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post #11941 of 12074 Old 09-17-2019, 01:16 PM
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Quote:
Originally Posted by Randomite View Post
For my model of projector (claimed 2000 lumens in the Brightest mode), projector central measured a lumen output of 1575 lumens in the brightest mode.

Chris Eberle of Home Theater Hifi Reported a peak brightness of 195 nits in this same mode, which at their screen size and gain (92"/1.3) works out to 1103 lumens.
I would trust measurements based the incoming lux rather than the nits off the screen, as the former eliminates the screen gain as a factor. Most screens have an actual gain lower than the advertised gain, sometimes significantly so, which will skew the calculated lumens.

Also, if the projector has a zoom lens the lumens will vary according to the throw distance. Manufactures will quote the maximum (usually at minimum throw distance) but Some reviewers measure at mid zoom.

Last edited by Dominic Chan; 09-17-2019 at 05:19 PM.
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post #11942 of 12074 Old 09-17-2019, 01:24 PM
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Originally Posted by Dominic Chan View Post
I would trust measurements based the incoming lux rather than the nits off the screen, as that eliminates the screen gain as a factor. Most screens have an actual gain lower than the advertised gain, sometimes significantly so, which will skew the calculated lumens.
OK, so when I plug in the "projector lumens" number on that calculator it is meant to be the measured lumens coming from the lens correct?
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post #11943 of 12074 Old 09-17-2019, 05:02 PM
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Originally Posted by Randomite View Post
OK, so when I plug in the "projector lumens" number on that calculator it is meant to be the measured lumens coming from the lens correct?
I just spent a lot of time learning about the nuances of this, so this is fresh in my mind and hopefully what I'm sharing is helpful.


With the meter as close as possible to the screen surface, take a lux reading facing the lens. The"effective" lumen output of the projector is given by:


lumens = lux * area of screen in sq meters

Things to bear in mind:


  • Meter distance affects the outcome. Meter really needs to be in the exact plane of the screen to use this formula which makes no allowance for other locations, inverse square law, etc.
  • Meter orientation affects the outcome. Obviously if your meter is not pointed straight at the lens you may get a lower reading. Take continuous readings and keep adjusting the alignment until you're getting maximum readings
  • Screen area is not really what determines lumens, it's image area, formula assumes your screen surface aligns perfectly with the active imaging area and captures all light emitted by the projector, no overspill, no anamorphic stuff.
  • Brightness uniformity affects the outcome. Measure from the center of your screen area unless you have a reason not to
  • User controls affect the outcome, max torch mode settings and super accurate night viewing settings won't give the same number. One presumes you are most interested in a calibrated mode.
  • How you generate "white" affects the outcome. Use a trusted pattern generator at video 100. (There exist "109% white" patterns etc which would obviously give a different result)
  • Reflected room light inflates the result, a window pattern (instead of a full field) will reduce this error
  • There could be other things I have omitted, I'd love to learn more if anyone has a suggestion
  • Final sanity check obviously you have a meter with a diffuser and you are telling HCFR to use it in "ambient" mode
Actually you don't need HCFR for this, any trustworthy lux meter will do. I found this old thread helpful:

https://www.avsforum.com/forum/24-di...projector.html
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post #11944 of 12074 Old 09-17-2019, 05:35 PM
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Originally Posted by omarank View Post
If we talk about the grey scale, I think the the lower levels would have one color temperature (assuming the display being calibrated) and the higher levels (from where the narrow primaries are engaged) would have perceptually different color temperature.
Unless the narrow primaries are brighter than the wider primaries (and you wouldn't do this if your aim is to reduce observer sensitivity), then the primary composition has nothing to do with light levels, just their saturation. So the neutral axis from black to white would be composed of only wide primaries. In fact the bulk of the gamut (80% ?)would be composed of only wide primaries.

