Reading and interpreting calibration charts and data for dummies - Page 4 - AVS Forum
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post #91 of 109 Old 09-08-2005, 02:38 PM
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Balancing RGB takes experience and really is about learning the art of science.. It is not just about following the dancing bars and dots - which ColorFacts also has BTW. The pretty pictures tend to make the newbie think it is easy - and I don't doubt that the software vendors want you to think that!

The first thing I do is find the clipping points. Usually for DLP this is Red for gains and Green for offsets. Then I work my way back to the target using the other colors. Knowing that luminance Y is mostly Green - if you do the gamma math you can balance out your gamma curve at the same time as grey scale and doing the clipping check maximizes contrast at the same time. Also knowing that dE is a perceptual error - you can try different moves on the target and see which has less perceptual error if you cannot make things perfect. Other technologies (LCOS, DLP, CRT, plasma) will have different natures - and it requires experience on those technologies to see what works best! LCD tends to have S shaped gamma curves, LCOS tends to be blue limited, CRT usually never has a flat blue across greyscale - and I am sure plasma has something wierd about them too - but I have never done any. When you cannot make it perfect - like all engineering fields you have to learn how to make acceptable compromises.

Combine that with what Bob is learning now - you have to understand the nature of your instruments and the compromises made in making them affordable - and figuring out how to use them for best accuracy, speed, and reliability.
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post #92 of 109 Old 09-16-2005, 08:35 AM
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Hey guys,

I don't have long, but did want to clarify a few things:

Greg wrote:
Quote:
Yeah, I agree that RGB% measurements don't have much useful meaning unless they are made relative to the display's actual primary colors, as I said way back earlier in this thread.
In ColorFacts, the RGB% levels *will* be relative to the display's actual primary colors, if those have been measured. If they have not been measured, the RGB% are relative to the color standard that is in use (HDTV, NTSC, etc).

However, the end result is *exactly* the same. When you get to 100%, 100%, 100%, you have reached the target (i.e. "D65"), no matter what set of primaries were used to calculate the RGB percentages.


Greg wrote:
Quote:
I realize the differences aren't huge, but if the purpose of the RGB percentages is to adjust the RGB gain and bias controls to calibrate the grayscale, it makes those RGB percentages less accurate to calculate them relative to an idealized set of RGB primaries (even if the errors are small).
True.

The idealized primaries are only used if the "real" primaries are not known. It's a shortcut designed to get you into adjusting the gray scale immediately if you don't intend to (or can't) measure the primaries.

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The only advantage for using some arbitrary set of primaries is that the user doesn't have to take time to let the measuring instrument "learn" the actual primaries. That's reasonable because the errors are usually pretty small...
Exactly.

Now, a completely different question is how the "Constrain Colors" option works.

The "Constrain colors to gamut" option in the Preferences creates an artificial "wall" at the edges of the color gamut (think WWF), and will not allow any colors to stray outside of this playground. If a color is measured and found to be outside of the gamut (theoretically impossible, but practically actually somewhat common), then that color is relocated to the closest point which is within the gamut (actually, on the very 'edge' of the gamut).

This feature was mainly for a customer in the Aerospace industry who wished to prioritize mathematical accuracy and purity over empirical data integrity.

Leaving this option OFF will make the software always show the raw readings that the hardware is providing, even if that means that a measurement plots outside of the gamut (and causes the RGB calculations, etc. to break down).

This option has *nothing* to do with calibrating gray scale, unless your gray scale is SO far off that it's outside of your RGB color gamut!

I hope that helps!

Mark

Mark Hunter
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Datacolor, Inc.
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post #93 of 109 Old 09-16-2005, 11:47 AM
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Thanks for the explanation - that was the answer we were looking for. Sounds like it does make more sense for the option to default to OFF - we got thrown for a loop because it defaulted to ON!
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post #94 of 109 Old 09-16-2005, 01:23 PM
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Same here - thanks Mark!

Jeff (aka umr :) ), does your software do the same thing? That is, if everything is at 100% then I hit the target?
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post #95 of 109 Old 09-16-2005, 01:59 PM
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Quote:
Originally Posted by krasmuzik
Thanks for the explanation - that was the answer we were looking for. Sounds like it does make more sense for the option to default to OFF - we got thrown for a loop because it defaulted to ON!
So Mark is saying grayscale wise it doesn't matter whether it's off or on. But for true CIE readings it needs to be off.

I didn't see that link on the internet that Mark said would pop up during Cedia, the link for the free CF upgrade? I have the Accupel coming next week am I'm going to run into any hookup snags as is or with the upgrade with my laptop? Right now I have both sensors (1eye/trichromat) accessible in the directory.

