DCamProf - a new camera profiling tool

[Tim Lookingbill]

I just became aware that Argyll's printtarg can output chart files, that is it's probably really easy to make your own targets if you have a good inkjet printer and a spectrometer to measure it. I shall test and update the docs accordingly. I think an inkjet print will have at least as good spectral qualities as an IT8 target on photo paper.

Maybe I'll add a target generator later on as Argyll's targen is more tailored for printer and scanner targets.

Would the use of the spectrometer and software determine whether certain colors in a custom target are beyond a display's ability to reproduce like say Pantone 313C (coated)? A Fuji Frontier drylab inkjet can reproduce it but currently there are no reference displays. No telling what other colors there are for creating a custom target that would fit nicely and improve current photographic workflow processes.

Has someone come up with a more robust color target with a specific selection of colors that can fit within both Raw capture and display gamut workflows that would result in the least "gamut crunching" color errors?   

[Bart_van_der_Wolf]

Has someone come up with a more robust color target with a specific selection of colors that can fit within both Raw capture and display gamut workflows that would result in the least "gamut crunching" color errors?

Hi Tim,

Gamut crunching only happens on profile conversions. It does not affect the creation of a camera profile IMHO, unless one seeks to make a dumbed-down target that only covers the Adobe RGB colorspace (at best). But such a profile that's based on such a limited target would not be able to reliably map colors outside the limited aRGB gamut.

It's much better to use a target with relevant colors that the camera is able to 'see', and covers it full gamut as far as relevant existing colors go, and sometimes only use part of its full gamut if the subject poses a limited challenge.

Cheers,
Bart

[Alexey.Danilchenko]

I just became aware that Argyll's printtarg can output chart files, that is it's probably really easy to make your own targets if you have a good inkjet printer and a spectrometer to measure it. I shall test and update the docs accordingly. I think an inkjet print will have at least as good spectral qualities as an IT8 target on photo paper.

Maybe I'll add a target generator later on as Argyll's targen is more tailored for printer and scanner targets.

There is little benefit of printing your own camera targets on common 4 colours inkjets. The  number of paints used for each patch will be limited to that of the original 4.

[Tim Lookingbill]

Hi Tim,

Gamut crunching only happens on profile conversions. It does not affect the creation of a camera profile IMHO, unless one seeks to make a dumbed-down target that only covers the Adobe RGB colorspace (at best). But such a profile that's based on such a limited target would not be able to reliably map colors outside the limited aRGB gamut.

It's much better to use a target with relevant colors that the camera is able to 'see', and covers it full gamut as far as relevant existing colors go, and sometimes only use part of its full gamut if the subject poses a limited challenge.

Cheers,
Bart

How has it been determined what colors the camera is able to 'see'?

I'ld have to assume this optimized for digital sensor color target would have to comprise a certain gamut shape and size that would be tuned to the math that can map colors to the display. Not sure if the shape of the gamut is more important than the size. The 3D model of most inkjets show some colors go outside the AdobeRGB gamut where most others are well within.

Can distinction between subtle differences of colors be improved by using a custom target that's more tuned to the real color gamut shape that describes how a digital camera 'sees' color?

[torger]

There is little benefit of printing your own camera targets on common 4 colours inkjets. The  number of paints used for each patch will be limited to that of the original 4.

I think many like me has the higher end pigment inkjets, which have several more inks. I shall do a more thorough spectral comparison later.

[Alexey.Danilchenko]

I think many like me has the higher end pigment inkjets, which have several more inks. I shall do a more thorough spectral comparison later.

A quote from stephen stuff to describe some of the problems.

Spectral problems

Camera profiling targets are commonly photographed in sunlight, which includes ultraviolet wavelengths (UV). Photographic papers can contain “fluorescent whitener additives” (or “optical brightening agents”) which makes these papers appear more blue. These papers and printed camera profiling targets can show colour shifts towards blue when there is ultraviolet in the light source.

