ArgyllCMS profiles: Black Point Compensation gives composite black when checked

Graeme Gill

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I discovered that ArgyllCMS created printer profiles tend to use composite black when BPC is checked.

BPC is nothing to do with ArgyllCMS, it's strictly a CMM thing.

The type of black that is created (in CMYK profiles) is up to you - you set it with your choice of black generation when you create the profile.
If you are referring to RGB profiles then the profile has no control over the black composition, it's determined by the print driver or printer or whatever does the RGB to CMYK+ separation.
 

pharmacist

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BPC is nothing to do with ArgyllCMS, it's strictly a CMM thing.

The type of black that is created (in CMYK profiles) is up to you - you set it with your choice of black generation when you create the profile.
If you are referring to RGB profiles then the profile has no control over the black composition, it's determined by the print driver or printer or whatever does the RGB to CMYK+ separation.

What about RGB-profiles ? I use RGB-profiles and when using perceptual content with BPC the black point is elevated (lighter) than the printer can produce with pure photo black. X-rite Colormunki profiles uses the photo black when BPC is checked. So I should uncheck BPC when printing with perceptual content with ArgyllCMS created printer profiles to obtain maximum black level (photo black instead of composite black estimated by the ArgyllCMS profile) ??? Why does Colormunki profiles uses the maximum possible black when BPC is checked and ArgyllCMS not ?
 

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Edit: I just read the manual about the choice of black generation, but I can only find something about CMYK profiles, not about RGB-profiles.
 

Graeme Gill

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An Argyll profile often has substantial output values when the input is absolute black. Maybe the aim is to neutralize black?

There's a couple of problems with just assuming that the extremes of a device range will give you the extremes of the color. One is that it's an assumption, nothing guarantees such behavior. It's quite possible that some device may produce a lighter black for RGB = 0 than some other RGB values. The whole point of profiling is not to assume anything about a device, but to actually measure it.

The second issue is inversion. Generally the aim of profiling is to be able to turn a color you want, into the device values that will create it. This means inverting the measured behavior. So if you have a situation where the device output is relatively unchanging over a segment (say that the output of K = 100% to K = 80% produces almost the same L value), then when you invert it, you have a bit of a problem if you pin black to be 100%K in that the device values have to jump suddenly as the L transitions from a value of black to a value just above black. This can be quite hard to represent accurately in the B2A ICC table. So a safer approach is to use as a black the device values that produce a black that is very very close to the best possible black. (Sometimes it's hard to figure what that value is, and hard to represent it accurately through the color management chain.)
 

Graeme Gill

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What about RGB-profiles ? I use RGB-profiles and when using perceptual content with BPC the black point is elevated (lighter) than the printer can produce with pure photo black.
ArgyllCMS perceptual profiles are intended to be created for a specific source colorspace. If you do that, you certainly wouldn't select BPC, since this may well mess up the overall mapping. (see https://www.argyllcms.com/doc/Scenarios.html#PP5 ).
Edit: I just read the manual about the choice of black generation, but I can only find something about CMYK profiles, not about RGB-profiles.
RBG has no black channel. You can't control the black generation in an ICC profile without at least 4 channels.
 

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Nice to hear directly from the creator of ArgyllCMS. I always thought you should choose RGB when creating the targets as most commercial printers are pseudo-RGB and you should not choose CMYK when creating the targets, but maybe I was wrong about this.
 

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There's a couple of problems with just assuming that the extremes of a device range will give you the extremes of the color.
I noticed that RGB printer profiles with "elevated blacks" always have either the R, G or B curve ending with an output value > 0 at an input L = 0. I understand your statement, but in these cases the actual black value when the device output curves R=G=B=0 delivers a lower output L value (thus a deeper black). When and why does Argyll choose to generate profiles with RGB curves not converging at 0? Do you favor a more neutral black over a lower L? I have never seen this with X-rite.
 

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The ArgyllCMS curve is still the upper curve, much more elevated and starting at a much higher level on the L* out axis, this confirms @pharmacist 's findings that pure black at L* in = 0 prints results in a much lighter black than printing with the ColorMunki profile provides.
I'd be interested to know why that is. Could you email me the ArgyllCMS .ti3 file as well as the X-Rite .txt file of the measurement data, so I can take a look at it ?
 

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Nice to hear directly from the creator of ArgyllCMS. I always thought you should choose RGB when creating the targets as most commercial printers are pseudo-RGB and you should not choose CMYK when creating the targets, but maybe I was wrong about this.
You need to choose the device space that is correct for the printer/workflow you are using. You can't profile any other way.

If you have a choice between an RGB and CMYK workflow, then you need to decide whether you want to take control over the separation and ink limits (which takes more time and expertise but may give you results you prefer), or whether you want to leave all that to someone else by using RGB. You get no control over black point, separation or ink limits, and the gamut may be compromised, but you may also have no choice if the printer natively has more than CMYK inks and your profiler doesn't support that, or you have no workflow into that native mode short of buying a RIP.
 

Graeme Gill

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When and why does Argyll choose to generate profiles with RGB curves not converging at 0? Do you favor a more neutral black over a lower L? I have never seen this with X-rite.
Without being able to see the measurement data and know what the profile making workflow is (i.e. the options used to create the profile), I really can't say. The aim generally is to create the darkest black point possible. But the main limitation may be the neutrality of the lowest L* value point. The colorimetric table will be colorimetric, so if you pump L*a*b* 0,0,0, into it, it will give you the color with the minimum delta E to that target, which may well not be the value with the lowest possible L*. (Some profilers may be choosing to create a colorimetric table which is not so colorimetric, and has tweaks in there to give a more perceptual result. ArgyllCMS doesn't currenty do that.)

If you want a more perceptual result where the black point neutrality is compromised to give a lower L* value, then you would need to create and use a perceptual table - the point of a perceptual table is to allow that sort of non-colorimetric manipulation. Note that ArgyllCMS doesn't attempt to generate a "generic" perceptual profile (whatever that is!), you need to make a specific choice of source gamut to create a gamut mapping necessary for a perceptual table.
 
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