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Author Topic: multiple native white points?  (Read 8260 times)
digitaldog
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« Reply #20 on: February 02, 2009, 12:28:02 PM »
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Sandy, I'm aware of the LED advantages of the NEC (I had one) so I agree, that's a totally different beast here. I thought we were talking about CCFL? Is there provisions outside of the native white point that is controllable san's LUTs?
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Andrew Rodney
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sandymc
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« Reply #21 on: February 02, 2009, 01:02:07 PM »
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Quote from: digitaldog
Sandy, I'm aware of the LED advantages of the NEC (I had one) so I agree, that's a totally different beast here. I thought we were talking about CCFL? Is there provisions outside of the native white point that is controllable san's LUTs?

To confirm, you don't need a LUT to change the whitepoint of a CCFL LCD display. A LUT is a possible design choice, but just one among many.

I don't know how to put this more clearly. The NEC document confirms the mechanism by which white points are changed in conventional CCFL panels, the Wikipedia article describes underlying technology, so at this point I'm not clear what it is that isn't clear. If you have a specific question around this, please ask it. Or if you have references that suggests that the above isn't the case, please post it.

Sandy
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jackbingham
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« Reply #22 on: February 03, 2009, 09:52:56 AM »
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"The closer to the linear response you are (in Calibration Tester), the better for quality of your images - minimum changes were done in videoLUT (that's good)."

This is only true if either you don't you care about a critical color match between screen and print or by shear coincidence the native color temp of the monitor matches the color temp of your viewing light. If you do not have a match and you want one, native is not the right choice. Suffering a little banding to get a critical color match is the better choice. Most current calibration solutions do a very good job of smoothing out these banding issues. This "problem" is now considerably out dated and should be cast into the dustbin of history.
And for the record this can only effect the view of your images on your monitor, not their "quality."
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Jack Bingham
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trinityss
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« Reply #23 on: February 03, 2009, 01:12:06 PM »
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I don't know if this might be of assistance, but I just want to point something else out: when calibrating with BasICColor, my LP2475W display exhibited some odd behaviour. Although it should not be possible, BasICColour would sometimes actually allow a hardware calibration via DDC -  it would treat the HP monitor as it does my Eizo, which does not need the additional step of monitor settings adjustment. (The first page of BasICColour's set-up options would have DDC option available, so I chose it to see what would happen.) It actually worked well for a while (even though it is not supposed to work at all), and then became unstable (weird flickering, random colour changes etc.) and so I reverted to regular non-DDC calibration of the HP. I have no explanation for any of this, alas - so just to say that the interaction between the monitor and the calibration software is worth considering as a possible (though probably unlikely) source of the trouble.

Hi,
 
I don't have this behaviour... i can only perform a software calibration => Software calibration (Video LUTs).
Maybe it is version related, i use 4.1.9


Kr,
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trinityss
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« Reply #24 on: February 03, 2009, 02:17:59 PM »
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Quote from: sandymc
No, no really - LCD displays are fundamentally very simple devices - the amount of light transmitted is controlled by the voltage on a pixel. So, simplistically 0 volts = black, 1 Volt = all light transmitted. The 8-bit signal (or more bits in high end displays) just divides that up proportionally, e.g., 0 = 0V, 255= 1 Volt.

When you change the whitepoint of the display, you're changing the voltage that equals 255. So, e.g.,

1. a 9500 whitepoint may mean: Red max voltage = 0.8V,  Green max voltage = 1V, Blue max voltage = 1V
1. a 6500 whitepoint may mean: Red max voltage = 0.9V,  Green max voltage = 0.9V, Blue max voltage = 1V

Max voltage always equals 255 (in an 8 bit display).

There's no need for a LUT in this process. The display might, depending on how it works internally, use a LUT, but that's not a necessary part of the equation.

Note the above is vastly simplified - e.g., I've left out gamma, black levels, contrast controls, etc. Also most LCD panels (for technical reasons) internally work with 0V = white and max volts = black, etc, but the principle remains the same.

