Is DisplayCal’s matrix bpc perceptually spaced?

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  • #36723

    Alex.T
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    Hi,

    When creating a simpler single curve + matrix profile you can use DisplayCal’s bpc checkbox which makes the profile contain a true black. In an XYZLUT + matrix profile I think this is also applied to the matrix part automatically. Is it a perceptually based bpc like in Photoshop with relative colorimetric + bpc, or is it simply assigning black to 0 without this. If it’s the latter does it lead to different results depending on the display’s chosen trc? e.g profile says 2.2 to true black whereas real display tracks 2.2 to 50% then lightens up to display black. If you chose a different trc, eg srgb or messed with black output offset percentage would this affect the profile’s bpc accuracy too?

    #36764

    Vincent
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    This is ArgyllCMS behavior, DIsplayCAL is a GUI. For modifications or suggested features use ARgyllCMS maillist and write the full parameter list in DIsplayCAL log of each ArgyllCMS command.

    #37072

    Alex.T
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    I checked in other forums and it seems that this is a DisplayCal feature from a while ago. If anyone else is looking for an answer too here is a quote from 2013 on avsforum by Florian, idk if I can link to it:

    “If by BPC you mean “black point compensation” option present in dispcalGUI in contrast to the “black point correction” option (they are very different things, black point correction is an Argyll CMS option that corrects black point hue, while black point compensation as implemented in dispcalGUI I’ll describe here briefly):

    I’ll be frank, what black point compensation in dispcalGUI does is it will take the characterization measurements and then blatantly lie about them by mapping the actual measured black XYZ values to 0, scaling all other values accordingly. This has the effect that when a profile created with that option enabled is used in a transform with a source profile that has “perfect” zero black, black crush is avoided even in “dumb” matching scenarios (ie. when there is no other more sophisticated gamut mapping used, like the gamut mapping options involving a source profile when creating a LUT profile, or the BT.1886 option when creating a 3D LUT). This of course reduces the profile accuracy somewhat, with the error introduced by BPC being zero at white, and then progressively increasing towards black. This effect is greater the lighter the actual measured device black was. The main reason I implemented this option was because that’s how most commercial display profiling software seems to operate, and for “dumb” matching scenarios it eliminates black crush (at the expense of accuracy like I mentioned). Because I wanted to prevent the actual measured display response being totally lost, the measurement data (TI3) that is embedded in the profile is always the unaltered one, so one can create a profile without BPC from the BPC-enabled one (the way this works in dispcalGUI is by setting the desired profiling options, then selecting “Create profile from measurement data…” in the options menu and finally selecting the existing profile).”

    #37081

    Vincent
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    I thought ArgyllCMS had an option to make TRC in ICC RGB 0 => L* 0 with a 0 value to colprof -B. Your quoted text does not clarify if that GUI option in DisplayCAL is just translated to a colprof param or changes resulting ICC. I guess that is the former, hence a ArgyllCMS feature.

    #37084

    Vincent
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    You were right, it is not using ArgyllCMS params to do that (log file):

    -v  -qh -aS -C XXXX -M XXXX -A XXXX -D XXX

    for a matrix single curve with BPC, DisplayCAL was baking TI3 before feeding it to colprof. If you check DisplayCAL log file for that calibration you would find the whole param set for each of your scenarios.

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