If RGB values are equal (6500K) why is luminance low? - AVS Forum
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post #1 of 28 Old 12-21-2012, 08:54 AM - Thread Starter
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Optoma HD8200, calibrating with a Colormunki Display (brand new) & HCFR v. 3.0.4.0. The Optoma has 2-point grayscale controls.

I have attached the various graphs with the reference measures from HCFR (pre-calibration values are the dashed lines).

I managed to obtain a nice tight linear grayscale but the luminance graph shows a drop in luminace post calibration. Furthermore, my gamma pre-calibration was 2.25 and fairly linear, now it is 2.39 and u-shaped; however it never drops below 2.22.

Also verified that red does not clip at 100IRE.

Can any of this be improved? If so, what values should I be looking to adjust?

Any help/ comments would be greatly appreciated......





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post #2 of 28 Old 12-21-2012, 09:14 AM
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Did you recheck brightness and contrast afterwards? You will always loose light output after you adjust grey scale properly (if that is what you are asking)
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post #3 of 28 Old 12-21-2012, 11:24 AM - Thread Starter
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Quote:
Originally Posted by airscapes View Post

Did you recheck brightness and contrast afterwards? You will always loose light output after you adjust grey scale properly (if that is what you are asking)
I did not, finished at 2 a.m. in the morning. I will re-check the brightness/ contrast tonight. Is the luminance affected by both brightness & contrast? I thought it was only the contrast setting which affected luminance and the brightness raises the curve up or down.......

Will this affect my gamma as well?
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post #4 of 28 Old 12-21-2012, 12:11 PM
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Quote:
Originally Posted by guida74 View Post

I did not, finished at 2 a.m. in the morning. I will re-check the brightness/ contrast tonight. Is the luminance affected by both brightness & contrast? I thought it was only the contrast setting which affected luminance and the brightness raises the curve up or down.......
Will this affect my gamma as well?

The way I think of it is the brightness moves the entire rang closer or father from black and contrast expands the range in both directions. This is why you have to recheck each after adjusting one.
The cuts/bias and drv/gains of the the 2 point are just RGB adjustments of Brightness and Contrast. So they effect brightness and contrast.
Both effect the overall light output of the projector. Green and Blue are where most of the visible light comes from so if the starting white point is 8000 once reduced to 6500 you will get a reduction in light output. If the brightness and contrast are not rechecked after/during the 2 point your gamma can be effected, as gamma is the mathematically calculated difference between the amount of light output at each stimulus between 0% (video 16) and 100% (video 235) If the range is compressed by the brightness and or contrast being to high or low the gamma curve can change. Also if the contrast is to high and you are clipping (running out of a color) it can cause dips/humps in gamma at the top end.

ChadB posted some patterns in another thread. One of them is a gamma pattern with squares from 10 - 100% If you have a way to display this pattern, before doing the 2 point it can be helpful to determine which end of the 2 point to start at. As you raise for example red gain you will see the boxes turn red, notice how much the grain effects the the lower end of the scale. Then reset to 0 and do the same with the cuts this time watching the top end.
Depending on which (cuts or gains) effect the opposite end of the scale more would be where to start. I did this the other night and found I could achieve the correct setting faster. Since you know that they effect each other, don't make your first adjustment perfect as the the opposite end will effect. You will find there is less back and forth need if you sneak up on the balance.

Another thing to do prior to the 2 point is to display a 100% patten and do a constant read. One at a time turn up the drives one click at a time to see which color stops climbing first. Most likely it will be red. Calman5 has the dynamic range screen that will show clipping and can be used to set contrast or verify you are not clipping colors after you use the patterns by eye.

Once you know the color that clips, and you start your 2 point, for example it is red, and red is the lowest color, bring the others down to red. Yes I know you are not supposed to touch green as it is your major light source and gamma could be effected, but this has worked very well with my DLP projector, your mileage may vary.

Hope that helps!
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post #5 of 28 Old 12-21-2012, 12:28 PM - Thread Starter
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airscapes,

Those are some pretty good tips and I will try them this weekend. HCFR is limited to basic measurements; you really need a good understanding of color science so that you can interpret the data.