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post #11945 of 12074 Old 09-18-2019, 12:53 AM
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Originally Posted by gwgill View Post
Unless the narrow primaries are brighter than the wider primaries (and you wouldn't do this if your aim is to reduce observer sensitivity), then the primary composition has nothing to do with light levels, just their saturation. So the neutral axis from black to white would be composed of only wide primaries. In fact the bulk of the gamut (80% ?)would be composed of only wide primaries.
Yes, you are obviously right. Actually I didn't put it in the right context. I was thinking in terms of having a smooth transition between the broad and the narrow primaries. I think for that to happen, we would require that all the primaries remain active for all colors (above a certain luminance level*). For less saturated colors, the broad primaries would have greater contribution and the narrow primaries would have very less contribution. For highly saturated colors, the narrow primaries would have rather large contribution and the broad primaries would have very less (reducing to zero at the extremes of the gamut). I believe something like this is needed so as to have smooth blending between colors produced by the two set of primaries. *And this would be needed only above a certain luminance, because almost all type of displays (except for OLEDs) exhibit a reduction in gamut size as the luminance level decreases. So, there is no wide gamut below a certain luminance level. And therefore, the blending/transition needs to be managed above a certain luminance level.

So, my point was that even if we take care of the transitions, there are chances that we may still be seeing hue shifts. Like if we manage the transitions (by using both the set of primaries) above a certain luminance level, there may be perceptual change in color temperature in the grey scale (as I mentioned).

But your point is clear and this idea is very interesting : broad primaries be used for all colors except for very saturated ones and narrow primaries be used only for the saturated colors. The manufacturers would have to address just one challenge of making the transitions seamless such that there are no visible hue shifts. I hope they are listening.
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post #11946 of 12074 Old 09-18-2019, 02:27 AM
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Originally Posted by omarank View Post
*And this would be needed only above a certain luminance, because almost all type of displays (except for OLEDs) exhibit a reduction in gamut size as the luminance level decreases. So, there is no wide gamut below a certain luminance level. And therefore, the blending/transition needs to be managed above a certain luminance level.
I don't believe so, no. If the primary colorants retain their spectral shape independent of luminance level, then the gamut is independent of the luminosity level. (That's the assumption made by most display chromaticity diagrams.)

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post #11947 of 12074 Old 09-18-2019, 02:59 AM
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Originally Posted by gwgill View Post
I don't believe so, no. If the primary colorants retain their spectral shape independent of luminance level, then the gamut is independent of the luminosity level. (That's the assumption made by most display chromaticity diagrams.)
I was talking about the gamut reduction (lowering of saturation) as the luma level drops, as shown in the attached image. I would imagine that the reduction in gamut/saturation with the decreasing luma levels implies that there is change in spectral shape of primaries too.

In fact, some studios who use OLED mastering monitors, use a LUT (often baked with Calibration LUT) to simulate this effect (as shown by LCD, CRT and projectors), because OLEDs maintain the gamut/saturation all the way down to black.

I am taking reference of this article by LightIllusion here.
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post #11948 of 12074 Old 09-18-2019, 07:36 PM
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Originally Posted by omarank View Post
I was talking about the gamut reduction (lowering of saturation) as the luma level drops, as shown in the attached image.
That's not a thing, as far as I'm aware. It depends somewhat on how you define "saturation" though. When it comes to color appearance, there are several metrics analogous to saturation, and some of them are defined in a way that has them change with luminosity level. From chapter 4 of the first edition of "Color Appearance Models" by Mark Fairchild:

Colorfulness:
Attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic.

Chroma:
Colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting.

Saturation:
Colorfulness of an area judged in proportion to its brightness.

By these definitions both Chroma and Saturation remain constant at different luminance levels (assuming the spectral shape remains the same), while Colorfulness changes with luminance level.

But all of this is irrelevant to the discussion - the relative saturation of a set of narrow primaries to a set of broad primaries remains constant over lightness (unless there is some real world factor intruding), and so any separation function can be independent of lightness.

