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Discussion Starter #1
First of all a small indication of what I do know about it. I know what the purpose of a CMS is and that it needs to be able to adjust saturation, hue and lumincance for both primary and secondary colors.

What I do not understand is that is how the CMS is implemented and what the impact of the different implementations is.


Some questions that I hope someone can answer or discuss with me:


1. Are the CMS implementations in VP's like those of Lumagen and DVDO 1D luts or 3D luts?


2. I've been reading thinks like [insert number here] point gamma calibration and greyscale calibration. What does this mean? I understand more is better but what does it mean?


3. More generally speaking, out of which components does a CMS exist? Is the basic product of any CMS creating a LUT (and applying it obviously)? If yes, there are only 2 possible implementations : 1D luts and 3D luts?


4. So, if you go with 1D luts, there is a lut for R, G and B, gamma and greyscale? (so 5 luts?). If you go with 3D luts, the aforementioned eurm... adjustment area's are comprised within 1 single 3D lut?


Sorry for all the questions. I've got an Iscan duo and would very much like to know how "good" it is with regards to other solutions. And knowing "it will do just fine" doesn't do the trick with me



thanks,

Jeroen
 

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Most video processor like the lumagen, duo and videoeq use a look up table internally for grayscale and a matrix for CMS.


These type of LUTs have an index and RGB values, but each channel is looked up individually. So if red at 200 is turned down to 190, then 200,200,200 is output as 190,200,200 and 200,16,16 is also 190,16,16. If red is oversaturated that's what the matrix is for.


A 3D lut has a unique index for r,g,b that means each channel is factored as part of the look up and rgb value is returned.
 

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Quote:
Originally Posted by Jeroen1000 /forum/post/20933534


1. Are the CMS implementations in VP's like those of Lumagen and DVDO 1D luts or 3D luts?

Since they operate over 3 planes of adjustment--hue, saturation, and luminance--then they must be considered 3D.

Quote:
Originally Posted by Jeroen1000 /forum/post/20933534


2. I've been reading thinks like [insert number here] point gamma calibration and greyscale calibration. What does this mean? I understand more is better but what does it mean?

Traditionally, displays have offered only the ability to adjust grayscale at 2 levels of stimulus--high end and low end. Gamma adjustments are generally even more course offering several preset curves only. What the Duo and Lumagen (and now some displays) offer is the ability to adjust at 10 or even 20 points of stimulus throughout the grayscale. This allows for a very precise of adjustment of white point and gamma.

Quote:
Originally Posted by Jeroen1000 /forum/post/20933534


3. More generally speaking, out of which components does a CMS exist? Is the basic product of any CMS creating a LUT (and applying it obviously)? If yes, there are only 2 possible implementations : 1D luts and 3D luts?

No, you could have a 2D--saturation and hue.

Quote:
Originally Posted by Jeroen1000 /forum/post/20933534


4. So, if you go with 1D luts, there is a lut for R, G and B, gamma and greyscale? (so 5 luts?). If you go with 3D luts, the aforementioned eurm... adjustment area's are comprised within 1 single 3D lut?

Not sure what you are asking here, and much of this has to do with terminology. An LUT is a Look Up Table. The device receives a signal, identifies it, then it uses a table of stored values to replace the image characteristics of the incoming signal with associated characteristics stored in the table. The CMS and gamma/grayscale adjustments in the Duo and Lumagen are the best available for consumers. There are no doubt more sophisticated solutions aimed at professional applications that will cost many thousands of dollars.
 

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Discussion Starter #4
Thanks for the replies. Regarding 4, I was reading this thread . Steve Shaw (light illusion) seems to talk about a some other kind of "true" 3D LUT.


There is also a good link explanation of things (I sort of understand) here.


So there appears to be a 3x 1D LUT and a "true" 3D LUT. I was thus wondering which type is present in consumer video processors and whether we as consumers should care about the 'true' 3D LUT Mr. Shaw is talking about.



If I understand Sotti, the answer would be a "3x 1D" LUT?
 

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Quote:
Originally Posted by Jeroen1000 /forum/post/20934880


Thanks for the replies. Regarding 4, I was reading this thread . Steve Shaw (light illusion) seems to talk about a some other kind of "true" 3D LUT.