This is my first attempt at calibrating so please bear with me. What do you think of the before& after graphs overall. The picture definately looks warmer with more natural colours to my eyes. Can you interpret the gamma "u-shaped" graph for me in laymens terms. Is this acceptable in the world of videp calibration?
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post #6 of 28 Old 12-21-2012, 01:05 PM
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Quote:
Originally Posted by guida74 View Post

airscapes,
Those are some pretty good tips and I will try them this weekend. HCFR is limited to basic measurements; you really need a good understanding of color science so that you can interpret the data.
This is my first attempt at calibrating so please bear with me. What do you think of the before& after graphs overall. The picture definately looks warmer with more natural colours to my eyes. Can you interpret the gamma "u-shaped" graph for me in laymens terms. Is this acceptable in the world of videp calibration?

I am no expert by any means and have never used HCFR.. but I am guessing that the brightness and contrast are not correct. The lower end of the range is to bright (to much light) and the upper end is not bright enough. I assume there are no gamma adjustments on your projector.
Again not sure how HCRF works, but in Calman, the 100% is what is used to calculate the gamma at the end of the run, so if you do a full run, then change something, the only way to get an accurate graph is to do the complete run again so the software knows what the light output is at each percentage.

Write down the setting you have now.
Reset everything to default
set your brightness and contrast with the AVS disk (17 just flashing) everything below 254 flashing, all bars flashing up to 254 on the color clipping chart.
Then put up a gray scale ramp if you have no way to display chads patterns and what I talked about to see the interaction of cuts and gains.
find out what color clips first and then proceed to do the 2 point.
After the initial first set of adjustments at 80/30, go back to the brightness and contrast patterns and reset both if need be.
Then continue with 2 point.
When you are done with 80/30 recheck Br/Cont and do a complete run. If you have any colors that peek or dip along the way, you want to try and level them out, which may mean you need to give up some small amount of perfection on the 30 and 80 points. Watch our DeltaE charts (I assume HCFR has them) keep them below 3. It is better to have all of them for instance under 2 than to have 30/80 .5 and 50 and 60% 3.5

Get the gray scale correct with the brightness and contrast correct. If your display has no gamma adjustment you are stuck with whatever the gamma is. There is no way to effectively change it without gamma controls or a 10 point adjustment.

Just took a quick look at your projectors users guide.
These are the setting I would use to start with to do the calibrations
Mode Cinema
Image advanced setting
Noise Reduction off
Gamma Film
B/W off
Color temp Warm (but measure each to see which is closest to D65 if you have not)
Pure engine off
dynamic black off
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post #7 of 28 Old 12-21-2012, 03:33 PM - Thread Starter
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Doug,

I turned all the advanced picture settings off like you said. I measures warm in other modes, cinema, reference, user etc. and they are all approx. the same value 8000K +/-.

So if red clips at +10 for example, do I bring it back down to 0 and increase/ decrease blue and green only? I don't quite understand what you mean by "Once you know the color that clips, and you start your 2 point, for example it is red, and red is the lowest color, bring the others down to red."

Or do I set red at the highest possible before clipping and then leave it alone and adjust only blue and green for the gains? I also read that you set green bias on the low end to the point where it just clips video black and then only use red and blue for the bias adjustments and leave green alone.

I tried this method a few nights ago and although the contrast ratio increased I found there was a greenish hue in skin tones.

This is proving to be more challenging than I thought.
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post #8 of 28 Old 12-21-2012, 04:17 PM
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I'm sorry I should have been clearer. You are finding the weakest color so you know to be conscious of it and not push it to hard. Start with everything at default (use the reset display if you are not sure) if 0 is default. Then proceed. If red was the one to clip first (you have the least amount of head room with red), and green or blue is high to start, you don't want to raise red to much if at all, you want to drop the others, if you seem to be dropping the other to much then bump red just a little but be cautious . Every display is different and it takes some time to figure out how it acts. You will find as you lower blue or green, red will rise. As you lower blue and green the light output also drops, this is why you have to keep checking contrast as you proceed.
raising contrast is like raising each of the RBG gains the same amount. So dropping for instance if you dropped green and blue 4 clicks you may need to raise contrast a click. Yes, it can be very challenging in the beginning, there is a lot to absorb and things interact and don't always work the way we think they do or should.
You can't hurt anything, you will do this many times before you get it right and grasp it all. Like I said, I am no expert but I have learned how my own equipment responds. Yours may not be the same.. Take your time, take notes and it will all come together at some point.