The one thing you need to avoid is setting the display to settings such that it is in saturation, e.g., voltages of above 1V in the example above. How you tell is easy - when you calibrate the panel, check that the transfer curve doesn't have a kink in it at the top or bottom.

Sandy


Hi Sandy,

Is your described theory valid for all panel types? I've found an article that i think confirms your theory:
http://monitordelcd.com/en/monitor/manuten...monitor-de-lcd/

"Controlling the level of voltage applied between the polarized and can vary the level of light that pass through the display.
...
If all the bits are 0 that subpixel is erased. If some other bits are 0 and 1 are, if the subpixel lights off eight times and very fast so that our eye enxergará a weaker brightness.
Since each subpixel (color) receives 8 bits at a time, he can make 256 levels of brightness"


So to summarize:
The level of voltage between the polarize filters (for each subpixel) determines the color temperature.
The 8 bits define the level of brigthness for R / G / B thus creating a color.

That makes sense checking the options while playing with the white point option.
If i switch between the presets sRGB/6500/9300 there is no change for the RGB values.

sRGB= 5432/5333
6500= 6245
9300= 7300/7100
maximum= 6360

So the maximum preset is when the voltage value is maximum?

But why do i need to change the RGB values in BasICColor to achieve a certain color temperature?
This is a totally other approach to change my white point...
Looks like i can change the white point using a hardware way (presets) and using a software way (RGB values)?

Can i conclude if all i said is true that i have 4 native white points? My 4 presets...

Kr,
« Last Edit: February 03, 2009, 02:21:41 PM by trinityss » Logged
sandymc
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« Reply #25 on: February 04, 2009, 05:30:50 AM »
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Quote from: trinityss
But why do i need to change the RGB values in BasICColor to achieve a certain color temperature?

Well, there are really two white points that you are dealing with (and nether of them is native white point!):

  • the white point of the display on its own (so the color you get when the input to the display is 255,255,255), which is what the previous discussion on this thread was about
  • the white point of the system as a whole - that is, the color that you will get when a color managed application asks for lab 1,0,0

Those are very often a bit different in a calibrated system. Reason is that the Display white point is often a bit out - so if set it to say the 6500 preset, it will actually give 6400. When you then calibrate with an I1 or whatever to 6500, the ICC profile that is created "tells" the color management system that to give pure white it should give values of 255, 250, 253 (or whatever).

End result is that while the white point of the monitor is 6400, the color temperature of a white that is displayed by a color managed application will be 6500.

Its best to have the monitor white point and the system white point you're calibrating to as close together as possible. Otherwise, a grey in a non-color-managed part of the screen will look different to a grey in the color managed part, which pretty much defeats the point of calibration. Most color calibration systems let you do this by setting a monitor's custom white point by moving the monitor's RGB controls.

You can see this difference quite easily on most calibrated systems, btw - in Aperture on a Mac, for example, the grey surround is not color managed, while the actual image is. So if you use the DigitalColor Meter tool to look at the RGB values of a white point adjusted grey in the image, and a grey in the surround, there will probably be a slightly different ratio of R to G to B.

Hope that helps.

Sandy
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Phat Photographer
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« Reply #26 on: February 04, 2009, 10:39:37 PM »
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As a possible aside, there is one disadvantage to using a white point other than native: the brightness is reduced as the monitor compensates by lowering the R, G or B to compensate.  It's neither good or bad, just a trade-off between using the native white point (brighter) or an adjusted one (which is truer white).
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Wayne Fox
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« Reply #27 on: February 07, 2009, 01:57:26 PM »
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Quote from: Phat Photographer
As a possible aside, there is one disadvantage to using a white point other than native: the brightness is reduced as the monitor compensates by lowering the R, G or B to compensate.  It's neither good or bad, just a trade-off between using the native white point (brighter) or an adjusted one (which is truer white).

I can see that, but since nearly every monitor has to be dimmed dramatically to be used correctly, does this really matter?  Plenty of headroom to tweak the backlight up slightly to compensate for the very slight dimming resulting from the LUT's adjustment to achieve the target white point.

Personally to get anywhere close to matching color in my viewing station and my monitor, my display profile is set at 6100k ... native and 6500k just aren't even close.
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