There is also a good link explanation of things (I sort of understand) here.


So there appears to be a 3x 1D LUT and a "true" 3D LUT. I was thus wondering which type is present in consumer video processors and whether we as consumers should care about the 'true' 3D LUT Mr. Shaw is talking about.

Like I said above in a traditional LUT + Matrix.

The matrix handles the hue, saturation and luminance (relative to white) of the primaries and secondaries. The LUT handles the white point and gamma.


A 3D LUT has every possible input combination specifically mapped to an output value. Obviously this is more flexible, but also more complex and not nearly as straightforward to build a UI for.


For a display that behaves in a reasonably linear fashion, there isn't a huge benefit for a 3D LUT. On displays that aren't or in cases where you want to have absolute control over the colorspace, you'll need a 3D LUT.
 

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Discussion Starter #6
Thanks Sotti,


I think I sort of understand. Is there any reason why they have opted for a matrix (instead of just serveral 1D luts for R, G and B respectively)?


I may not have the correct mathematical vocabularly in english, but a 1D lut looks very much like a matrix to me.

Granted, I would not be asking this if I'd have a full understanding. I think details like these make AVS an excellent source for those who like techie stuff


So, It would seem I'm a bit confused what the difference is between a LUT and a matrix.


hmm, I see you have actually explained this. I'll have to reread very carefully...

Quote:
Most video processor like the lumagen, duo and videoeq use a look up table internally for grayscale and a matrix for CMS.

These type of LUTs have an index and RGB values, but each channel is looked up individually. So if red at 200 is turned down to 190, then 200,200,200 is output as 190,200,200 and 200,16,16 is also 190,16,16. If red is oversaturated that's what the matrix is for.


A 3D lut has a unique index for r,g,b that means each channel is factored as part of the look up and rgb value is returned.

To eliminate a possible language miscommunication, I'm just asking the following:

The sentence beginning with "These type of LUTs" is referring to " a the matrix for CMS", correct?
 

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Quote:
Originally Posted by Jeroen1000 /forum/post/20935387


Thanks Sotti,


I think I sort of understand. Is there any reason why they have opted for a matrix (instead of just serveral 1D luts for R, G and B respectively)?


I may not have the correct mathematical vocabularly in english, but a 1D lut looks very much like a matrix to me.

I admit I would not be asking this if I'd have a full understanding. I think details like these make AVS an excellent source for those who like techie stuff



I'm a bit confused what the difference is between a LUT and a matrix.

In order to adjust hue and saturation, you'd need to adjust the opposite primaries. It's not something you can do with a 1D LUT.


Lets make up a little LUT here.

This is a basic pass through LUT.
R020406080100
G020406080100
B020406080100
 020406080100

Lets say Gamma was a little low and it was also a little too green we'd have a modified LUT that would look more like:

R018385679100
G01736537796
B01939577899
 020406080100

Of course a real LUT would have every value 0-100, I dropped the interim values so we'd have something easier to look at.


So this make all three primaries gamma correct they now ramp linearly and white is D65. So if you have a white input of 80,80,80 you'd get an output of 79,77,78. That's all you can do with this type of LUT. A 100% red signal would be an input of 100,0,0. In order to decrease it's saturation you'd need to add both Green and Blue. So you can see that this data structure doesn't have anyway to determine if the blue look up is simply for red saturation or where in the color space it is. It simply says blue input =0, output = 0.


So that's where the matrix comes in, which is a little to complex to describe here, but it takes the inputs and can recognize when it's dealing with 100% saturated red, or something a little closer to white and correct saturation, hue and luminance as the colors run out from the white point.


The 3D LUT instead of having 2 indexes (r,g or b, then 0-100) and storing just one value per cell has 3 indecies(0-100,0-100,0-100) and stores 3 values per cell, so instead of storing 300 single value it stores 1,000,000, triple values. Of course in the digital world it's not 0-100 it's either 8bit (0-255) or 10bit (0-1023). So for a 10 bit 3dLUT you'd have 3 axis of 1024 which means 1,073,741,824 30 bit value instead of for a 10bit 1D LUT that would have 3072 10 bit values.
 