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post #96 of 109 Old 09-16-2005, 03:11 PM
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Quote:
Originally Posted by maxleung
Same here - thanks Mark!

Jeff (aka umr :) ), does your software do the same thing? That is, if everything is at 100% then I hit the target?
Yes.
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post #97 of 109 Old 09-18-2005, 04:54 AM
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This has been a very informative thread and it got me thinking about how calibration must keep up with new technologies.

Genoa ColorPeak and TI's BrilliantColor are expanding the color gamut that the displays that are using this technology can produce.

How does a calibrator accurately handle these new display technologies?

Here is Genoa Color's explanation of their technology.

Pardon me Bob if I'm too off topic with this question, but I think it is relevent enough to benefit all of us.

Steve
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post #98 of 109 Old 09-18-2005, 11:20 AM
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hex gamuts rather than trigamuts are certainly interesting from a research point of view.

But wide trigamut has not even been accomplished for HDTV - which is barely an improvement over SDTV - so you can forget about hex gamuts for a long time.

A display composed of a hex gamut - only adds complexity and unused capability to the display - as long as there is no software to drive it.

All media currently is either RGB or YCbCr - which is a trigamut. The source media and interconnects would have to entirely change throughout the industry if hex gamuts are to be adopted for more than making pretty pictures.

What they will do is add more calibrator challenge if there are no controls to put the colors back where the HD/SD standards say they should be. The calibrator will leave and you will still be left with an unnatural looking picture.

So until the display manufacturers get it together with the broadcasters and filmmakers - they are doing nothing more than colorizing the picture. Which is nothing new - their marketing departments have always operated on the mistaken idea that "different" means "better".
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post #99 of 109 Old 09-19-2005, 05:07 AM
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Thanks Kevin,

I see, the improvements of video will have to be industry wide, from source to display for best results, and with that will be standards for calibrators to reference to.

It looks like companies like Genoa Color are getting the ball rolling by bringing to market a method of taking existing input RGB or YCbCr data and transferring this information through a multiple primary converter (post processed), resulting in a widened gamut.

Based on the Genoa Color White Paper, I assume there would be no difference in the way a calibrator would calibrate this type of display.

I guess it reminds me of Dolby creating 5 channels out of only 2 channel sources. Not the ideal compared to 5 discrete channels, but does create a relatively believable soundstage.

Steve
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post #100 of 109 Old 09-19-2005, 11:10 AM
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Not the same thing as Dolby ProLogic. Dolby Prologic encoded the extra channels using 'inaudible" phase differences into stereo - which then extracts extra channels. This is actually rather similar the way that color was added to B&W TV signal.

Adding more colors is not the same thing as adding more audio channels! Rather adding more colors is more like a manufacturer enforced audio equalizer setting - does not sound so attractive anymore does it?

It will take more than a startups white paper to change the entire video industry. YCbCr is only a media compression format - it is not a camera and display format. That is a major problem to change considering that someone would have to invent cameras/displays that are not native RGB.

Just look how long HDTV is taking - ten years from release and ten years in development. And that includes a wide gamut HDTV standard that expands the trigamut to more saturated colors all the way around - that was never adopted.


The only issue for calibrators is having the controls to calibrate back to the standard tri-gamut - which means the expense of the "enhancement" is wasted - just like when you buy an audio equalizer only to set it to neutral. The problem is getting color point controls for calibrators often does not happen even now - with oversaturated primaries or yellow segments.
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post #101 of 109 Old 09-19-2005, 11:16 AM
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The genoa paper does not post the 5P primary values - but let me guess from the charts. I will post my zoomed color target so you can see how far they are off from the standard.
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post #102 of 109 Old 09-19-2005, 12:58 PM
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Here are the perceptual charts based on Genoas white paper. I posted here as it gives me a chance to add some review words to my charts - maybe making them more understandeable.

krasmuzik GENOA

They first propose a Wide Gamut - the advantage here is that gamma is not changed because Cyan is a deeper towards Blue and thus you do not get brighter Whites - but Cyan gets an F---

The natural greyscale point defined by the RGBCY primaries is thus grossly blue - give it an F---

But that means Magenta is deeper towards Blue as well - give it a D-

Green is deeper and more Cyan - give it a D+

Blue and Yellow are just a bit overdone - give them a B. Red is overdone - give it a C (note many display like Sim2, Infocus SP7210, Optoma H79 are similar reds)

So if you were watching an HD source on this gamut - the picture would look very strange. Especially skies and grass - it would look like these SciFi shows that colorize things to make it look like another planet.

But Genoa realizes that and have a High Bright gamut - which is closer to HD spec.

