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post #61 of 136 Old 04-23-2009, 04:10 PM
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Originally Posted by gregr View Post

So do I understand that you have a CMS to make these x,y and Y adjustments to the primaries and complementary colors but it doesn't have enough range to adjust the green correctly?

Because what people prefer for color can be very subjective and what they find most pleasing is often based on what they have previously watched, which may not have been accurate. Or because what you think was accurate actually wasn't.

Exactly. I have a Samsung LN46A630. It has advanced CMS. But its green primary xy cannot get to the rec709 point. So do you think I'd be better off calibrating the secondary xy points to rec709, or calibrating to the computed secondary xy points based on my primaries (like I said, my only primary xy that's not at rec709 is green).

And by "better off" I mean seeing what the director intended. When I say that calibrating to computed secondary xy points yielded better skin tones, I don't mean brighter or flashier. I mean more realistic, natural fleshtones (often that means slightly more pale), with no green tint to people's beards, no jaundice looking people, nobody too red or sunburn looking either. they look more real.
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post #62 of 136 Old 04-23-2009, 05:09 PM - Thread Starter
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Originally Posted by dave999z View Post

Exactly. I have a Samsung LN46A630. It has advanced CMS. But its green primary xy cannot get to the rec709 point. So do you think I'd be better off calibrating the secondary xy points to rec709, or calibrating to the computed secondary xy points based on my primaries (like I said, my only primary xy that's not at rec709 is green).

And by "better off" I mean seeing what the director intended. When I say that calibrating to computed secondary xy points yielded better skin tones, I don't mean brighter or flashier. I mean more realistic, natural fleshtones (often that means slightly more pale), with no green tint to people's beards, no jaundice looking people, nobody too red or sunburn looking either. they look more real.

I just downloaded the manual to see what kind of CMS it has. It appears to segment the input colors into six independent sectors. The answer to your question probably depends on how the CMS handles the transitions between the segments. The flesh colors should primarily land in the yellow or red sectors. If you freeze the image (use a DVD player) of a closeup of someone's face, does changing the green saturation affect the flesh color? If not, I would try to get the yellow and red segments as close as possible to Rec 709 or Rec 601 and see how the flesh tones look. If the green sector setting has an effect on the flesh tones then you may have to balance that by moving the yellow segment back out (more saturation).

But BEFORE you do any of the above, make sure the color decoder is orthogonal using the NATIVE primaries as explained above, else nothing is going to make any sense in the rest of your adjustments. AND make CERTAIN that the color temperature (and grayscale) are not being affected by the CMS adjustments. i.e. check the color temp and grayscale AFTER you adjust the CMS. If the flesh colors aren't looking right the answer could be in the grayscale or the color decoder. The other possibility (actually a pretty good one in a product like this) is that the CMS is non-linear. To determine that you need to make the xyY measurements with 75% RGBYCM color windows and also with 100% color windows (you should also do it with 50% color windows but you probably don't have a source of them). You should get the same xyY results with both 75% and 100% (and 50%) color windows (Y as a percentage of white). If the CMS isn't linear (a fair chance here it isn't) then that will never allow you to get accurate color and a lot more seat-of-the-pants compromising will be necessary to get reasonable color.

It's certainly odd that the CMS doesn't have enough range to get green into position. Even when you reduce the green component of the green sector? That is a clue to me that the CMS may not be linear.

Did you follow all of that? If not feel free to ask questions before you do a lot of work.

Greg Rogers
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post #63 of 136 Old 04-23-2009, 05:27 PM
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first, thanks so much for this input on my particular situation. i'll try to absorb that. but for now...

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It's certainly odd that the CMS doesn't have enough range to get green into position. Even when you reduce the green component of the green sector? That is a clue to me that the CMS may not be linear.

The green slider for the primary color Green in the CMS only affects the luminance (Y) of Green. Move it way up or down and all that changes is Y, while xy stays exactly the same. And my current setting for Green is R=22, G=44, B=0. So, I cannot take away any more blue. I know lots of others have reported this issue with these Samsung sets. I can nail the Rec709 for the Red and Green primaries. Just can't do it for Green... I can increase Green's x position by adding red (though that lowers Green's y position a bit). But there is absolutely no way to increase Green's y position. It's max y position is pinned slightly below the Rec709 position.
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post #64 of 136 Old 04-23-2009, 05:44 PM
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Originally Posted by gregr View Post

If you freeze the image (use a DVD player) of a closeup of someone's face, does changing the green saturation affect the flesh color? If not, I would try to get the yellow and red segments as close as possible to Rec 709 or Rec 601 and see how the flesh tones look. If the green sector setting has an effect on the flesh tones then you may have to balance that by moving the yellow segment back out (more saturation).

All I can do is decrease the Green saturation (by adding equal amounts of R and B), I cannot increase the Green saturation (because I'd have to subtract equal amounts of R and B, but B is already at zero).

When I pause on an image of a face and decrease the Green saturation, I do not really see the fleshtone changing. Maybe by the time I increase R from 22 to way up to like 82 and simultaneously B from 0 to 60, I see a very slight change in fleshtones (I may be seeing a yellowish shadow go slightly more red, but it's barely noticeable), but not really. (Note that my secondaries are currently calibrated to the computed secondary xy targets, not to Rec709.) I'm not sure exactly what that experiment tells me?
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post #65 of 136 Old 04-23-2009, 05:56 PM - Thread Starter
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Originally Posted by dave999z View Post

But there is absolutely no way to increase Green's y position. It's max y position is pinned slightly below the Rec709 position.