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I would imagine that the reduction in gamut/saturation with the decreasing luma levels implies that there is change in spectral shape of primaries too.
Why ? The physics doesn't/shouldn't change with luminance levels.
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In fact, some studios who use OLED mastering monitors, use a LUT (often baked with Calibration LUT) to simulate this effect (as shown by LCD, CRT and projectors), because OLEDs maintain the gamut/saturation all the way down to black.
So do other display technologies.

It's possible though that this is referring to other effects - as the light levels fall to really low levels (scotopic levels), our color vision disappears, because the LMS cones stop responding, while the rods continue to work. But this is a human effect, not the display.
Quote:
I am taking reference of this article by LightIllusion here.
It's poorly explained, but I guess they are referring to the fact that LCD displays have a fixed minimum black level, so of course the effective spectral characteristic of the primaries "broadens" as you approach black. This is simply the result of the minimum black being added to the primary values. Exactly the same effect will occur for OLED displays if you measure them at a distance in a real world environment with ambient light - the "off" state is not a perfect absorber, so a small amount of ambient light will be reflected for black, diluting the primaries, and desaturating near black. (In the diagram the percentage brightness seems way off for this though.) This has little to do with primaries and everything to do with the black level of the display. Yes you can simulate a less than perfect black level on a display that has a better one.

This effect may not even be interpreted as a saturation loss in a subjective sense - it could be perceived as something else, such as being occluded by a "fog" for instance.

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post #11949 of 12074 Old 09-19-2019, 01:03 AM
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Originally Posted by gwgill View Post
Why ? The physics doesn't/shouldn't change with luminance levels.
I think I didn't articulate it well. I agree with all that you have said and learnt something new too (about the effect of fixed minimum black). But my point is different (I believe). Let me take an example (and this is from actual measurement data of my Sony LED TV; and I have seen this behaviour on almost all displays that I have calibrated):

Please see how the measured chromaticities change for Green primaries, for instance, with decreasing levels:
(0,255,0) -> (0.3003,0.5929)
(0,191,0) -> (0.2983,0.5937)
(0,102,0) -> (0.2963,0.5912)
(0,63,0) -> (0.2929,0.5852)
(0,51,0) -> (0.2904,0.5767)
(0,38,0) -> (0.2858,0.5558)
(0,25,0) -> (0.2756,0.4985)
(0,12,0) -> (0.2505,0.3307)

And there is a similar behaviour for Red and Blue primaries too. The gamut triangle is reducing in size as the luma level decreases (the measured chromaticities show an inward movement in the CIE diagram with the decreasing luma levels). I was calling that as lowering of saturation. And I believe the inward movement of chromaticities (reduction in gamut) would happen when the spectral shape of the primaries change. I am not sure if we can attribute this to the minimum black effect, as the measured black of this Sony TV is 0.0287 nits (I can share the profile data if you prefer) which is quite low and the gamut reduction started at much higher levels. Alternatively, could it be possible that there is an inward movement of the chromaticities of the primaries with the decreasing luma levels, but the spectral shape is unchanged?

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post #11950 of 12074 Old 09-19-2019, 08:22 AM
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Quote:
Originally Posted by omarank View Post
I think I didn't articulate it well. I agree with all that you have said and learnt something new too (about the effect of fixed minimum black). But my point is different (I believe). Let me take an example (and this is from actual measurement data of my Sony LED TV; and I have seen this behaviour on almost all displays that I have calibrated):

Please see how the measured chromaticities change for Green primaries, for instance, with decreasing levels:
(0,255,0) -> (0.3003,0.5929)
(0,191,0) -> (0.2983,0.5937)
(0,102,0) -> (0.2963,0.5912)
(0,63,0) -> (0.2929,0.5852)
(0,51,0) -> (0.2904,0.5767)
(0,38,0) -> (0.2858,0.5558)
(0,25,0) -> (0.2756,0.4985)
(0,12,0) -> (0.2505,0.3307)