Many have told me, never lower green, but on my particular display, bring red up does not work well. You will have to experiment and see what works best.
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post #9 of 28 Old 12-22-2012, 12:46 PM
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If there IS a "weak" color (one that runs out of gas before the others), you really only have to worry about it at 100%... maybe 95% (if you are doing a 20-step grayscale calibration), and in very very severe cases, it might affect 90% a little.

d65 does not define Luminance in any way. It defines ONLY color. We most often refer to calibration coordinates in terms of xyY coordinates... the x & y determine d65. Y is expressed in fL for calibration purposes (most commonly used in North America). So if every step of your grayscale measures d65 you are only addressing the x&y coordinates. The fL or Y coordinate can be ANY value and the point will still be d65. At 100% white you might have 35 fL and a perfect d65 measurement. At 75% white (with a 2.25 gamma target), you might have a perfect d65 measurement but the Y/fL number would be more like 17.5 fL.

Gamma is your "luminance factor" for grayscale calibration. You what each step of the grayscale to hit d65 or as close to it as possible, but you also want each grayscale step to hit the luminance target that you set by telling your calibration software what your gamma target is. Unless there is some overriding need for a different gamma, I use 2.25 for the gamma target because that is right in the middle of the range of gammas used in editing movies before the discs are mastered (2.2-2.3). Gamma is not a single number for the entire grayscale, though it could be expressed that way... for calibration purposes, you want each step in the grayscale to have the same gamma (2.25 is my choice). So to REALLY calibrate grayscale, you need to see at least 2 graphs... one that shows the relationship of red, green, and blue and another graph that, somehow, shows how close luminance is to your 2.25 gamma target at each grayscale step. You then need to use your grayscale controls to get both the color and luminance as close as possible to where you want them to be. If the video display has only 2 adjustment points for red, green, and blue, you don't have much control over luminance and the focus tends to be on d65 while gamma errors can be fairly large. You really want to try to minimize both color and gamma errors. That's best done with a TV that has 10-point grayscale controls (Samsung) or 20-point controls (LG) or an external video processor that has 10 or 20 grayscale adjustment points. Lacking 10 or 20 point grayscale controls means you'll have a much harder time getting xy and Y correct for each grayscale step. 10 or 20 point grayscale controls typically have hue, saturation and Luminance controls for each step... hue and saturation get your xy coordinates where they need to be go achieve d65. The luminance control gets your Y where it needs to be.

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post #10 of 28 Old 12-22-2012, 02:09 PM - Thread Starter
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So in my RGB levels graph (image 3) blue is +20% and red is -10%, green is tracking very close to 6500K. In the luminance graph (image 1) the OTB luminance is on target at all the IRE's 10 thru 100. Is the luminance correct because of the high blue in the mix?

In realize that all displays RGB high/low controls interact differently, in general how would you adust the RGB gains/ cuts in this scenario?

Adjust brightness.
Adjust contrast.
Set gamma curve/ offset next? (I have no idea what the curve type and offset means)
OR
Adjust grayscale, then gamma?
Recheck brightness, contrast between each adjustment of course..........

I have set my brightness by displaying a 0IRE field and dropping the brightness until there is no further reduction in brightness, various people on these forums have said that this is the preferred method for adjusting brightness on lamp based displays. So why would brightness require re-adjustment after a grayscale calibration if that is the deepest black my projector can produce?
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post #11 of 28 Old 12-22-2012, 03:27 PM
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Quote:
Originally Posted by guida74 View Post

Recheck brightness, contrast between each adjustment of course..........
I have set my brightness by displaying a 0IRE field and dropping the brightness until there is no further reduction in brightness, various people on these forums have said that this is the preferred method for adjusting brightness on lamp based displays. So why would brightness require re-adjustment after a grayscale calibration if that is the deepest black my projector can produce?