Ah.... the green is less saturated than Rec 709. Surprise! I just guessed it was way more saturated than Rec 709 because that is so common these days. So what is the x,y position of green?

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post #66 of 136 Old 04-23-2009, 06:30 PM
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I believe my A650 behaves very similar to Dave's A630, so I will share with you the color points I measure.

The native green of the set is not so much undersaturated as the wrong hue. The native green point has an (x,y) of (.280,.594). It only becomes undersaturated when one tries to make it approach the Rec709 spec; on my set I have this now at (.300,.588) when measuring 75% colors.

The linearity of the set is not ideal. This does not show up as much at green as it does at red. Comparing the green 75% stimulus (.300,.588) to 100% stimulus (.301,.590), these measures seem roughly within measurement error of the EyeOne. The corresponding red points are (.630,.337) at 75%, (.635, .337) at 100%, a bigger difference than I see in just measurement variation.
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post #67 of 136 Old 04-23-2009, 06:55 PM - Thread Starter
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Originally Posted by Bill Mitchell View Post

I believe my A650 behaves very similar to Dave's A630, so I will share with you the color points I measure.

The native green of the set is not so much undersaturated as the wrong hue. The native green point has an (x,y) of (.280,.594). It only becomes undersaturated when one tries to make it approach the Rec709 spec; on my set I have this now at (.300,.588) when measuring 75% colors.

Thanks Bill. That isn't bad at all. If the Y value is correct (0.7152) that's a dE of just 3.9, which is pretty negligible.

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The linearity of the set is not ideal. This does not show up as much at green as it does at red. Comparing the green 75% stimulus (.300,.588) to 100% stimulus (.301,.590), these measures seem roughly within measurement error of the EyeOne. The corresponding red points are (.630,.337) at 75%, (.635, .337) at 100%, a bigger difference than I see in just measurement variation.

That's a bit more error in red than I would like if I were targeting Rec 709 (it's closer to SMPTE C), but it's not too bad a shift for red (a shift of about 3 dE from 7.7 to 10.4) if the Y value is correct and holds between 75% and 100%. Do you know how much the Y values are changing? If they are changing very little than I would consider this relatively linear, at least between 75% and 100%.

I need to go out for a while. I'll respond again later if there is some Y value info.

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post #68 of 136 Old 04-23-2009, 07:25 PM
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The linearity is not bad as regards Y, the largest variation in Y shows up at blue instead of red. For a more complete set of (x,y, scaled Y) measures:

Code:
                75%                100%
red      (.630,.336,.2221)   (.635,.337,.2224)
green    (.300,.588,.7062)   (.301,.590,.7069)
blue     (.151,.064,.0765)   (.153,.067,.0797)
As you may infer from the numbers above, I calibrated for the actual gamut of where the primaries lie. In part, I went this direction after looking at Lindbloom's equations, where he describes the gamut as defined entirely by the white point and the x,y coordinates of the primaries. But with the user controls, there is only so much granularity available, especially in the control over Y, and there is the measurement variation of the EyeOne. Regardless, the measures indicate the change in scaled Y depending on whether 75% or 100% stimulus patterns are used. Some of the difference, I'm sure, comes from variation in the grayscale, as it is calibrated to be balanced at IRE75/IRE80, so some error creeps in at IRE100. As you noted above, all of these points are very close to the targets, just not perfectly on, so whichever approach is used we are talking about small differences here.

As you suggest, the native gamut is big enough that one can hit the SMPTE-C primaries exactly. And the native green is not very far away from the PAL primary.

I did appreciate your recommendation to use the calculator on the native color gamut first to check for linearity in the basic CMS system. That step in the process had not yet occurred to me, although I did something like that when I was checking if there were any color decode errors in my player or set.
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post #69 of 136 Old 04-23-2009, 08:03 PM
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Originally Posted by dave999z View Post

Exactly. I have a Samsung LN46A630. It has advanced CMS. But its green primary xy cannot get to the rec709 point. So do you think I'd be better off calibrating the secondary xy points to rec709, or calibrating to the computed secondary xy points based on my primaries (like I said, my only primary xy that's not at rec709 is green).

Dave, to be honest, I don't think you'll visually see a difference between calibrating to your "custom" gamut secondaries/luminance values vs. targeting the rec709 reference. I was just looking at your measures that you posted in the other thread (which I'll reply to in more detail in that thread) and green is sitting at .300, .584 - essentially, it is just slightly undersaturated - this means that magenta's optimal location is pretty much the same as the reference hd709 magenta. - .321, .155 vs. .321,.154 assuming a D65 white point (it's .325,.157 using your measured 100% white point). Cyan is shifted slightly on the x axis but once again, it's only .002. As for luminance, while the Y values are slightly different for all colors, they are once again extremely close. For example, blue is .0703 (using your measured primaries and D65 white point) vs. .0722 which is a difference of 2.6%. The rest are even closer I believe. These numbers change slightly when I plug in your measured white point at 100% (since you used 100% color window patterns for measures) but they don't change drastically since your white point is fairly close to D65.