The Red is a less pale Orange - causing Magenta to be pale as well - combined with a paler Blue. Give them all a B - but you can see they made the dark colors pale because it improves brightness.

The lighter colors Cyan, Yellow, and Green - they have oversaturated a bit - because Cyan and Yellow are used to create white they can afford to do this with the brighter colors. The hues are pretty good - also give them a B.

As a result the greyscale is not bad - give it a B+ for being just a bit Cyan.

I have seen worse color/greyscale points on other projectors - as a calibrator though I strive for an A - as I can see the difference to the point of annoyance. I would consider a B projector OK if you don't care about videophile stuff and are budget limited, but only an A projector would be highly recommended.

You can see the compromise in the gamma. The gamma curve is higher value and the blacks are crushed (or whites are popped depending on your viewpoint). I would give gamma a B as well.


So bottom line - the real point of these CMY colorwheels is to improve brightness - at the expense of a videophile calibration on greyscale/colors/gamma. In other words the usual game - market the big numbers - not the videophile numbers. Using them for wider gamuts makes little sense - no brightness improvement - but wierdly pushed colors for HD sources.
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post #103 of 109 Old 09-19-2005, 01:18 PM
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Quote:
Originally Posted by gregr
But something is clearly wrong on the chart at the start of this thread. The yellow measurement is outside the line between the green and red measurements, and that is physically impossible unless the projector has special processing to adjust the primaries separately from the complementary colors, and the 7210 doesn't have that type of processing.
Greg,

The ScreenPlay 7210 does use color correction processing to independently correct for primaries, secondaries, and white point. The corrected yellow indeed does not lie directly between the green and red primaries when viewed on a CIE 1931 chart like the one at the beginning of this thread. This non-triangular gamut is intentional.

Some people say we are "cheating" because we choose a yellower green than some others do, however the reason behind this choice is to preserve contrast. Adding yellow to the green moves the white point to a warmer temperature. If you turned off all processing, the natural white point in the ScreenPlay 7210 would be very close to D65. Because of this, you lose no contrast when you properly setup your projector for 6500K. Based on all of the feedback I have been able to gather (as well as my own personal preferences), this boost to contrast is more important to relative picture quality than the absolute saturation of green.
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post #104 of 109 Old 09-19-2005, 04:05 PM
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Quote:
Originally Posted by Bob Williams
Greg,

The ScreenPlay 7210 does use color correction processing to independently correct for primaries, secondaries, and white point. The corrected yellow indeed does not lie directly between the green and red primaries when viewed on a CIE 1931 chart like the one at the beginning of this thread. This non-triangular gamut is intentional.
Thanks for the info Bob. I didn't see anything about user adjustable primaries and complementaries in the data sheet so I figured it had no capability for this kind of independent color correction. If it does have user control over the primary and complementary colors please confirm that.

I've never measured a 7210 but this will keep me from tearing my hair out puzzling over what happened to the yellow measurement if I ever do. :)

Greg Rogers
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post #105 of 109 Old 09-19-2005, 04:09 PM
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Bob,

"Based on all of the feedback I have been able to gather (as well as my own personal preferences), this boost to contrast is more important to relative picture quality than the absolute saturation of green."

Not to be a curmudgeon, but why then hasn't Infocus used an adjustable or dynamic iris to get more of that desirable contrast?

Noah
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post #106 of 109 Old 09-19-2005, 05:15 PM
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gregr

If you get BobWilliams to divulge any supersecret service codes - I want in on them :D. The accessible service menu only has the typical ADC YPbPr controls and only for HD - not 480i.

Even if it was NEC HT1100 style primary/secondary control of sliding along the triangle over to the next secondary/primary is something - though not as good as insanely more expensive projectors.

All of the ScreenPlay's I have measured have always had perfect hues on secondaries - I just want to try some tricks....
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post #107 of 109 Old 09-19-2005, 06:55 PM
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Quote:
Originally Posted by noah katz
Bob,

"Based on all of the feedback I have been able to gather (as well as my own personal preferences), this boost to contrast is more important to relative picture quality than the absolute saturation of green."

Not to be a curmudgeon, but why then hasn't Infocus used an adjustable or dynamic iris to get more of that desirable contrast?
The optical design in the ScreenPlay 7210 is at its maximum contrast already, so no additional iris will make it any higher.
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post #108 of 109 Old 09-19-2005, 08:11 PM
 
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Welcome back to the high dollar forum Bob!! I haven't seen that smiling face for a while. I thought you quit the forum from being brain fried from all the questions. I see you frequent the "other" forum.

Scott
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post #109 of 109 Old 09-30-2005, 02:15 PM
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good one for calibration forum
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