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Author Topic: DCamProf - a new camera profiling tool  (Read 719978 times)

torger

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Re: DCamProf - a new camera profiling tool
« Reply #1440 on: February 08, 2017, 03:47:59 am »

I'm pausing GUI development for a while to look into color appearance in tungsten light.

95% of the visual color testing and tuning of DCamProf has o far been made in daylight/flash conditions. I myself and most users use the cameras in this condition almost exclusively, so it's been all good. However recently I've got some interest in looking over color appearance in the tungsten light (StdA) range. It seems like it's maybe not as good as it could be there.

I have a bit of research to do, but here's what I know so far. The problem with StdA is that it's way different than the standard illuminant D50 used for screens and printing. A camera profile must convert all colors to D50 as the connection space for the color pipeline uses the D50 whitepoint. So the challenge here is to model the color appearance in StdA and convert each color to a corresponding D50 color. There's already well-established standard models for that, chromatic adaptation transforms (CATs), and the current best is CIECAM02's CAT02 which DCamProf uses today to figure out how colors appear in StdA and convert that to D50. Broadly speaking the largest difference is that reds appear a bit brighter and some colors a bit warmer in tone.

However this is often not enough in terms of warmth, I often find myself setting a creative white balance to a warm tone to better mimic the color appearance. It also seems (I don't know this for sure though) that camera preset white balance for tungsten is often put a bit high in temperature to offer some residual yellow tinting effect. This indicates that there's "partial chromatic adaptation" going on, that is our eyes have only partially adapted to the StdA white and still experience the whites (and all other colors) as tinted slightly yellow.

Something around here it seems to break apart a little, modeling colors after 100% adaptation in the profile, and adjust-to-taste for partial adaptation using white balance maybe doesn't really cut it. Unlike in D50 conditions we got to accept in a larger extent that we can't do anything accurate, as there's too many parameters in human chromatic adaptation, there's short-term effects, long-term effects, and with some mixed lights thrown in there it gets even worse. However, it may be possible to make something better than the current, where it seems to me that to get the warmth I find suitable for some of the saturated colors I need to put so strong off-white white balance that the whites become too tinted. The CAT02 does model partial adaptation which I will experiment with a bit, not sure yet if that will be enough though, or if I'll have to make manual adjustments to it.
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jrp

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Re: DCamProf - a new camera profiling tool
« Reply #1441 on: February 08, 2017, 02:30:26 pm »

Nikon seem to have come to similar conclusions to you.  The have introduced, in addition to a pure, grey is neutral, white balance additional auto white balance options that retain some warmth that can make portraits look more pleasing and the look closer to what you remember.

Whether it is worth doing anything other than generating neutral white balance and letting the user warm pictures in post according to their own taste, is an open question.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1442 on: February 09, 2017, 02:53:00 am »

I haven't had time to do all testing yet, but it seems like just adjusting the white balance to suitable off-white is pretty close to the result of a CAT partial chromatic adaptation model, for example setting to 80% chromatic adaptation StdA CAT02 is for most color sub 0.5DE from setting white to RGB 1.03,0.98,0.87. This indicates that I won't get much different result from working with the CAT rather than just changing white balance alone.

If so, I think there may be a need to make an own custom CAT just for StdA, because I see some problems in the results, mostly in red-brown colors - simply the hues are clearly different from the real thing, not warm enough and too much green in them. Chromatic adaptation is messy to work experimentally with though so I shouldn't get too fast to conclusions.

I'm a bit surprised though that the CAT from D50 to StdA does not seem to be that accurate, I haven't come across anything in the literature that says that it has these kinds of problems, so it might be some other thing I'm observing. I've looked at each step though, and it does seem like the color error is introduced in the CAT.