And there is a similar behaviour for Red and Blue primaries too. The gamut triangle is reducing in size as the luma level decreases (the measured chromaticities show an inward movement in the CIE diagram with the decreasing luma levels). I was calling that as lowering of saturation. And I believe the inward movement of chromaticities (reduction in gamut) would happen when the spectral shape of the primaries change. I am not sure if we can attribute this to the minimum black effect, as the measured black of this Sony TV is 0.0287 nits (I can share the profile data if you prefer) which is quite low and the gamut reduction started at much higher levels. Alternatively, could it be possible that there is an inward movement of the chromaticities of the primaries with the decreasing luma levels, but the spectral shape is unchanged?
This is an interesting discussion. I've seen that LightSpace article before, but never associated the phenomenon with "black polution" that Graeme Gill uses to explain this.

When gamma is taken into consideration, 0.0287 nits is not that low - it corresponds to about (6,6,6) in 8-bit representation, assuming (255,255,255)=100 nits. If you add that linearly to (0,25,0) that gives (6,31,6) which corresponds to a desaturated green of (0.30,0.50) which seems consistent with what you measured.

EDIT: I'm not sure if the math above is valid. Should the black polution be added after the gamma?

Last edited by Dominic Chan; 09-19-2019 at 08:35 AM.
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post #11951 of 12074 Old 09-19-2019, 07:17 PM
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Quote:
Originally Posted by omarank View Post
Please see how the measured chromaticities change for Green primaries, for instance, with decreasing levels:
(0,255,0) -> (0.3003,0.5929)
(0,191,0) -> (0.2983,0.5937)
(0,102,0) -> (0.2963,0.5912)
(0,63,0) -> (0.2929,0.5852)
(0,51,0) -> (0.2904,0.5767)
(0,38,0) -> (0.2858,0.5558)
(0,25,0) -> (0.2756,0.4985)
(0,12,0) -> (0.2505,0.3307)
Without a real device to examine in detail I'm just speculating, but these don't seem to be huge changes, and may well be real world type effects where the display technology is such that the primary spectral shape does change a little with luminance. Note that the gamma encoding means that something like RGB 12 may be a very low level. You could also be seeing some inaccuracy in the measurement instrument. For instance something like an i1D3 may start to see quantization errors at very low light measurement levels.

All irrelevant though - there's no fundamental reason why the relative gamuts of narrow and wide primaries should change with luminance. In designing a 6 primary display, such practical issues as the behavior of the primaries vs. luminance would be part of the design challenge and technology tradeoffs.

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post #11952 of 12074 Old 09-20-2019, 12:07 AM
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Needing a little more guidance with my color calibration. When using the Masciola test patterns I have the option to choose 50/50 rec 2020, and rec2020/p3. When choosing the corresponding hcfr color references, rec2020/p3 is at -15% luminance for all colors, where as rec2020 is -8% on red and magenta, but the rest are very close. Which pattern set and hcfr setting is proper to use? The only way I can get rec2020/p3 lined up with proper luminance is to raise my red luminance to +28 on my OLED causing major red crush. The other problem I have is that my OLED definitely uses p3 as it's coordinates, so there's literally no way to line up green when using 50/50 rec2020. Should I set the color coordinates with rec2020/p3 and the luminance with 50/50rec 2020? This is very confusing.....someone help a guy out.

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post #11953 of 12074 Old 09-20-2019, 12:37 AM
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Quote:
Originally Posted by gwgill View Post
In designing a 6 primary display, such practical issues as the behavior of the primaries vs. luminance would be part of the design challenge and technology tradeoffs.
Yes, right.

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Originally Posted by Dominic Chan View Post
When gamma is taken into consideration, 0.0287 nits is not that low - it corresponds to about (6,6,6) in 8-bit representation, assuming (255,255,255)=100 nits. If you add that linearly to (0,25,0) that gives (6,31,6) which corresponds to a desaturated green of (0.30,0.50) which seems consistent with what you measured.
I am not sure if this analysis is correct. Gamma starts from the black level of the display (in my understanding). As per the above analysis, the first 5 levels should be clipped. But they aren't as checked with Pluge patterns.
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post #11954 of 12074 Old 09-20-2019, 12:54 AM
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Quote:
Originally Posted by Dominic Chan View Post
This is an interesting discussion. I've seen that LightSpace article before, but never associated the phenomenon with "black polution" that Graeme Gill uses to explain this.
I had read about the phenomenon in these terms before on Light Illusion's website when I started using 3DLUT - maybe 3 years or a bit more ago - but I can't find the article now (it was described as backlight contamination).