That is not really the best way to do it, using a proper pattern is best (first pattern on the AVS disk basic patterns) 17 should flash, 16 should not.
As mentioned before, when you turn up or down the cuts, you change brightness. They are one in the same .. brightness moves all RGB up or down together cuts move each color separately up and down. Same goes for gains and contrast.
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post #12 of 28 Old 12-23-2012, 05:12 AM
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On gamma graph table, right click and check RGB. It'll tell you which color went wrong.
Also download this excel. It makes everything easier
http://www.avsforum.com/t/1111066/new-calibration-tool-excel-download
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post #13 of 28 Old 12-23-2012, 05:48 AM - Thread Starter
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Here it is........
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post #14 of 28 Old 12-23-2012, 07:00 AM
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It seems to me that you didn't properly check your brightness/contrast settings.
Like I said above download the calibration aid excel.
- Enter 100% IRE Y value and enter Gamma target ; then increase Brightness so that Y reading of the 10 IRE window pattern measures as close as possible to the number in the excel.
- Adjust RGB low/highEnd using 30/80 IRE windows again.
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post #15 of 28 Old 12-23-2012, 07:50 AM - Thread Starter
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I will re-chech the brightness/ contrast settings and report back with my results. I thought the correct setting for brightness was to adjust where dithering is eliminated at 0IRE?
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post #16 of 28 Old 12-23-2012, 08:09 AM
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Quote:
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I will re-chech the brightness/ contrast settings and report back with my results. I thought the correct setting for brightness was to adjust where dithering is eliminated at 0IRE?

That works well for my DLP and coincides with the AVS pattern (the bar with 16 is video 0 look at that for the dithering) and 17 should still be visible), but if contrast changes so does the point the mirrors stop moving. It can also, depending on the projector, be to low (17 not flashing) and crush shadow detail. So set brightness, then contrast, then recheck brightness, if you change it on the recheck, recheck contrast again.. interaction..
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post #17 of 28 Old 12-23-2012, 08:13 AM - Thread Starter
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Will do. Thanks for everyone's input/ recommendations thus far......
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post #18 of 28 Old 12-26-2012, 07:07 AM - Thread Starter
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So I had another calibration run the other night and implemented some of your comments/ recommendations (check brightness/ contrast) each time I modified RGB high/ lows and the gamma is now more linear, luminance is still slightly under from 50% to 90% gray; can't achieve target luminance in the upper range for some reason. All in all I think it is an improvement over the previous calibration. I also tried playing with the gamma in the user menu; there is a curve type and offset (adjustable range -7 to +7 for each) if someone knows what combination of these values is required to raise the luminance in the 50% grays and higher I would appreciate your comments/ recommendations.

Here are my graphs.

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post #19 of 28 Old 12-26-2012, 07:44 AM
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Quote:
Originally Posted by guida74 View Post

S. I also tried playing with the gamma in the user menu; there is a curve type and offset (adjustable range -7 to +7 for each) if someone knows what combination of these values is required to raise the luminance in the 50% grays and higher I would appreciate your comments/ recommendations.

Figuring out how the controls work on each display is the job of the Calibrator! eek.gif You will have to make changes and take readings in a controlled manner to understand how they work, if they even work at all...
Looking at the users guide for your projector, I would choose the film gamma curve with any adjustment left at 0 and perform an 11point run 0-100%.
Save that chart, now make an adjustment (user guide does not show what the adjustments look like) but if you have a choice of adjusting the curve low/med/high then make the adjustment for the high end as that is where you have the problem.
Do another run and see which way it moved things.. repeat..

From the looks of the charts, your image should be very nice even with a little dip at the top end..
A small dip in gamma at the top end is not really going to be all that visible like it would be if it were at the dark end of the range.
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post #20 of 28 Old 12-26-2012, 07:53 AM - Thread Starter
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The gamma setings are adjustable, curve type and offset. I am tring to figure out what each of those terms mean with respect to gamma adjustment.
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post #21 of 28 Old 12-26-2012, 08:11 AM
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Quote:
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The gamma setings are adjustable, curve type and offset. I am tring to figure out what each of those terms mean with respect to gamma adjustment.
Gamma is the speed in which the image comes up out of back to full brightness.
This is represented as a curve 0 at the bottom up to 100% at the top. There are different curves for different types of content.
Film and HDTV uses 2.2 to 2.3 and that is what you want. PC Photography etc use other Curves.
The curve is achieved by the difference in the amount of light output between stimulus 10% to 20% to 30% etc. You have a set amount from 0 to 100 so how that is carved up between the 10 steps alters the way the curve looks and the image looks. You want a curve that is closest to how the HD signal was mastered. Then the adjustments alter that curve up or down ..some display will give you adjustments in different parts of the curve (low/mid/high) and some (10 point or more) will let you alter it in 10 or more places.
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post #22 of 28 Old 01-11-2013, 02:13 PM - Thread Starter
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Well I had another go at calibrating my HD8200 and it looks like things are better this go around, gamma is linear up to 80% then drops off, colour temperature is linear and luminance is at or close to reference values. The only way I could get the luminance correct was by increasing the contrast control, problem is that white is now clipping (above and below 235) when I display the AVSHD pattern for contrast. If I reduce the contrast to have everything up to 235 flash my luminance drops and gamma goes wacky. Any suggestions, or is this a tradeoff? Is my picture being degraded by clipping whiter than white and levels below white?
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post #23 of 28 Old 01-12-2013, 10:23 PM - Thread Starter
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BUMP