The design idea for improvement I have currently is to use a CAT and you specify a partial adaptation you want at profile design time. If you don't specify full adaptation the software calculates an ideal off-white white balance for you that you can use when shooting, and the profile is optimized specifically for that off-white white balance. It's still unclear though if it's necessary to optimize for a specific off-white or if the difference will be insignificant from always designing for 100% adaptation.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1443 on: February 11, 2017, 04:38:54 pm »

Findings so far regarding color rendering for tungsten light; cameras are generally more dependent on LUT correction in the StdA range than regular daylight. This means that if you set another white balance than the one the profile was designed for the LUT offset will be wrong and you get the wrong corrections. For a daylight profile it usually doesn't matter much as the matrix is already good and the LUT has very broad corrections. With StdA it matters a bit more.

This is a bit of a problem as in StdA you typically want to have a creative white balance, usually a warm one, as discussion in the previous posts. I'm thus looking into the possibility to offset the LUT so it matches a pre-defined warm white balance setting.

I've also done some matching-color-by-eye tests, and the CAT02 indeed does seem to quite well simulate the color appearance changes when you change from D50 to StdA. However, regardless for how long I sit under StdA, white keeps a warm tone. I reach about 80% adaptation I would say. I've asked others to look and they get the same result. I also have discovered some weakness is the CAT02, which may be a side-effect of this partial adaptation thing, I don't know. To the eyes reds get more saturated and gets a bit orange added to them, and orange-skin-brown gets an overall saturation increase, which CAT02 doesn't model. It's not huge differences, say ~2 DE or so, so my guess is that it's to small to be included in the CAT02 model. However while small, those changes hits a sensitive areas, including skin tones, and it's not just that the CAT doesn't make it as accurate, it makes is subjectively less pleasing too with the cooler under-saturated colors.

I will thus look into making an adjusted CAT to be used when modeling StdA color appearance.
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jrp

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Re: DCamProf - a new camera profiling tool
« Reply #1444 on: February 12, 2017, 01:50:07 pm »

One things that I often find is that if I use the Auto WB in Lightroom, I get a less pleasing picture when shooting under tungsten or mixed light settings, than the camera (Leica SL's) chosen balance, even when the latter is not quite right.  So I come back to an earlier point: do you want the profiler to create a pleasing picture, or a "correct" one that can be adjusted to taste.  Regrettably, I suspect that you will need to implement both options and see what people find, in real life.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1445 on: February 12, 2017, 03:20:48 pm »

Here comes some example images.

There's two new things I've implemented demonstrated in these images.

1) A new CAT which makes some adjustments to CAT02 for the tungsten (StdA) color appearance simulation.
2) Possibility to offset the LUT so that it uses a preset creative white balance (warm tone for StdA) instead of white.

These features are useful for tungsten lighting conditions, you don't need them for daylight conditions. The example image shows a CC24 and a cello. It doesn't happen too much to the cello with the CAT change, so you have to look at the CC24 mostly. I actually had a more diverse test scene with more objects but I happened to start put together the various combinations on this image and I don't want to redo it again :-). The cello is however an excellent object to test that the creative white balance is rightly tuned.

The creative white balance was hand-tuned so that the color appearance of the objects would appear as true to the real scene as possible. That white balance ended up being very close to the in-camera tungsten preset, so that is what is used in the end in these images. I think it's often the case that tungsten in-camera WB is set at say 3200K rather than the 2850K of StdA, and this seems suitable to simulate the partial chromatic adaptation that takes place.

So on to commenting the test images. The first row shows with white balance picked from the CC24. While it sort of looks right when briefly looking at the CC24, it becomes rather obvious that an object rich in red like the cello doesn't get the right hue and looks a bit desaturated. This is not a CAT problem, but that we need to use a different white balance as discussed above. I think gray cards / colorcheckers are often mis-used in these conditions, many think that the "correct" way is always to pick the white, which works fine in daylight conditions but makes tungsten scene colors look way too cool.

We can observe the differences between the old (CAT02 original) and new (CAT02 + adjustments) CAT. The new provides slightly increased saturation orange-brown-skin, and in red a further saturation increase plus a slight hue shift towards orange.