That's the reason the CIE diagrams for the native gamut of LCD displays (including projectors) end up with the primaries desaturating towards the backlight / lamp native whitepoint.

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post #11955 of 12074 Old 09-20-2019, 01:31 AM
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Originally Posted by omarank View Post
I am not sure if this analysis is correct. Gamma starts from the black level of the display (in my understanding). As per the above analysis, the first 5 levels should be clipped. But they aren't as checked with Pluge patterns.
Most displays actually follow the “Gamma with black compensation” to avoid clipping at the bottom. You can think of it (approx) as the black level being added to the output.
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post #11956 of 12074 Old 09-20-2019, 01:49 AM
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Originally Posted by Dominic Chan View Post
Most displays actually follow the “Gamma with black compensation” to avoid clipping at the bottom. You can think of it (approx) as the black level being added to the output.
That actually depends on the colour standard being used.
Rec709 and HLG are relative.
PQ is absolute.

Edit to add: and the de-saturation of gamut on LCDs has nothing to do with 'black level contamination'.
It is a physical attribute of LCD technology, and is almost definitely due to light leakage between/across the pixel cells.
(Cross colour contamination, due to the use of a backlight.)
Self illuminating pixels will not suffer this.

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post #11957 of 12074 Old 09-20-2019, 05:44 AM
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Originally Posted by Light Illusion View Post
That actually depends on the colour standard being used.
Rec709 and HLG are relative.
PQ is absolute.
BT.2390 specifies how the shadows should be tone-mapped according to the black level. Here’s an example for an EETF from 0 – 10 000 cd/m2 to 0.01 – 1 000 cd/m2; the EETF modifies the “absolute” PQ EOTF.

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post #11958 of 12074 Old 09-20-2019, 06:06 AM
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Indeed - that is all built into LightSpace for PQ based calibration.

It's interesting to see how many HDR displays ignore black level roll-off, and just clip...

And also to compare the different shadow compression that gets applied when displays with different black levels do have black level roll-off.

Some really do end up with seriously compromised shadow detail.

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Just following up on my question last night. So, 50% rec2020 patterns or rec2020/p3 patterns? When using the corresponding work flow in hcfr the color luminance is way different. With rec2020/p3 I have to turn up the red and greenluminance so much that it causes clipping. I know that's not how it should be. My c8 most definitely uses p3 in rec2020 container as it's default hdr10 mode, so calibrating green at 50/50rec 2020 is just not possible. Someone please help me. What patterns do I use to calibrate to the correct luminance?
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post #11960 of 12074 Old 09-20-2019, 11:24 AM
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Quote:
Originally Posted by chronitis View Post
Just following up on my question last night. So, 50% rec2020 patterns or rec2020/p3 patterns? When using the corresponding work flow in hcfr the color luminance is way different. With rec2020/p3 I have to turn up the red and greenluminance so much that it causes clipping. I know that's not how it should be. My c8 most definitely uses p3 in rec2020 container as it's default hdr10 mode, so calibrating green at 50/50rec 2020 is just not possible. Someone please help me. What patterns do I use to calibrate to the correct luminance?
Are you talking about 50% saturation or 50% amplitude? HCFR automatically uses 50% amplitude with ST.2084, but AFAIK you have to select the 50% saturation point yourself for BT.2020.


Before you adjust CMS you need to ensure your greyscale is correct, including the luminance at each point.