Metamerism is another problem for photographic and printed targets. The processes are optimised to produce colour, usually from just three of four colourants, that look natural to the human eye but actually might be composed of quite different spectra. This is a problem if the camera spectral sensitivities are different to the human eye. ColorChecker targets are made using multiple different pigments, giving reflective spectra that are more representative of the real world.

In fact if the solution would be as easy as to print your own target with however good colour coverage you need then anyone would not have problem obtaining good profiles. Alas this is not what I see happening yet.

[torger]

DCamProf's color separation diagram gives an idea what colors a camera can "see", most modern cameras can differentiate colors in most of the human gamut, well beyond limits of say AdobeRGB.

Profiles generally distort extreme colors though and may move them outside valid space so they get clipped to a nearby color. Extremely saturated colors is a narrow special ccase so few have problems with that.

If you really want to cover an extreme gamut with your profile, designing using SSF is the answer, which you can do with DCamProf.

Spectrometer data will indeed give the profiler full information of the color. Consumer spectrometers are a little limited in range, often cover only 400-700nm which can be a problem in some special cases.

[torger]

A quote from stephen stuff to describe some of the problems.

Already well-known and discussed in DCamProf's docs. I need more testing but initial results suggest that OBA-free paper and pigment inkjets do a good job rivaling what you find in cc24 or munsell book of colors. There are probably some problem colors though,  need to look at the spectra of all patches to give a full report. The bad reputation comes from old print systems and basic consumer papers.

I'm currently travelling so I can't test that much.

[Bart_van_der_Wolf]

How has it been determined what colors the camera is able to 'see'?

Isn't that the purpose of DCamProf, to allow encoding colors that the camera can see? Whether and how that translates into humanly visible colors is another exercise, and whether and how our output modalities reproduce them is yet another. If we cannot adequately encode the input, there is no hope for getting good output.

I'ld have to assume this optimized for digital sensor color target would have to comprise a certain gamut shape and size that would be tuned to the math that can map colors to the display. Not sure if the shape of the gamut is more important than the size. The 3D model of most inkjets show some colors go outside the AdobeRGB gamut where most others are well within.

Yes, that's why an inkjet printed target may not be the best basis. Remember the exercise that Bruce Lindbloom did to find the parameters for his BetaRGB colorspace. It comprises many important colors that may need to be encoded, but as few more extreme ones as possible, to improve the quantization step precision.

Can distinction between subtle differences of colors be improved by using a custom target that's more tuned to the real color gamut shape of a digital camera 'sees' color?

Not all colorspace coordinates represent humanly visible colors, and they are thus by definition not 'colors'. What matters is that a camera can capture and distinguish between as many 'colors' as possible. This is not going to be perfectly possible due to the Luther-Ives condition (more here), so some form of perceptual mapping will ultimately be needed.

The goal of creating a camera profile is to get a solid basis for the conversions that are to follow. Accurate for important colors, smooth transition to intermediate colors, able to reduce metameric and color constancy issues. But that is more an input or scene referred profiling than the ACR centric output referred profiling that gives us hue twists and other trouble.

I prefer clean scene referred profiling, making use of an adequate but not overly large working space, and a perceptually based output processing, e.g. using a CIECAM like color appearance model for output. I am not sure if an inkjet print will be challenging enough for a camera sensor, although it does allow to produce some saturated Cyans, Yellows, and Magentas. Apparently those dyes or pigments are available in nature, so we should also be able to encode those.

Cheers,
Bart

[torger]

Afaik only Argyll and now DCamProf allows custom targets, which is a cost saver if you already have good printing gear, plus that commercial targets are focused on ease of use, minimize risk of reflection and other lighting errors rather than maximizing profile performance. Custom targets are definitely worth investigating deeply.

Target design is only a small part of the problem though. Profiling is an unsolvable problem (infinite spectral variation etc), it's always a compromise and therefore just as much an art as a craft. Many (most?) people prefer "looks" rather than neutrally accurate too, so it all boils down to taste. The best design method/target for you may not be the best for me.