Here are your dELUV values from HCFR with 709 as the reference:

R G B Y C M
0.8 5.3 2.2 3.1 2.4 4.2

As Greg says, if your dELUV values are <= 5, then you're doing pretty good. Looking at your color measures again and CIE chart, in order to target 709, about all you could do is move magenta and yellow both a bit to the left - very slight. As for color luminance values, the only one I'd change to better match 709 is blue - increase it slightly but since the overall dE value is 2.2 (and the luminance error is -2.2%), the change would most likely not be visually noticeable.

hope this helps,


--tom
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post #70 of 136 Old 04-23-2009, 08:42 PM
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Originally Posted by gregr View Post

Now I'm the one that is confused. Are you trying to adjust a TV using just its Color and Hue (Tint) controls? The calculator is really aimed at doing a display calibration with some type of a color management system (CMS). It you only have Color and Hue controls you are extremely limited in what you can do.

YES! You have it now. If the lines connecting primaries to their corresponding complementary colors ALL intersect at the white reference, then your YCbCr->RGB color decoder is working in a standard manner (it is "orthogonal" in technical terms) and there are no other "odd" non-linearities in the system. This a pre-requisite to achieving totally accurate color using YCbCr (digital) or YPbPr (analog) input signals. (If you input RGB signals, then this should always be true because there is no color decoding.) Once you have the color decoding correct, it is then necessary to have the correct RGB primary x,y values and the correct white point (and a constant gray scale) to achieve totally accurate color. And that requires a CMS or a display with standard primaries.

If you only have Color and Hue controls (no CMS) then the only things you can do to improve color accuracy if you have non-standard primary colors is to "slightly" modify the white reference point to shift the complementary colors closer to the standard, and/or to use the Hue control to simultaneously rotate all of the complementary colors closer to their standard targets. The calculator can help you minimize the errors when making those adjustments. When you are done with that you will probably not like the flesh colors on every piece of video you watch. You would then adjust the Color control to optimize the appearance of the flesh tones, and you might find the Color control would be a little different for different source material (such is the nature of trying to compromise a non-accurate system).

If you have TV with service level adjustments for the color decoder, you could also modify the color decoding (so it's no longer orthogonal - as defined above) which can also help to improve the overall color accuracy in a system with the wrong primary colors. But that is definitely not something anyone should do unless they have great experience in these matters, because A) you are more likely to make things worse than when you started even if it looks better to you on a few things, and B) if you aren't careful keeping exact track of what you did, you may never get back to the factory setup. This is a real recipe for disaster - you were warned!

I have read the next several posts, and I am in the same Samsung boat as Dave and Bill. My A-750 has a fairly capable CMS even without going into the service menu. The main problem is with green's x coordinate being too low. Each primary color has 3 different sliders.

Take green for example. It has a slider for red, green and blue. Changing the green slider changes green's Y value, and since this starts off in the middle of the slider at 50 (slider ranges are all 0-100) this means the Y value can be set to essentially anything it needs to be. Changing the red or blue sliders for the green primary change the xy coordinates of green. These sliders start at a default setting of 0, so green can only be "pulled" towards them. Consequently green's main problem of too low x value is somewhat correctable by adding some red. This "pulls" green towards red, but the line between green and red do not pass through the proper Rec709 position for green. The line passes below, i.e., with a lower y value. The green primary can be moved to the perfect x coordinate, but at the expense of an incorrect (too low) y coordinate. Undoubtedly there is a spot between these two extremes where the dE of green would be lowest. Perhaps this would be a worthwhile thing to experiment with. Remember green Y (and all other Ys for RGBYCM) can be adjusted close to perfection.

The main problem with the Samsung CIE is this green issue. The secondary effect of the green issue is that all secondaries are effected, but mostly magenta and cyan are very much effected by this. In the situation above where green is pulled all the way to the correct x coordinate would result in orthogonal (did I use that correctly?) Rec709 magenta. But the line between blue and green would be inside/to the right of the Rec709 CIE, and cyan would be effected (x too high). Leave green where Samsung put it and the issue is whether cyan should be placed on the line from Samsung blue to Samsung green or on the line from Rec709 blue to Rec709 green. We can get cyan to either of these points with the Samsung CMS, and as already stated we can get the Y values to whatever we need them to be.

The main thing I just cant get my head around is how important convergence of the lines connecting primaries and secondaries at the white point is. I'm not sure, but I don't think your calculator includes this in any of the dE calculations. Consequently moving the secondaries to Rec709 xy coordinates will result in lower dE values even though these (Rec709) secondary xy coordinates are not orthogonal relative to the primaries' (non-Rec709) xy coordinates.

Adjust the primaries' Y values and the secondaries' xyY values to Rec709 and the calculator yields low (as low as possible) dE values, but there is no convergence at white.