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Discussion Starter #8
Yes, but you could do this with three 1D luts? I think that this way you can arrive at the same result, you just do it differently. I'm letting this sink in a bit now



edit: You have added some more details. Disregard the above comment if it addresses my Q
 

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Quote:
Originally Posted by Jeroen1000 /forum/post/20935387


The sentence beginning with "These type of LUTs" is referring to " a the matrix for CMS", correct?

No I was referring to the LUT portion of typical CMS systems which are a LUT + a Matrix.


The matrix looks more like this.
 RGBCMY
R100000100100
G010001000100
B001001001000

It takes an input and fans it out and says if you take the red input and multiple it across the red column , the Blue input multiply it across the blue column, same for green. Then you sum the components from each column. It's actually significantly more complicated than that so you can account for the secondaries, but that's sort of the gist of it.
 

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Since we only calibrate greyscale at 11 points (for example) how are all the intermediate values (say 21-29%) derived. Interpolation?
 

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Discussion Starter #12
Fantastic effort. Thank you. Nevertheless, I'm very glad one does not need to worry about this when using calibration software. That would really ruin it haha



@Geof,I think linearity (as in a linear display response) is assumed and that they are indeed interpolated then. But I'm still slowly getting my head around this so I might me miles off.
 

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Quote:
Originally Posted by Geof /forum/post/20935642


Since we only calibrate greyscale at 11 points (for example) how are all the intermediate values (say 21-29%) derived. Interpolation?

Yes they are usually something proprietary, but some kind of cubic spline.
 

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Cubic spline makes sense. Thanks!
 

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Discussion Starter #15
Just wanted to report I get it now! However, how does this work with computer monitors? You have the 1D lut in the video card OR screen (not likely because then it would be a very expensive screen).


But, this 1D lut only takes care of gamma and greyscale. What takes care of the gamut errors? In other words, how do you add a CMS to a computer monitor?
 

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Quote:
Originally Posted by Jeroen1000 /forum/post/20940759


Just wanted to report I get it now! However, how does this work with computer monitors? You have the 1D lut in the video card OR screen (not likely because then it would be a very expensive screen).


But, this 1D lut only takes care of gamma and greyscale. What takes care of the gamut errors? In other words, how do you add a CMS to a computer monitor?

On an OS wide level, nothing takes care of the gamut errors. The gamut just is what it is.


There are ICC profiles that document what that gamut is. ICC aware applications like photoshop can use that ICC profile to render images in their target colorspace as long as the monitor is capable of it.


ICC is a fairly robust container for gamut correction data. It can either be matrix style or 3D LUT information. If only it was implemented on an OS wide level I think users would be much happier.
 

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Discussion Starter #17
So no graphic card is capable of doing the gamut correction? And there are no "CMS-boxes" that can be placed between the graphics card and the monitor?


Hmm, but basically, a Computer can do just about anything a VP can, so I'm assuming there is software capable of doing gamut correction?


Those ICC profiles sound nice but all depends on how much software decides to use the profiles. I'm not sure this can be trusted...
 

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Quote:
Originally Posted by Jeroen1000 /forum/post/20941033


So no graphic card is capable of doing the gamut correction? And there are no "CMS-boxes" that can be placed between the graphics card and the monitor?


Hmm, but basically, a Computer can do just about anything a VP can, so I'm assuming there is software capable of doing gamut correction?


Those ICC profiles sound nice but all depends on how much software decides to use the profiles. I'm not sure this can be trusted...

None of the graphics card currently have independant CMS, some have global hue, saturation controls, but those aren't much good.


Any of the standalone VP can be placed inbetween a computer and PC. I have a videoEQ on mine



ICC applications are reliable for accuracy, but there aren't many available beyond the Adobe Creative Suite.
 

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Or you can use just a little 3D lut program on your pc for a fraction of the price. e.g. UpsilonMixer


Or as free alternative yCMS.
 

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Discussion Starter #20
A Video EQ is nice, although a bit expensive for just a PC monitor (seeing the very good color critical monitors have a CMS, or it does look like they are starting to do that anway's).


Thanks for the hints for software CMS applications, those look both interesting and promising!
 
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