The second row shows when we apply the creative white balance. The problem we get here is that the correction LUT assumes that white is white rather than slightly yellow, which means that when we use a creative white balance the LUT gets offset and makes the wrong corrections. This has always been a problem, but for daylight it's minor as temperature changes is often less aggressive, and the LUT corrections are most certainly so. Cameras usually need stronger LUT corrections for tungsten than daylight. The difference between compensated and uncompensated is still not huge though, which is fortunate, as we can only design for one specific white balance in any case. One may need to overlay the images to really see what happens, a slight saturation loss in some colors and a little bit green cast. The exact effect will depend on camera.

The final row shows when both creative white balance is set and the LUT white is adapted to match that, bottom right is thus the new result with both new features, while middle left is the result you get with the currently available DCamProf.

The separate image shows what happens if you use a D50 profile instead of a proper StdA profile. Generally loss of saturation and a slight green cast of warm colors.

While working on this I also noted that if you shoot in light in-between tungsten and daylight, it seems like a dual-illuminant profile is almost necessary to get good results (unless you make a profile for exactly that light of course), but if you only have one profile it's better to use one made for D50. This is not that surprising considering the large differences in matrix/LUT, plus the CAT on top.
« Last Edit: February 12, 2017, 03:25:40 pm by torger »
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jrp

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Re: DCamProf - a new camera profiling tool
« Reply #1446 on: February 12, 2017, 06:07:56 pm »

I don't have a cello to hand, but the first row looks most natural to me; the others are a bit too red (looking through Chrome web browser on a Mac).
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1447 on: February 13, 2017, 02:51:42 am »

I don't have a cello to hand, but the first row looks most natural to me; the others are a bit too red (looking through Chrome web browser on a Mac).

String instruments vary quite a lot in color, and I'm told that this cello was the reddest in the orchestra so it's a reason it looks "too red" :-). It's my own photo and I have the objects at hand so I've been able to tune against the real thing. Actually my own hand-tuned white balance made it a tiny bit redder still, but as the fixed in-camera tungsten preset was so close I used that instead. It should also be said that the lamp is at about 2700K, which is in the lower range of what typical indoor light is, eg the light is redder than in many cases.

It is a little bit messy to match tungsten scene with what you have on screen due to the widely different whitepoint, you can't really have them side by side. I had mine set up behind my back so I turned around, let eyes adapt, remember the colors as good as possible and turned back to the screen, let eyes adapt. I used other objects too and my own hand to include some skin, and I'm rather confident that the match is good.

It would have been better to show a skintone sample. The reason I had a cello in there in the first place is that I got reports from a user which had troubles with string instruments in photos of an orchestra not being red or vibrant enough. That particular problem turned out to be mostly related to setting the appropriate white balance (and using an StdA profile rather than D50), but the new modified CAT also helps, especially the less red instruments.

All that said I think there will be quite varying tastes on how off-white you should make the white balance for simulating the color appearance under tungsten. With the appropriate CAT the inter-color relationships are correct regardless of the whitepoint, so it doesn't matter that much, and I expect users come to different conclusions on what's the best approach, and it depends a bit on viewing condition. The white balance was tuned in a dark room condition (the only practical when having the real scene in the same room), and when viewing it in a brighter daylight-lit room like I do now I'd say that indeed the warmness is a little overdone and I would probably back off a little, but certainly not all the way back to a pure white.

There is one thing I've thought about related to this, I don't think it's necessary but I'm not 100% settled, and that is if you may want to make a special CAT that lets white be whiter but keeps redness of the saturated colors. That is you could have the cello as red as in the bottom row, but the whites (almost) as white as in the top row. The reason for that kind of twist would be exactly the stuff discussed above, that is when the photo is viewed in bright conditions you may want to make the whites purer, but still not desature/cool the reds-browns. I wouldn't handle that as part of the CAT though, but sort of a special offset white-balance handling.
« Last Edit: February 13, 2017, 03:11:33 am by torger »
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Tim Lookingbill

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Re: DCamProf - a new camera profiling tool
« Reply #1448 on: February 13, 2017, 05:43:35 am »

The second row actually looks the best to me maintaining more color balance relationship. It also has more color gamut where sampling the cyan patch in sRGB shows a low red (17red) and the others show it as more sky blue with red being over 40 in sRGB. If tungsten is suppose to be a yellowish to orangish warm hue cyan shouldn't look sky blue but even more intense cyan which is a bit greenish blue instead of magenta tinted light blue. There are many varieties of tungsten bulbs I'm sure exhibiting different hues so I'm going by regular soft white household bulbs which to me show more yellowish orange than yellowish red where my camera records more of a red bias just with the default ACR profile.