Anyone?
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post #24 of 28 Old 01-12-2013, 10:51 PM
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You have to forget about 6500K it is virtually worthless because green does not affect 6500K much at all... for practical purposes, 6500K defines the balance of red and blue with only a SLIGHT influence from green. I don't even look at color temperature while calibrating. If I hit d65, color temperature, by definition, will be perfect. So you x & y targets are color (neutral gray for grayscale measurements) and your Y measurement is what you use to dial in Gamma so you have the same Gamma at each point along the grayscale....

Gamma is not a single number for the display... you can express it that way, but you can also calculate a gamma for every step in the grayscale... 90%, 80%, 70%... 20%, etc. What you want is for every step in the grayscale to have a gamma of 2.25 (my preferred target because studios use 2.2-2.3 on the displays they use when creating movies). So if you have every grayscale step close to d65 and every step in the grayscale has a calculated gamma close to 2.25, you have a good calibration.

RGB Balance does not take Luminance/Gamma into account in any way. It only tells you if your xy (color coordinates) are accurate (little or no variation in RGB balance) or not. RGB Balance is not the same as RGB COORDINATES... 2 very different things. You really don't want to get into RGB COORDINATES when calibrating as RGB space is very counter intuitive for most people. xyY or Luv or Yuv are much easier to comprehend and work with for calibration purposes.

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post #25 of 28 Old 01-13-2013, 05:25 AM - Thread Starter
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Doug,

Looking at my first graphs vs. the ones in post #22 would you say that the later is a better and more accurate calibration? The HD8200 has 2 pt. greyscale controls and adjustments for gamma curve type and offset, no 10 pt. controls for either.

I was hoping someone could give me a technical explanation as to why gamma is linear up to 80% then falls off between 80% grey and 100% white? Is this deviation in gamma linearity perceptible to the human eye in this range?

Here are my x y Y values for my calibration, any further comments are greatly appreciated as I am learning about the calibration process.

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post #26 of 28 Old 01-13-2013, 10:04 PM
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You can't fix the gamma problem you have at 90% (and to a smaller degree at 80%) without 10-point gamma controls. If the projector doesn't have that type of grayscale controls, and only has 2 adjustment points, you just aren't going to be able to fix that problem... it's a result of an internal design issue in the projector. If you do have 10-point grayscale controls, all you have to do is make the correct adjustment to fix 80% and 90%.

Assuming the graph is telling you 90% is too bright, the adjustment you'd make depends on how the 10-point controls work. If they are RGB controls, you'd lower all of them by the same amout... say by subtracting "6" from each one. There may be some fine tuning of 1 or 2 individual colors required, but reducing them equally SHOULD make the step get darker. If the 10-point controls adjust hue, saturation, and luminance/brightness, then you'd only need to adjust the luminance/brightness control lower. But some fine tuning with hue and saturation may be required if the reduction in luminance/brightness is not perfectly linear for all 3 colors.

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post #27 of 28 Old 01-14-2013, 04:26 AM - Thread Starter
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That's what I thought; without 10pt grayscale controls the gamma in the upper end is not fixable with the 2pt controls. Would you say that the 2nd set of calibration graphs are an improvement or inferior to the initial calibration run I performed?

In real world movie/ video content how much information is actually presented in this 80% to 100% range? A spike or dip in gamma within the 30% to 80% range would most likely be more evident and detremental to the overall picture vs. one at say 80% to 100%?
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post #28 of 28 Old 01-14-2013, 10:13 AM
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If I was going to tune that last calibration a bit, I'd see if I could make red and blue a little darker at 80% and 90% without causing any problems with lower steps. I'd rather see the gamma go to 2.3 at 70% and 80% then drop to 1.9 at 90% than have the gamma stay at 2.2 and drop to 1.8 or lower at 90%. You did avoid leaving a plus green error which is good... you don't want excess green anywhere... that gets pretty obvious in images.

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