[AlterEgo]

Already well-known and discussed in DCamProf's docs. I need more testing but initial results suggest that OBA-free paper and pigment inkjets do a good job rivaling what you find in cc24 or munsell book of colors. There are probably some problem colors though,  need to look at the spectra of all patches to give a full report. The bad reputation comes from old print systems and basic consumer papers.

I'm currently travelling so I can't test that much.

I just measured a regular IT8 target from Wolf Faust (my is quite old, but spent most of the time in dark non humid no extreme temperatures storage) with i1pro2, here are the spectral files = https://app.box.com/s/o4gwrk0d5slw7y1g0nu4bhs1fevg775d that is a bottom level of what can be printed if you have access to proper printer/ink/paper, no ?

[torger]

DCamProf only cares about human gamut currently, it's not good for IR cameras for example. You can probably do SSF design with IR but color must be related to an observer, and currently only human observers are covered. How a color we can't see should look is not so easy to define ;-)

Also note that DNG profiles unlike ICC by design clips the gamut to prophoto. DCamProf's native format is unbounded though so the upcoming ICC support will not be clipped.

The prophoto clipping can be an issue in scientific applications, but hardly for ordinary photography.

[AlterEgo]

Afaik only Argyll and now DCamProf allows custom targets

ProfileMaker for example too - just supply .tiff and matching .cie

[torger]

Thanks AlterEgo that it8 data will be great for reference. I do expect that a pigment inkjet target will be at least as relevant as that. However the criticism above also targets photographically printed targets like Faust's It8, and suggests special printing like used for cc24 is required for adequate results.

Worth investigating.

[GWGill]

I just became aware that Argyll's printtarg can output chart files, that is it's probably really easy to make your own targets if you have a good inkjet printer and a spectrometer to measure it.

Note that input targets created using a printing process have their limitations compared to targets created using a wider range of pigments.

The chart ideally should represent the spectra of what you are trying to capture, and something created with 4 or so inks has limited coverage of possible metamers.

Those capturing something like original artwork have had good success in creating custom test charts composed of the types of pigments that they intend capturing.

[GWGill]

Would the use of the spectrometer and software determine whether certain colors in a custom target are beyond a display's ability to reproduce like say Pantone 313C (coated)?

Perfectly possible. You need to be aware of what you are assuming with regard to white point adaptation though. (i.e. because reflective colors are measured standard to D50 illuminant, and most displays are D65).

Has someone come up with a more robust color target with a specific selection of colors that can fit within both Raw capture and display gamut workflows that would result in the least "gamut crunching" color errors?   

Not following you there. The gamut of an input device test chart has nothing to do with how out of gamut colors get handled further down the chain. Limiting the input test chart merely makes the camera/scanner characterization less accurate in the part of the gamut you haven't tested. Colors beyond that will still be captured by the input device. All colors are within a cameras gamut if its exposure is properly set (i.e. a camera doesn't have a gamut in the same sense an output device has.)

[GWGill]

How has it been determined what colors the camera is able to 'see'?

I'ld have to assume this optimized for digital sensor color target would have to comprise a certain gamut shape and size that would be tuned to the math that can map colors to the display.

A camera doesn't have a gamut. If one of the channels hits saturation, you reduce exposure. If everything is black, you increase exposure.

What a camera has instead is a spectral mismatch to a human observer. There are colors that are metamers to a camera that are not a match to a human observer, and vise versa. Unless it's spectral sensitivities are exactly the equivalent of the standard observer, a matrix or other color profile can only approximately convert a camera RGB into the human XYZ values, and can do nothing about metameric mismatches.

A consequence of this is that any particular profile will allow a camera to produce imaginary colors, as well as there being colors that the camera can never produce (i.e. the human spectral locus and profiled camera spectral locus will not match).