Adjust the primaries' Y values and the secondaries' xyY to the values your calculator produces when actual native Samsung RGB xy and white xyY values are input, and you get higher dE values but you also get convergence at white and orthogonal relationships between primaries and their secondaries. In the case of the Samsungs, this results in the biggest change in magenta, which is shifted way up the blue-red line towards red.
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post #71 of 136 Old 04-23-2009, 09:54 PM
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Originally Posted by kjgarrison View Post

Adjust the primaries' Y values and the secondaries' xyY to the values your calculator produces when actual native Samsung RGB xy and white xyY values are input, and you get higher dE values but you also get convergence at white and orthogonal relationships between primaries and their secondaries. In the case of the Samsungs, this results in the biggest change in magenta, which is shifted way up the blue-red line towards red.

Well put. And I think this option is what I'm going to stick with for now. It has really yielded the most realistic (or, rather, the least unrealistic) skin tones. I hope eventually to understand why that is.
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post #72 of 136 Old 04-24-2009, 12:21 AM - Thread Starter
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Originally Posted by Bill Mitchell View Post

The linearity is not bad as regards Y, the largest variation in Y shows up at blue instead of red. ...

The shift in Y for blue actually reduces the error at 100% compared to if the Y value stayed the same as for 75%. From a practical point of view errors less than dE=3 are nearly insignificant in real video images, so the blue primary errors are negligible. A shift of 1 dE, such as occurs for green is practically undetectable in real video images, and even a shift of 2 dE would be very hard to spot except under really special conditions. So the linearity here is really very good (congrats to Samsung for doing that right!) but you would like to be just a bit closer on the red primary to comply with Rec 709. The red primary is really in the middle between Rec 709 and SMPTE C. Red relative to SMPTE C - dE=5.8 at 75% and dE = 8.2 at 100%. I added the dE values compared to Rec. 709 below.

Code:
                75%                100%
red      (.630,.336,.2221) dE=6.8  (.635,.337,.2224) dE=4.9
green    (.300,.588,.7062) dE=4.6  (.301,.590,.7069) dE=4.1
blue     (.151,.064,.0765) dE=1.2  (.153,.067,.0797) dE=1.8
Quote:
Originally Posted by Bill Mitchell View Post

Some of the difference, I'm sure, comes from variation in the grayscale, as it is calibrated to be balanced at IRE75/IRE80, so some error creeps in at IRE100.

Yes, grayscale errors always help to create color errors. But this is not bad performance at all for an LCD display.

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post #73 of 136 Old 04-24-2009, 12:57 AM - Thread Starter
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Originally Posted by kjgarrison View Post

I have read the next several posts, and I am in the same Samsung boat as Dave and Bill. My A-750 has a fairly capable CMS even without going into the service menu. The main problem is with green's x coordinate being too low. Each primary color has 3 different sliders.....

Yes, once someone told me what product you guys were trying to calibrate I downloaded the manual and saw the type of CMS controls it had. You confused me earlier when you said you couldn't change the x,y point of green. That made me think you had no CMS. But obviously you can change the x,y point of green, just not to where you really want it. Anyway, the errors in the numbers that Bill provided are not too bad as I discussed above. The biggest error is really in red in Bill's case, but that could vary with different units and different cal's. Nevertheless, if you are getting values close to dE=5 or less (as is generally the case here) you are doing pretty good. dE of 3 or less for the primary colors is really good, especially if the CMS adjusted gamut is linear (as it is here). I sometimes see people calculating dE values to 2 or 3 decimal places and that is just silly. No one can recognize those kinds of differences, and one decimal place is enough to check the linearity itself.

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Originally Posted by kjgarrison View Post

Leave green where Samsung put it and the issue is whether cyan should be placed on the line from Samsung blue to Samsung green or on the line from Rec709 blue to Rec709 green. We can get cyan to either of these points with the Samsung CMS, and as already stated we can get the Y values to whatever we need them to be.

The errors are small enough that I would always adjust for the best agreement with the Rec 709 targets. There is no need to worry about discontinuity issues across the CMS segments with errors this small.

Quote:
Originally Posted by kjgarrison View Post

The main thing I just cant get my head around is how important convergence of the lines connecting primaries and secondaries at the white point is. I'm not sure, but I don't think your calculator includes this in any of the dE calculations. Consequently moving the secondaries to Rec709 xy coordinates will result in lower dE values even though these (Rec709) secondary xy coordinates are not orthogonal relative to the primaries' (non-Rec709) xy coordinates.

It's not important that the lines intersect at the white reference point when you are adjusting a CMS. You should be adjusting for minimum dE error vs the standard target at each primary and secondary. There is no practical significance to where the lines intersect in that case. The significance to where the lines intersect describes the performance of the color decoder, which can only be assessed relative to the native primaries, or if the pseudo primaries are created using a simple RGB to RGB linear matrix transform (sometimes called a 3-axis CMS). But when using a 6-axis CMS (which is what this is) there is no longer any significance to straight lines intersecting at the white point in the final calibrated result. (Don't let this confuse you, but the 6-axis CMS is really a non-linear transform made up of 6 linear color gamut segment transforms. The lines pass between segments and therefore are no longer straight lines, but rather two line segments with a break point at the white reference, so they really do still intersect at the white reference point.) So forget about the straight lines when using a 6-axis CMS, although if you get the primary and complementary colors perfect, and the white reference point perfect, they will be straight lines again.

Greg Rogers
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post #74 of 136 Old 04-24-2009, 04:31 AM
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Originally Posted by gregr View Post

It's not important that the lines intersect at the white reference point when you are adjusting a CMS. You should be adjusting for minimum dE error vs the standard target at each primary and secondary.

