Basic DCI/XYZ questions on values

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  • #3405

    Manuel Weber
    Participant
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    Hi Displayors

    There is something, for my understanding, I’d be glad to get a reaction from you people.
    It is concerning the DCI XYZ Colorspace and conversion from/to that field. I’ll make some assumptions:

    I learned that DCI uses the 100% XYZ range encoding of color coordinates for their material, though in cinema, everything will be “scaled” to (something equal to) P3 in realtime.

    So going to cinema means, not going into the “target” P3 space, but instead going to “raw, 100%” XYZ in gamma 2.6 and E Whitepoint and let the projector handle the rest.

    True?

    Given something like a sRGB space already has its XYZ coordinates, I conclude that these sRGB ZYX coordinates of colors must in fact be understood in bundle with the actual sRGB Primaries/Whites, and in combination of both there can be made a reliable statement for a conversion. Without the Space definition as a kind of “multiplicator” to the color values, they are meaningless. So a colorspace acts like a limiter, with which it is possible to “break out” to other spaces. This would be the reason that any topic (like Whitepoint Coordinates) in rec709 just is not the same as in other Spaces, because these three XYZ numbers just don’t mean the same thing even if they are based on the same logic.

    True?

    But if yes:
    How can it be that we target DCI gamma 2.6, if it is mathematically incorrect to do color scaling based on something other than a linear gamma? Greyzone for me.

    Thanks!
    Best Manu

    #3416

    Florian Höch
    Administrator
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    Hi,

    I learned that DCI uses the 100% XYZ range encoding of color coordinates for their material

    The DCI XYZ colorspace is just used as a container, normally it only encodes up to P3 primaries because that’s the most cinema projectors can handle – meaning that most of the available encoding space of DCI XYZ isn’t actually used, but it provides huge headroom for future advancements in gamut (e.g. Rec. 2020), which can then also be encoded in a DCI XYZ container.

    So going to cinema means, not going into the “target” P3 space, but instead going to “raw, 100%” XYZ in gamma 2.6 and E Whitepoint and let the projector handle the rest.

    Sounds about right.

    Given something like a sRGB space already has its XYZ coordinates, I conclude that these sRGB ZYX coordinates of colors must in fact be understood in bundle with the actual sRGB Primaries/Whites, and in combination of both there can be made a reliable statement for a conversion.

    All RGB color spaces (and despite its name, the DCI XYZ also falls into that category) can only be made sense of in conjunction with the primary and whitepoint XYZ color coordinates as well as the tone response curve.
    The difference between sRGB (or Rec. 709 where it derives the primaries) is that 100% red in sRGB gives you the Rec. 709 XYZ color coordinates, while 100% DCI XYZ “RGB” red gives you the XYZ color coordinates of an imaginary (non-existent in real life) color. If you’d encode the Rec. 709 100% red in DCI XYZ, it would result in a combination of RGB values that are considerably less than 100% in any of the red, green and blue channels.

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