[torger]

A camera doesn't have a gamut in the sense of a printer, as it's an input profile, but it does have limitations of what colors it can differentiate (and capture). This can be visualized with the DCamProf color separation diagram, here an example of a Canon 5Dmk2:



The darker color the worse color separation (related to the standard observer), and black it can't separate at all, ie all local colors are registered as one and the same.

The camera's SSF rarely stretches out as wide as the CMF so you get limits on maximum red and maximum violet, and in the case of older cameras you don't have red/blue overlap so you have some metamerism, modern cameras have very little metamerism though although the SNR will vary of course. The SSF shapes differ greatly from the CMF so you get some minimas here and there seen as dark spots in the diagram.

The diagram doesn't involve a profile, just the SSFs. A modern camera can generally separate colors near or at the locus, but to get a match you need strongly non-linear correction (LUT) and the strong non-linearity can hurt performance of normal colors so it's generally not a good idea to try to make a profile with maximum gamut.

A profile optimized for a smaller gamut will generally cause chaos for extreme colors, even moving them into invalid positions so they are clipped away, visualized in this diagram:



where you see short error vectors for normal colors and looong ones for extreme colors, some stretching outside the gamut. I'm sure many that has SSFs for their cameras will be tempted to make the "ultimate" profile which makes the camera perform in almost the whole human gamut but that's probably a mistake as it will most likely hurt performance of the normal colors that are 100% of the colors in 99% of the scenes you shoot.

[torger]

Note that input targets created using a printing process have their limitations compared to targets created using a wider range of pigments.

The chart ideally should represent the spectra of what you are trying to capture, and something created with 4 or so inks has limited coverage of possible metamers.

Those capturing something like original artwork have had good success in creating custom test charts composed of the types of pigments that they intend capturing.

I'm very aware of this, however it seems to me than noone has really tested this to depth the past 10 years or so. Pigment inkjets today doesn't have 4 inks, they like six colors and 3 neutrals, and for each new ink generation improvements have been made in metamerism etc. I shall make a thorough spectral analysis when I get some time, I'm sure there are problem colors but I'm also sure today's pigment inkjets perform much better now than consumer printers did when the criticism towards printed test targets was formed, so I think there are reasons to re-consider this option.

Also special-printed test targets like a CC24 or munsell's book of colors has limitations. I have compared with spectral databases from skin and nature and real spectra of course show much larger variation than you can capture in a few patches. With DCamProf you can profile against those spectral databases directly using camera SSF, but due to the natural limitations of camera profiling it's not that sure it will be that much better. Likewise I don't think it's that sure that a special-printed target will be that much better than a pigment inkjet printed target, and the home-made target has the advantage that it can if you desire be made much more saturated (cover a larger chromaticity gamut) than typical commercial matte test targets do.

Apart from traditional real-world testing with your eyes, with DCamProf it's possible to make comparisons virtually, you can measure your home-made target with a spectrometer, and make a target from a spectral database, and use the spectra from a CC24 or other commercial spectra for one more, and then make profiles from SSFs for all those and compare how well they match various spectra. So there's good opportunity to make some serious testing. I'm still in for making some more features in the software though so I don't have that much time to do all those tests myself.

[torger]

Here's attached a few patches from canon pigment lucia ink on oba-free baryta paper, CC24 spectra, some real reflectance spectra from nordic nature (leaves, flowers etc), and finally some patches from IT8 Faust spectra provided by AlterEgo.

For the Baryta it seems like we have a colorant gap around 470nm and 570nm, and it's less smooth than CC24.

I think these plots indeed does show that the CC24 does match natural spectral shapes better than printed spectra, but that the printed spectra is certainly not as bad as it could be. The printed spectra also show higher saturation colors than the CC24 and nature so it will therefore make steeper shapes. A better/easier comparison would be to try matching the CC24 colors with the printer and see how those shapes look, I may do that later when I have some time.

On a first look the IT8 spectra looks pretty good, smooth. Certainly not obvious that it would be worse than the spectral shapes provided by CC24. There's the rise of all spectra towards 720+nm, but I don't think that matters that much as the sensitivity of the observer is low there.