Greg, not to take this off the topic too much or start a heated discussion but I was wondering if you could comment on the "School 2" approach that says always match the secondaries/luminance based on your custom primaries unless your CMS allows you to move your colors to the precise locations of the standard target. For example, in reading Tom H's articles recently in Widescreen Review, he essentially outlines this approach and I guess I'm wondering if I'm misinterpreting what he is really saying or if there are really two schools of thought here. I understand all the math and actually wrote my own color gamut calculator (along with various error calculations) awhile ago but I've never quite wrapped my head around the two schools of thought here or if there really are two schools of thought. Thanks much for any insight.

cheers,


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post #75 of 136 Old 04-24-2009, 07:18 AM
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Quote:
Originally Posted by gregr View Post

...
It's not important that the lines intersect at the white reference point when you are adjusting a CMS. You should be adjusting for minimum dE error vs the standard target at each primary and secondary. There is no practical significance to where the lines intersect in that case. The significance to where the lines intersect describes the performance of the color decoder, which can only be assessed relative to the native primaries, or if the pseudo primaries are created using a simple RGB to RGB linear matrix transform (sometimes called a 3-axis CMS). But when using a 6-axis CMS (which is what this is) there is no longer any significance to straight lines intersecting at the white point in the final calibrated result. (Don't let this confuse you, but the 6-axis CMS is really a non-linear transform made up of 6 linear color gamut segment transforms. The lines pass between segments and therefore are no longer straight lines, but rather two line segments with a break point at the white reference, so they really do still intersect at the white reference point.) So forget about the straight lines when using a 6-axis CMS, although if you get the primary and complementary colors perfect, and the white reference point perfect, they will be straight lines again.

Thanks for the comment, Greg. I was trying to work through the implications of that in my head. Clearly, if one had a CMS with only 3 control points, the primaries, without any white point adjustment, it would be critical to use the Y values associated with the actual primaries to get them to add up to the intended white point. But given that we have six control points, seven if you count the white balance control, we are not simply stretching a triangle between three points, rather we are configuring six triangles each of which can stretch a little differently. (I say stretch, because I am always using this mental picture of a tent to represent the Y component, where the tent poles are at white and the six color points.) It is a complex process to model and to understand all the implications.
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post #76 of 136 Old 04-24-2009, 10:27 AM
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Thank you for your patience Greg.

Sometime in the next few weeks, when I finish a big project, I'm going to do two calibrations. One with "method 1" (minimize dE, which is your recommended method), and one with "method 2" which is the one your calculator yields with (let's call it) "Samsung primaries" (perhaps slightly modified in the case of green if that yields a lower dE for green). I'll post pictures, unless everybody has ColoHCFR in which case I'll just post the files.

I actually have done both of these previously (except for tweaking green away from Samsung green to try to get a lower dE), and my opinion of the color performance of my TV is that method 2 looks better. My main "test" of color is human skin. That is one thing that I believe I can tell when it's not right (especially when it's green tinted or sunburned looking). As to the other colors, who can tell if the green of a field of grass is "off". It doesn't bother even if it is off a little. Same for sky blue or stop-sign red. But skin is a different story.
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post #77 of 136 Old 04-24-2009, 10:53 AM - Thread Starter
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Thanks for the comment, Greg. I was trying to work through the implications of that in my head. Clearly, if one had a CMS with only 3 control points, the primaries, without any white point adjustment, it would be critical to use the Y values associated with the actual primaries to get them to add up to the intended white point.

Yes, all that is really needed is a simple 3-axis (RGB) CMS that includes a 3x3 linear matrix multiple to convert one set of RGB values relative to one (standard or pseudo) color gamut to another set of RGB values relative to another (native) color gamut. The math is simple and the hardware implementation is easy. The only "catch" is that the transform must be executed on linear RGB values and not the R'G'B' gamma corrected values of the input signal. That potential (cost savings) error is the primary source of CMS non-linearities in bad implementations. The luminance balancing of the pseudo-primaries can be done in several ways, but simply remixing the grayscale using the resulting pseudo-primaries is sufficient. That automatically produces the correct complementary xyY values as well. An added bonus is that the user (or calibrator) only needs to adjust 3 primary colors for x,y (plus the grayscale) and not 6 colors for x,y and Y (plus the grayscale). Of course the YCbCr->RGB color decoding must also be correct (orthogonal) in the display.

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But given that we have six control points, seven if you count the white balance control, we are not simply stretching a triangle between three points, rather we are configuring six triangles each of which can stretch a little differently. (I say stretch, because I am always using this mental picture of a tent to represent the Y component, where the tent poles are at white and the six color points.) It is a complex process to model and to understand all the implications.

The 6-axis CMS is really a marketing "feature" in a display. (In an external processor, a 6-axis CMS can be a benefit to help correct for bad color decoding or other non-standard color processing in a display, but in a display all of those things are already under the control of the manufacturer.) As I said above, it is easier to implement, and easier to correctly adjust (i.e. it is likely to produce better results in actual practice), a simple 3-axis CMS. But what manufacturer is willing to sell a display that only has RGB gamut adjustments to mess up when its competitors have RGBYCM gamut adjustments to mess up? (The answer was Yamaha, that provided both in its 1080p front projectors.)

