Display Calibration Calculator - Page 2

I've answered this before but I'll try again.

1. The FIRST purpose of the AccuPel Display Calibration calculator is to adjust the display for STANDARD YCbCr/YPbPr to RGB Color Decoding. Most front projectors will have standard YCbCr/YPbPr to RGB color decoders, but Tint (Hue) and Color controls modify the color decoding. (Many non-front projectors have non-standard color decoders, but may include as many as 6 additional specific color decoder adjustments in a service menu.)

In this case you should enter your measured primaries and gray reference in the Target Values at the top of the Color Gamut screen and press Compute. The calculator will compute the Y values of the primary and the x,y,Y values of the complementary colors with standard color decoding. You can enter your measured primary and complementary color values at the bottom as you adjust the color decoding and the calculator will compute the dE (CIELUV 1976 - my preference) deviation from the Target values each time you press the Compute button.

2. The SECOND purpose of the AccuPel Display Calibration calculator is to adjust the display's total colorimetry to match a Target display colorimetry "as closely as possible". If the display (or an external processor) has a fully functional CMS system, then I recommend the display's Color Decoder first be adjusted for Standard color decoding (#1 above), and then the CMS processing be adjusted to match the Target display colorimetry. This is the ideal/preferred case. However, many displays have no CMS or an incomplete CMS, and in that case it may be necessary to adjust whatever controls are provided (partial CMS and/or Color Decoding) to match the Target display colorimetry "as closely as possible".

You must decide what Target display colorimetry you want to try to match. You may choose a Target display colorimetry based on a set of Standard primaries (Rec. 709, SMPTE C, or EBU) or you may enter a set of Custom/Measured primaries and a gray reference. If you enter Custom/Measured primaries or a non-D65 gray reference and then press the Compute button, the calculator will compute Target x,y,Y values based on Standard Color Decoding. If you enter measured x,y,Y values of the primary and complementary colors at the bottom, the dE (CIELUV 1976) deviation from the Target values you chose at the top will also be calculated when you click the Compute button.

3. The THIRD purpose of the AccuPel Display Calibration calculator is to adjust the display's Grayscale tracking as closely as possible to D65. You can enter the x.y values of the grayscale and the calculator will compute the dE (CIELUV 1976) values of the grayscale.

4. The FOURTH purpose of the AccuPel Display Calibration calculator is to adjust the display's Gamma curve to a Target Gamma curve. You may select any Gamma value from 1.80 to 2.20 in 0.05 increments and the calculator will compute the Target Y values from 5% to 90% based on your measured 100% Y value. It will also compute the actual incremental Gamma values for measured values that you enter. You can also enter a black level (0%) or a Full-Field Contrast Ratio to compute the Gamma values relative to a non-zero black level.

The latest version of the AccuPel Display Calibration Calculator is version 1.23. The only differences from previous versions are the labeling of the SMPTE C and EBU standard primary buttons, and a 4th decimal place was added to the computed color gamut Y Target values because SMPTE standards also use 4 decimal places.

I still don't think you are understanding what I am trying to convey (may be my fault). You have to decide what set of primary and complementary Target colors you want to match. The calculator can not select that set for you. It doesn't know what you like or don't like, or what you believe about color accuracy or don't believe. It is preprogrammed with 3 standard choices (Rec 709, SMPTE C, EBU) that you may wish to use (I would). OR you can choose to use any Custom set that you want by just entering the primary targets and gray reference yourself. If you do that it will calculate the complementary color targets for you. So it is up to you to pick the target colorimetry that you want to use. Then it will help make adjustments by telling you the dE deviations of your measured/adjusted values from the Target set you chose to use.

If I am reading your posts correctly (maybe I'm not) you are choosing to use the display's native primaries to produce your final Target display colorimetry. I wouldn't do that, but that is up to you. (I would only use the display's native primaries to be sure (adjust) that the color decoding was standard, then I would use a standard set of primaries for the display's final Target colorimetry.)

Yes, to adjust/verify that a color decoder performs standard color decoding you need to use the "non-standard" native primaries to generate the Target values.


OK, then you are using the Rec 709 Targets at this point in your description. You are not using a Custom set of Targets based on the native primaries at this point in what you have described.


At this point you have changed from using the Rec 709 Targets to Targets based on your own native display primaries.


If you want to match one of the standard colorimetries (Rec 709, SMPTE C, or EBU) as closely as possible (which may not be very close for your display depending on what adjustments it has) then you need to use those as the Target values and not the native primaries that you copied into the Target section above.

That verifies/demonstrates that it has standard color decoding.


Any display with standard color decoding should work that way.

Yes, the first thing I would do is confirm/adjust the color decoding to be standard if possible. For one reason, there are some sources and some displays that use incorrect color decoding matrices (design errors). They mix up the standard-definition (Rec 601) and high-definition (Rec 709) matrices. So it's a good idea to first confirm that the correct matrices are being used. (If they should happen to be wrong there are often manual settings that can be changed to use the right matrix, and sometimes they are wrong because someone incorrectly set one of these manual selections.) Secondly, in non-front projectors the decoding matrix is sometimes very non-standard (by design - for bad reasons) but there may be controls in a service menu to adjust it to standard performance - although those controls are often difficult for an inexperienced person to understand how to use. Third, the simple Color/Hue controls may be incorrectly set in a service sub-menu and the default setting in a user menu may not be correct (that isn't too common except in the second case above). Most important to remember, if the color decoding is not standard, the color accuracy will be wrong even if you adjust a CMS to match standard primary and complementary colors.


If there are no CMS controls, only Color/Hue controls, then in most cases the only thing you can do to improve the color matching to a standard is to adjust the color decoding (Color/Hue) controls and the reference white point. But its still a good idea to first check the color decoding as I described in #1 (and above) to make sure that you don't have a correctable matrix mismatch problem. But after you have eliminated those possibilities you are basically left with only the Color/Hue controls (or other color decoder controls that might be in a service menu - usually only found in non-front projectors) and the reference white point. You can move the reference white point "some" to improve the overall color accuracy and your eyes will adapt to a non-D65 white reference, but if you push it too far you will notice the difference especially on black and white source material.

If the CMS is implemented using a linear 3x3 RGB to RGB matrix conversion from the native primaries to the "corrected" pseudo primaries, then that is true because setting the white reference point to D65 correctly sets the primary Y values, and the complementary xyY values are a linear combination of the corrected RGB values. However, if the CMS is done using LUTs (Look Up Tables), or non-linear transforms, then anything can happen.

I'm sorry if anyone got the impression that I was annoyed last June 2008 by the questions being asked. I've literally answered hundreds (maybe a thousand or more) questions on this forum over the years and I try not to get annoyed with anyone asking a question unless they persist in arguing for some point they don't understand after it's been explained to them multiple times by myself and others. That wasn't the case in this thread, so I don't remember being annoyed with anyone.

The best explanation I can give for the purpose of the calculator is in one of my replies above where I enumerated 4 uses of the calculator. If that confused anyone just ask a specific question about that posting and I will try to clarify. I'm not sure where the explanations need to be "dumbed down - a lot" to make them more understandable.


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?

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!

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.