I wonder how this 2700K tungsten profile would render 2700K-ish cloudless sunset landscapes? Thanks for the samples, Anders.

 
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1449 on: February 13, 2017, 07:12:41 am »

Here's an animated GIF switching between the standard CAT02 and my modified CAT02 for 2700K, making the differences more apparent than the side-by-side frames in the previous post. The one with the brighter red is the modified CAT, it's swapped every two seconds. You need to click the image to make it to animate.

This was generated with the dcamprof si-render command from a spectral image using a black-body illuminant. Here white-point is kept at 1,1,1.

For reference I've also attached the plain render under D50 (no CAT).

To recap, what we show here is how colors appear differently under tungsten compared to daylight, even with 100% chromatic adaptation, that is "color inconsistancy" phenomena. To simulate this important if you want colors to look realistic when you make a tungsten camera profile. CAT02, the standardized CAT in the CIECAM02 color appearance model, generally does a good job but I think the brightening/warming effects of the reds and saturation increase of orange-brown-skin is not strong enough, and this is what I've targeted with the modified CAT.
« Last Edit: February 13, 2017, 07:37:39 am by torger »
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1450 on: February 13, 2017, 10:30:00 am »

Note that this is a work in progress, I'll rest a bit and get back to it, and make a deeper look into skin colors which is the most sensitive to not mess up. The likely result of that is that there will be some scaling back of the effect, ie the final differences will be smaller than it is in the above samples.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1451 on: February 15, 2017, 02:12:31 am »

Note that this is a work in progress

Indeed, had some setbacks. It seems like it won't work out that well for many (possibly most) cameras. The key problem with tungsten lighting is that cameras are made to match colors in daylight/flash conditions, which means that in tungsten you either must have poor color matching or strong LUT correction. There's nothing wrong with a strong LUT correction if it just rotates and increases perceptual distance (DE) between the colors. However if it has to make strong compressions, that is put colors closer together than they are in the matrix, you will likely get tonality issues.

Gradients may still look smooth, but colors in a compressed range may look a bit flatter, and if this happens in the skintone range it can be a serious problem.

Doing manual optimization and tradeoffs these issues can be avoided, but the automatic optimizer still have issues here. I'm probably going to look into a LUT compression limiter parameter and see if that works out. In any case, when the LUT is properly relaxed you may very well end up with a result that is not that similar to what the CAT thinks the colors are, the difference between the old CAT and the new modified discussed above is often ~0 after LUT relaxation.
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Alexey.Danilchenko

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Re: DCamProf - a new camera profiling tool
« Reply #1452 on: February 15, 2017, 03:58:18 am »

that cameras are made to match colors in daylight/flash conditions
Is that really correct though? Typical sensor CFA are not daylight balanced - closest to daylight balanced were Kodak sensors used in ProBacks and Phase One P20 and P25. The red channel on those is stronger than green and blue is a bit weaker than green which in daylight results in WB closer to unity. In reality though, since daylight is of a wide variety, real unity is rarely achieved.

Most modern sensors SPD are tuned for something else though - not daylight. Typical daylight WB on a lot of modern sensors requires lifting both red and blue (the latter usually a bit more). Cannot say what lighting this is designed for - possibly a compromise between tungsten and daylight?
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Ernst Dinkla

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Re: DCamProf - a new camera profiling tool
« Reply #1453 on: February 15, 2017, 05:08:26 am »

Is that really correct though? Typical sensor CFA are not daylight balanced - closest to daylight balanced were Kodak sensors used in ProBacks and Phase One P20 and P25. The red channel on those is stronger than green and blue is a bit weaker than green which in daylight results in WB closer to unity. In reality though, since daylight is of a wide variety, real unity is rarely achieved.