Once you go to a 6-axis CMS there are lots of ways to implement it, but a simple basic approach is to divide the color gamut into 6 segments, each centered around one of the primary or complementary colors. Each segment shares boundaries with its adjacent segments. You can compute the angle of the incoming video signal very easily to determine which segment the signal falls into at any instant. Then each segment can be treated separately as its own 3-axis linear transform similar to the simple case above. (That is why I asked earlier if the green segment adjustments affected the out-of-segment flesh colors in this display.) It is necessary to deal with potential noise effects along the boundaries between segments, and again you must do all of the calculations using linear RGB signals and not gamma corrected RGB signals. White/gray signals (equal RGB component values) are treated as a special case, which can then retain the original calibrated grayscale if done right - but other things can also be done with white/gray calibration. The realtime hardware implementation of a 6-axis CMS system is more complex and can be done with a complete 3-D LUT (lookup table) which is normally not used because of cost implications in a product like this, or can be done with a pseudo 3-D LUT and interpolation (less expensive), or it can be done with parallel multipliers and other computational logic. The human interface functions to allow the CMS to be adjusted can sometimes get more complex to design/implement than the actual CMS implementation functions. The human interface in the Samsung is simple and effective.

(In answer to the question asked above by thomasl) Since the 6-axis CMS has independent linear color gamut segments (in general) the minimum overall color accuracy error is achieved by minimizing the color error of each segment at its reference color (primary or complementary color). Lines connecting primaries to opposing complementary colors will be straight lines if the CMS can be adjusted to the ideal targets and reference white, but if not, then the connecting lines will consist of two segments that break at the white point. There is no advantage (indeed there is an accuracy penalty) to manipulating the segment reference colors by introducing larger errors in their x,y positions just to create straight primary-complementary connecting lines.

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post #78 of 136 Old 04-24-2009, 10:57 AM - Thread Starter
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Greg, not to take this off the topic too much or start a heated discussion but I was wondering if you could comment on the "School 2" approach that says always match the secondaries/luminance based on your custom primaries unless your CMS allows you to move your colors to the precise locations of the standard target. For example, in reading Tom H's articles recently in Widescreen Review, he essentially outlines this approach and I guess I'm wondering if I'm misinterpreting what he is really saying or if there are really two schools of thought here. I understand all the math and actually wrote my own color gamut calculator (along with various error calculations) awhile ago but I've never quite wrapped my head around the two schools of thought here or if there really are two schools of thought. Thanks much for any insight.

cheers,


--tom

Tom, attend "School 1" and minimize color errors. See my answer at the end of my post just above this one.

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I think the best way to answer your main question is to ask why you would want to aim at something other than the standard (Rec. 709 or Rec. 601)? If the optimum color accuracy is achieved by matching one of those standards, why would you want to aim for anything else? If you don't have the adjustments necessary to exactly achieve the standard colorimetry, why would you think exactly achieving a wrong colorimetry would be better than getting as close as you can to the right colorimetry? By adjusting for the minimum xyY errors (measured by dE) relative to the standard colorimetry you are effectively minimizing the errors within the 3-D (xyY) color gamut. And that is one of the 4 uses I explained for the calculator.

I'm not sure what you meant when you said "distorted" triangle, but it seems like that is influencing your thinking. Any set of three primaries creates a color gamut triangle in an x,y plane, so I don't see how any choice is "distorted". But the objective to color accuracy is to not only match the x,y coordinates of the target primaries (and complementary colors), but to also match the correct luminance (Y) of the target primaries (and complementary colors). The latter is just as much a contributor to the perceived color accuracy errors as the x,y values. So we need to think in 3-dimensions to measure the color errors and that is what dE does for us. If I have the ability to adjust the Y values of the primaries/complementaries independent of their x,y values, then I should do so with the purpose of minimizing the color accuracy errors relative to the standard, not to simply put the Y values where they would have been on a display with the wrong primaries. Again, what would the point be of exactly achieving a wrong target, rather than getting as close as possible to the right target?

Greg - I think some of the confusion here lies in which tool/set of controls are being used at a given point in time. The distinction here needs to be made between a CMS and a color decoder. I have argued (and I believe that you have as well) to use the native gamut of the display to set the targets for the complementary (secondary) colors when adjusting/calibrating a color decoder. Some try to adjust the decoder controls to make the secondary colors better align with the given target standard. Personally, I feel this later approach just shifts color error around, rather than really driving a net decrease in total error in the system.

When using a CMS, of course, you calibrate to the defined targets, within the limits of what the CMS and the native gamut of the display can do.

Bill

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Sometime in the next few weeks, when I finish a big project, I'm going to do two calibrations. One with "method 1" (minimize dE, which is your recommended method), and one with "method 2" which is the one your calculator yields with (let's call it) "Samsung primaries" (perhaps slightly modified in the case of green if that yields a lower dE for green).

That would be great to see. When you do it, please make clear what you mean by "Samsung" primaries, because there are two options: (i) the default Samsung xy primary points (all of which differ from 709), or (ii) the "as close to rec709 as Samsung can get" xy primary points (in which case only Green differs from 709, but is closer to the 709 point than it is if left in the default Samsung position).

