.g. coming from darker patches). Truely random noise can be reduced by averaging multiple images/captures/readouts.

Cheers,
Bart
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