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post #11961 of 12074 Old 09-20-2019, 11:52 AM
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Quote:
Originally Posted by chronitis View Post
Just following up on my question last night. So, 50% rec2020 patterns or rec2020/p3 patterns? When using the corresponding work flow in hcfr the color luminance is way different. With rec2020/p3 I have to turn up the red and greenluminance so much that it causes clipping. I know that's not how it should be. My c8 most definitely uses p3 in rec2020 container as it's default hdr10 mode, so calibrating green at 50/50rec 2020 is just not possible. Someone please help me. What patterns do I use to calibrate to the correct luminance?
Hi,

REC.2020 is the only colorspace you have to aim for HDR10 calibration, because displays can't cover that colorspace completely, we are using lower Saturation level (50%) and lower Stimulus Level (50%) patterns.

Because most of the consumer HDR displays are not covering 100% the DCI-P3 primaries (inside REC.2020 container), this is why we are using REC.2020 with 50% Saturation patterns for HDR CMS, because xy of 50% Saturation REC.2020 is undersaturated from 100% Saturation of DCI Primaries (inside REC.2020 container), so it's a target that you can reach when the CMS controls of the display are working as expected and 50% Stimulus has luminance target for 92.24 nits.

After your calibration using REC.2020 with 50%/50% patterns, then you can evaluate to see how your calibration tracking the P3 primaries inside of REC.2020 container.

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post #11962 of 12074 Old 09-20-2019, 03:37 PM
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Just following up on my question last night. So, 50% rec2020 patterns or rec2020/p3 patterns? When using the corresponding work flow in hcfr the color luminance is way different. With rec2020/p3 I have to turn up the red and greenluminance so much that it causes clipping. I know that's not how it should be. My c8 most definitely uses p3 in rec2020 container as it's default hdr10 mode, so calibrating green at 50/50rec 2020 is just not possible. Someone please help me. What patterns do I use to calibrate to the correct luminance?
Hi,

REC.2020 is the only colorspace you have to aim for HDR10 calibration, because displays can't cover that colorspace completely, we are using lower Saturation level (50%) and lower Stimulus Level (50%) patterns.

Because most of the consumer HDR displays are not covering 100% the DCI-P3 primaries (inside REC.2020 container), this is why we are using REC.2020 with 50% Saturation patterns for HDR CMS, because xy of 50% Saturation REC.2020 is undersaturated from 100% Saturation of DCI Primaries (inside REC.2020 container), so it's a target that you can reach when the CMS controls of the display are working as expected and 50% Stimulus has luminance target for 92.24 nits.

After your calibration using REC.2020 with 50%/50% patterns, then you can evaluate to see how your calibration tracking the P3 primaries inside of REC.2020 container.

So what's up with the difference in color luminance between the two settings? Which one is correct? I don't think clipping red and green is a desirable end result. Is this just the way it is?
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post #11963 of 12074 Old 09-20-2019, 03:39 PM
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Is there a guide for idiots like myself who thinks they know something, and gets humbled every other day by something new?
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post #11964 of 12074 Old 09-20-2019, 04:16 PM
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Is there a guide for idiots like myself who thinks they know something, and gets humbled every other day by something new?
It would be easier for others to provide suggestions if you post your HCFR data files for both the 50% BT.2020 and 100% P3 measurements, preferably the "pre-adjustment" versions.
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post #11965 of 12074 Old 09-20-2019, 05:37 PM
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Is there a guide for idiots like myself who thinks they know something, and gets humbled every other day by something new?
It would be easier for others to provide suggestions if you post your HCFR data files for both the 50% BT.2020 and 100% P3 measurements, preferably the "pre-adjustment" versions.
How do I configure hcfr to measure for 50/50% rec2020 slides? Do I aim for the 50% mark within the rec2020 cie diagram? DCI p3 in rec2020 is pretty straight forward.
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post #11966 of 12074 Old 09-20-2019, 05:43 PM
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How do I configure hcfr to measure for 50/50% rec2020 slides? Do I aim for the 50% mark within the rec2020 cie diagram? DCI p3 in rec2020 is pretty straight forward.
Do it from the saturation screen:
Menu -> Measures -> Saturations -> Primary & Secondary Colours; and then go through the individual colours to adjust.