Most modern sensors SPD are tuned for something else though - not daylight. Typical daylight WB on a lot of modern sensors requires lifting both red and blue (the latter usually a bit more). Cannot say what lighting this is designed for - possibly a compromise between tungsten and daylight?

One would expect 4000K but I recall a claim near 3300K for older Canon sensors.

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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1454 on: February 15, 2017, 08:19:10 am »

Is that really correct though? Typical sensor CFA are not daylight balanced - closest to daylight balanced were Kodak sensors used in ProBacks and Phase One P20 and P25. The red channel on those is stronger than green and blue is a bit weaker than green which in daylight results in WB closer to unity. In reality though, since daylight is of a wide variety, real unity is rarely achieved.

Most modern sensors SPD are tuned for something else though - not daylight. Typical daylight WB on a lot of modern sensors requires lifting both red and blue (the latter usually a bit more). Cannot say what lighting this is designed for - possibly a compromise between tungsten and daylight?

They all need a matrix of course, but with one a modern camera matches a color checker pretty good, while doing less well in tungsten -- with CAT at least. That's my experience from profiling cameras, I don't have a super-large amount of cameras tested in that range though as most profiles I do are D50-D65 ones.

The thing is that a daylight spectrum is quite even, no exaggeration in any part of the spectrum, while tungsten is very slanted. I think it would be unwise to optimize for tungsten, at least the 2850K ones (StdA). The CAT complicates it too a bit, especially if I go ahead adding non-linearity to it, as the camera then no longer can make a linear match.

So far I haven't profiled a camera that haven't been more difficult to deal with in StdA than daylight, but temperature scale isn't linear and there's a big difference between 2850K and 3300K or 4000K, so it could very well be as you say that many cameras ideally operate in that sort of light.
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daicehawk

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Re: DCamProf - a new camera profiling tool
« Reply #1455 on: February 15, 2017, 05:21:05 pm »

They all need a matrix of course, but with one a modern camera matches a color checker pretty good, while doing less well in tungsten -- with CAT at least. That's my experience from profiling cameras, I don't have a super-large amount of cameras tested in that range though as most profiles I do are D50-D65 ones.

The thing is that a daylight spectrum is quite even, no exaggeration in any part of the spectrum, while tungsten is very slanted. I think it would be unwise to optimize for tungsten, at least the 2850K ones (StdA). The CAT complicates it too a bit, especially if I go ahead adding non-linearity to it, as the camera then no longer can make a linear match.

So far I haven't profiled a camera that haven't been more difficult to deal with in StdA than daylight, but temperature scale isn't linear and there's a big difference between 2850K and 3300K or 4000K, so it could very well be as you say that many cameras ideally operate in that sort of light.
My little experience with cameras says that extra warmth of camera BB for tungsten is caused by trying to achieve accepatble non-magenta skin tones. You may prove wrong, but it seems that in general all hues look OK after demosaicking without an input profile but skin tones are almost always magentish, it is a common trend in DSLRS. There are many points to be discussed on getting accurate\pleasing (as in non-magenta) skin tones and keeping the neutral axis neutral. That is my problem with Argyll - built matrix profiles with good skin tones rendered the gray midtones greenish. 
As for the tungsten - CIECAM IMO works good, the most difference between the upper and second row is the gamma curve (lower in the first row, check the neutral axis) hence the saturation loss and "out of balance".
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jrp

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Re: DCamProf - a new camera profiling tool
« Reply #1456 on: February 15, 2017, 06:33:44 pm »

If dcamprof manages to solve this problem ... the sky's the limit
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1457 on: February 17, 2017, 03:25:18 am »

I've been working on a number of issues. One is a new LUT relax algorithm that is more robust if there are some patches struggling in the opposite directions. I had problems on this with my test camera in tungsten which caused too aggressive compression in the skin-tone range, making tonality suffer. The new relax algorithm will handle those situations better.