If that makes any sense.
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I actually have done both of these previously (except for tweaking green away from Samsung green to try to get a lower dE), and my opinion of the color performance of my TV is that method 2 looks better. My main "test" of color is human skin. That is one thing that I believe I can tell when it's not right (especially when it's green tinted or sunburned looking). As to the other colors, who can tell if the green of a field of grass is "off". It doesn't bother even if it is off a little. Same for sky blue or stop-sign red. But skin is a different story.

I agree that flesh colors are very important, perhaps the most important single noticeable feature. But dave999z said above that changing the green segment (as an example) had no effect on the flesh tones except perhaps at some extreme adjustment limit. So why adjust any segment that doesn't affect the flesh tones to create more error in that segment? Sorry, but I just don't get it. (I can see the red color error on a stop sign, or a bottle of Heinz Ketchup, or green error on fields where the green primary is out of whack - see the JVC DLA-RS2) Anyway, Caucasian flesh colors normally fall into the yellow or red segments. If you adjust those segments for minimum error and still don't like the skin colors, then you need to consider the reference white point and the grayscale tracking as the possible reason, or perhaps the accuracy of your color measuring equipment, or some other explanation. But the bottom line is this - when you are happy with the color, regardless of how you got there, stop tweaking and start watching!

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post #82 of 136 Old 04-24-2009, 11:42 AM
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Greg - I think some of the confusion here lies in which tool/set of controls are being used at a given point in time. The distinction here needs to be made between a CMS and a color decoder. I have argued (and I believe that you have as well) to use the native gamut of the display to set the targets for the complementary (secondary) colors when adjusting/calibrating a color decoder. Some try to adjust the decoder controls to make the secondary colors better align with the given target standard. Personally, I feel this later approach just shifts color error around, rather than really driving a net decrease in total error in the system.

Bill, yes, that crystallizes exactly what I was thinking and am wondering about - if the school 1 vs. school 2 is simply related to how to treat the color decoding controls on the display (optimize for the display's native gamut or optimize to a target standard). When a CMS is involved, does anyone advocate getting the primaries as close as possible and then adjusting the luminance and secondaries to match that new pseudo-gamut created via the CMS? i.e. one should always get the lines straight from the primaries to the secondaries.

It seems to me that optimizing the color decoder controls (color/tint usually) based on the native gamut and then using a CMS to try and get all primaries/secondaries as close to the chosen target standard as possible (both location and luminance) seems to be a reasonable approach. I should mention in re-reading Tom H's articles, I believe this is the gist of what he is advocating as well but I don't want to put words in his mouth.

cheers,


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I agree that flesh colors are very important, perhaps the most important single noticeable feature. But dave999z said above that changing the green segment (as an example) had no effect on the flesh tones except perhaps at some extreme adjustment limit. So why adjust any segment that doesn't affect the flesh tones to create more error in that segment? Sorry, but I just don't get it.

Here's why (I think)... the issue is not Green. Under either method, we're going to calibrate Green to be as close to the 709 standard as possible. The issue is what to do after that with the secondaries. We can either calibrate Yellow and Magenta to as close to the 709 standards as possible, or we can calibrate Yellow and Magenta to the values computed by your calculator. And the Yellow and Magenta xyY will have an effect on skin tones. So, that's the choice.
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(In answer to the question asked above by thomasl) Since the 6-axis CMS has independent linear color gamut segments (in general) the minimum overall color accuracy error is achieved by minimizing the color error of each segment at its reference color (primary or complementary color). Lines connecting primaries to opposing complementary colors will be straight lines if the CMS can be adjusted to the ideal targets and reference white, but if not, then the connecting lines will consist of two segments that break at the white point. There is no advantage (indeed there is an accuracy penalty) to manipulating the segment reference colors by introducing larger errors in their x,y positions just to create straight primary-complementary connecting lines.

Thanks Greg for the explanation of this and how 3-axis and 6-axis CMSes work. I can see why you wouldn't want to just create straight lines from the primaries to the secondaries.

cheers,


--tom
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Greg - I think some of the confusion here lies in which tool/set of controls are being used at a given point in time. The distinction here needs to be made between a CMS and a color decoder. I have argued (and I believe that you have as well) to use the native gamut of the display to set the targets for the complementary (secondary) colors when adjusting/calibrating a color decoder.

Bill, Absolutely! I made exactly that point a couple of times earlier in this thread - read back farther. As I said earlier, you must first adjust the color decoder for orthogonal performance with the native primaries, then adjust the CMS.

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Some try to adjust the decoder controls to make the secondary colors better align with the given target standard. Personally, I feel this later approach just shifts color error around, rather than really driving a net decrease in total error in the system.

I agree it shifts the color errors around. If you have no CMS, then shifting the complementary colors with the Hue control (not the Color control which leads to bigger problems) may improve the overall color accuracy, but it may also make it worse at critical colors like fleshtones. It really depends on the positions of the primaries and which complementary color is optimized. Improving yellow can often improve fleshtones, while optimizing cyan at the expense of yellow may make them worse. What it really boils down to is trying to compensate for a fundamentally inaccurate system with inadequate adjustments is a subjective crapshoot, and that is what you sometimes end up with.