Last edited by Dominic Chan; 09-20-2019 at 06:43 PM.
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post #11967 of 12074 Old 09-20-2019, 06:58 PM
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That actually depends on the colour standard being used.
Not in this discussion no - we're talking about the native display characteristic, rather than the target (calibrated) shape.
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Edit to add: and the de-saturation of gamut on LCDs has nothing to do with 'black level contamination'.
It is a physical attribute of LCD technology, and is almost definitely due to light leakage between/across the pixel cells.
(Cross colour contamination, due to the use of a backlight.)
Good to know.

But LCD black level would appear to be the dominant reason for saturation decreasing as input approaches zero. (See page 11).
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post #11968 of 12074 Old 09-21-2019, 12:45 AM
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Edit to add: and the de-saturation of gamut on LCDs has nothing to do with 'black level contamination'.
It is a physical attribute of LCD technology, and is almost definitely due to light leakage between/across the pixel cells.
(Cross colour contamination, due to the use of a backlight.)
Do you mean the leakage is from side to side - ie you're trying to light up just the red subpixels, but that some amount of that light is somehow travelling sideways into the green or blue subpixels? Or from the green and blue subpixels just not being able to fully block the backlight? it sounds like you're suggesting the first.

I also see the issue on projection systems where the R,G,B cells are located on separate panels relatively far away from each other.

It should be possible to work out how much of the desaturation is due to the black level and how much is unaccounted for and would be attributable to other factors.

Eg in the case of displaying primary red, you are actually displaying primary red + the green and blue "black" component. In a system where the backlight or lamp power is constant (ie Dynamic Dimming is off) the green and blue "black" components due to those subpixels maximum blocking capabilities remain constant while the red primary as a % of the total nits on screen decreases in ratio relative to that black, which will cause desaturation.
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post #11969 of 12074 Old 09-21-2019, 01:01 AM
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But LCD black level would appear to be the dominant reason for saturation decreasing as input approaches zero. (See page 11).
It's a really interesting paper, thanks for sharing it. I see they did exactly that test and did the math to remove the black point influence which resulted in the primaries being almost fully saturated all the way through the 52pt ramps after adjustment.


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post #11970 of 12074 Old 09-21-2019, 01:22 AM
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Do you mean the leakage is from side to side - ie you're trying to light up just the red subpixels, but that some amount of that light is somehow travelling sideways into the green or blue subpixels? Or from the green and blue subpixels just not being able to fully block the backlight? it sounds like you're suggesting the first.

I also see the issue on projection systems where the R,G,B cells are located on separate panels relatively far away from each other.

It should be possible to work out how much of the desaturation is due to the black level and how much is unaccounted for and would be attributable to other factors.

Eg in the case of displaying primary red, you are actually displaying primary red + the green and blue "black" component. In a system where the backlight or lamp power is constant (ie Dynamic Dimming is off) the green and blue "black" components due to those subpixels maximum blocking capabilities remain constant while the red primary as a % of the total nits on screen decreases in ratio relative to that black, which will cause desaturation.
If you look at the RGB Separation graph in LightSpace you can see the issue.

The cause can be pixel colour overlap, as on projectors with three separate RGB sources.
Or simple backlight leakage.
Or leakage between pixel colour.
(The illumnation from one colour exciting the colour next to it.)
Or contamination from the backlight being greater than zero at black, and having a colour cast.

Our test have shown the major effect is cross colour contamination, as having backlights with different levels and different colour cast makes little difference to the gamut de-saturation effect, other than changing the colour towards which the de-saturation tracks.
With most white led backlights that is 'blue'.

You can test this by altering the backlight level, and plotting the effect on the gamut de-saturation/RGB Separation graph.

(I'm not sure I'm happy with suggesting 'calculating' the effect of a black offset is a valid verification approach...)

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