I've also looked a bit more on screen-to-reality match, and that's ongoing. I think the dim room conditions I'm having in my home office at night this time of year is disturbing a bit, and the adaptation back and forth to the 6500K whitepoint on screen is a bit wide gap, especially when experimenting with tungsten. I'm getting less certain about the CAT02 lack of red saturation in StdA color appearance, I still think there is some lack of it, but not to the same extent as in the first rounds of experiments. I need to work more on whitepoints and test different screens etc to make sure the tuning is not disturbed by some viewing condition problem.

The off-white LUT anchor feature for StdA is probably not going to be in. It seems to me that in the end a suitable off-white creative balance is about 200K warmer than neutral-white, and that together with the better relax algorithm make the LUT shift so small it's not really worth messing with offsetting it, but it's not final on that either.

Regarding hues in StdA I think that for most cameras the case will be that you will see more of the native properties of the camera's color filters than you do in daylight. Due to the very slanted illuminant spectrum the specific choices of how they've balanced the sensor color filters will show through more. While you in theory can stretch with the LUT and create anything you want, in practice you don't want to do that as it will hurt tonality due to excessive local bending and compression. A good LUT is one that is pretty close to the matrix result.
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torger

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Re: DCamProf - a new camera profiling tool
« Reply #1458 on: February 17, 2017, 01:23:59 pm »

Worked a bit more with adapting whitepoints, even setting the screen extremely low to match my tungsten lamp at hand. The screen does look overly yellow with such a low whitepoint, which is about the mysteries of chromatic adaptation to displays which seem to be partial (that's why many use 6500K to match 5000K prints). Raising a bit I got a better perceptual match.

With (perceptually) matched screen whitepoint the CAT02 seems to be pretty much spot on, no modification required. So it seems I'll be backing out that modification too. I'm not sure yet what I'll end up with, the only modification that seems to stick so far is the improved auto-relax algorithm.

It's not over yet though. I have not yet fully understood the phenomenon that makes you set a warmer white balance to better represent the warmth of a tungsten scene, and that could still have an effect on how we want to deal with the CAT.
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daicehawk

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Re: DCamProf - a new camera profiling tool
« Reply #1459 on: February 17, 2017, 03:43:14 pm »

It is partial adaptation for sure. That is why I do not see any sense to recreate specifically the  tungsten "feel" for A conditions. In our memory the only "natural" color we have is basically the skin tone (and foliage to a smaller degree) since all the other significant colors such as sky, sea water, purple flowers, even lemons, peaches, oranges, rainbows - are memorized under all kinds of daylight. The skin tone on the other hand, is just getting WORSE under tungsten as in redder\bluer. That is the main issue. Since I do not believe in LUTs (to many chances to get into weird colors in real scenes) and "accurate" matrices (since any matrix is still a compromise due to non-L-I CFAs to be made between hue, saturation and lightness accuracy), I feel that the ultimate goal is to get a "reliable"and "mallable" matrix for any daylight (and probably an additional for A), which:
 1. recreates accurate hues of the NATURAL memory colors (skin tone being of the highest priority for non-heavy postpro PP wedding\fashion guys)
 2. provides equal perceptual saturation\lightness for all hues (a precondition for the matrix to give "mallable" material for postpro)
 3. is normalized (keeps greys greys)
 4. allows some freedom for under\overexposure.
Actually what I have found opening RAWs of different DSLRS is that most natural hues except the skin tones are quite OK without an input profile. The skin tones though are reddish\purplish. The problem with the profiling based on dE verification is that there is no weighting what patches are really of the utmost importance to be accurately reproduced and what deviation from the target is acceptable. For example, a Caucasian skin tone shifted a little bit too green from the (usual rendering) too red gives orange which means too tanned  but ok. On the other side, a little too blue from the usual too red gives purple which is a disaster, though people seem to get tolerance and even used to that!
All these thoughts and observations lead me to the conclusion that even matrices should be tweaked by hand. The DNG profile editor comes in very handy, but not without its own limitations like having a base profile and non-linear inbuilt curves which are affetcing the primary tweaking. I would prefer an opportunity to have like an 'identity" matrix and a linear curve to start with, not an ACR profile embedded to the DNG.

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