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When using a CMS, of course, you calibrate to the defined targets, within the limits of what the CMS and the native gamut of the display can do.

Absolutely, but there seems to be a strong pull in the force here for people to "toe a straight line".

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there seems to be a strong pull in the force here for people to "toe a straight line".

It's just empirical. It yielded better flesh tones. I would think targeting the standard would give the best result. But, for me at least, it didn't. I am not complaining. I am just hoping to understand why, because... well, I want to.
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I agree it shifts the color errors around. If you have no CMS, then shifting the complementary colors with the Hue control (not the Color control which leads to bigger problems) may improve the overall color accuracy, but it may also make it worse at critical colors like fleshtones. It really depends on the positions of the primaries and which complementary color is optimized. Improving yellow can often improve fleshtones, while optimizing cyan at the expense of yellow may make them worse. What it really boils down to is trying to compensate for a fundamentally inaccurate system with inadequate adjustments is a subjective crapshoot, and that is what you sometimes end up with.

As a hobbyist who actually still doesn't own a display with a CMS , I can say that moving the secondaries around with just a tint control is crude at best. Maybe it's just me but I seem to be annoyed most by yellow being too far off either toward red or toward green. So, I've typically adjusted things so yellow is at least optimized. Usually this has led to just setting Tint so two (with yellow being one) secondaries are optimized - i.e. moving the Tint control may improve the one other secondary but at the expense of the other two. Luckily, all my displays except for one have fairly decent primaries compared to 709 and have decent overall color decoding thus nothing is way out of whack.

cheers,


--tom
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post #88 of 136 Old 04-24-2009, 12:53 PM - Thread Starter
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We can either calibrate Yellow and Magenta to as close to the 709 standards as possible, or we can calibrate Yellow and Magenta to the values computed by your calculator. And the Yellow and Magenta xyY will have an effect on skin tones. So, that's the choice.

That is a choice created by misusing the calculator.

I don't know how many more times I have to say this - The Calculator is designed to help you calibrate a CMS to get "Yellow and Magenta to as close to the 709 standards as possible". That is why the Rec 709 (and SMPTE C, and EBU) color standards have default buttons in the calculator.

It is really a MISUSE of the calculator to use it to adjust a CMS to get "straight-lines" connecting the primary and complementary colors when the primary colors are incorrect. There is no rationale for doing that. If you do that, you are choosing to calibrate to a custom target with no rationale for why that would give you more accurate color. That is a choice you would have invented - not the calculator. (The calculator allows you to create that choice because it allows custom targets. There are reasons you may want to use it with a custom target - to use other color gamut standards, or with the measured native primaries to test external processors, etc. But it was not intended to be used to force "straight lines" while calibrating a CMS because there is no logical justification for doing that.)

The Calculator is also used to adjust a color decoder with the display's native primaries. That is a different use - an additional step before adjusting the CMS.

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greg - i think some of the confusion here lies in which tool/set of controls are being used at a given point in time. The distinction here needs to be made between a cms and a color decoder. I have argued (and i believe that you have as well) to use the native gamut of the display to set the targets for the complementary (secondary) colors when adjusting/calibrating a color decoder.

bill

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bill, absolutely! I made exactly that point a couple of times earlier in this thread - read back farther. As i said earlier, you must first adjust the color decoder for orthogonal performance with the native primaries, then adjust the cms.


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bill, absolutely! I made exactly that point a couple of times earlier in this thread - read back farther. As i said earlier, you must first adjust the color decoder for orthogonal performance with the native primaries, then adjust the cms.

JUST when I thought I was getting it along comes this interaction. To me the bold part of bear5k's post IS method 2. Then Greg agrees with it!

Then Greg says to "first adjust the color decoder" (with "method 2"). What is the distinction between adjusting the color decoder and changing things in the CMS? How do I adjust the color decoder???? and how does this help with adjusting the cms if the cms is to be adjusted to a standard (Rec709)?
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Then Greg says to "first adjust the color decoder" (with "method 2"). What is the distinction between adjusting the color decoder and changing things in the CMS? How do I adjust the color decoder???? and how does this help with adjusting the cms if the cms is to be adjusted to a standard (Rec709)?

The two basic color decoder controls are the color (saturation) and tint (hue) controls. Traditionally, these have been used to control the conversion (decoding, as it were) of component video (analog: YPbPr, digital: YCbCr) into RGB. You can set these controls visually, using filters and appropriate test patterns, or you can set them using a meter (a spectro is best, but some colorimeters will do). The mechanics of setting the decoder are covered in some of the more novice-oriented test discs, as well as in the help files of some calibration software. The same math that lies at the heart of Greg's calculator is what sits at the heart of the Color Target Editor in CalMAN.

One other source of confusion on this subject is Tom Huffman's big CMS thread that is stickied at the top. He and I have had some unfortunate public exchanges on this subject, but when Tom says:
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4. Set the color control at the point where Red measures closest to 21% of the white reading.

He is not advocating the same methodology as what is outlined here for setting the color decoder. Instead, this is using the color decoder as a supplement to whatever CMS you are using, and sticking to colorimetry of Rec 709 as the targets for adjusting the decoder. The fact that Tom continues to talk about setting color and tint as being a separate step from setting the color decoder (and separate from Saturation and Hue), I fear really only adds to the confusion.

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