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Thread: Alpha misinterpretation in SE 2.0?

  1. #1
    Registered User Dufusyte's Avatar
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    Question Alpha misinterpretation in SE 2.0?

    There are several types of alpha: straight alpha, premultiplied alpha, white matte alpha, black matte alpha, etc. See this excellent article on alpha.

    SE 1.x often did not interpret alpha correctly (I think it always assumed alpha was unpremultiplied) leading to dark fringes and other anomalies on some clips.
    Quote Originally Posted by SBowie View Post
    I'm told it is quite a complex issue, but I see it marked as a "Show Stopper" in the bug database so it's not being taken lightly.
    Have these alpha issues been resolved in SE 2.0?

  2. #2
    'the write stuff' SBowie's Avatar
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    I think I answered this elsewhere recently (and so has John), but in any case the answer is unfortunately no. I know for certain some cases have been addressed, but I'm told the issue (or variants of it) can reappear in some other places.

    Personally, whether it only supported either premult or straight, I'd be happy enough. The trouble is the inconsistency in different places in the software.
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  3. #3
    Production Wizard radams's Avatar
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    Thanks Steve for clarifying this.

    Cheers,
    Ray Adams
    FX and Production Supervisor

  4. #4
    'the write stuff' SBowie's Avatar
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    I'll be glad when clarifying it means "To get the result you want, do this ...", Ray. In the meantime, my apologies.
    --
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  5. #5
    It is a broad question unfortunately.

    As Steve says, some have, but i suspect there are still some cases where it still happens.

    If you see a case like this, please report it along with a sample clip/still with instructions so that we can repeat it and we will try to address it as quickly as we can.

    Thanks,
    John Perkins
    Software Engineer

    "No, it's my natural color..."

  6. #6
    Registered User Dufusyte's Avatar
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    Lightbulb

    There are two aspects to alpha management:

    1. Determine the correct type of alpha for a clip. This can be done by:

      a) The application takes a good guess based on an algorithm

      b) or the application allows the user to specify it (e.g., in Properties)
      And naturally if the user specifies it, then this takes precedence over the guessing algorithm.

    2. Now that we know the alpha type, we correctly process the clip based on the alpha type.

    Step 2. (correctly processing a clip once you know its alpha type) is actually the easy step, and can be verified by the developers using one clip of each type of alpha type (a straight alpha clip, a premultiplied black matte clip, and a premultiplied white matte clip. No need to support other color mattes as that is beyond the call of duty).

    Step 1a (guessing the alpha type) is always imperfect, and you might even just assume all clips are one type, and if not then allow the user to specify the correct alpha type in step 1b.

    Step 1b (allowing the user to specify an alpha type for a clip) is probably the most useful both for the user and for the application programmer. It allows the user to specify the alpha type. It allows the application to know the correct alpha type. I strongly recommend implementing a dialog or Property where the user can specify the alpha type.

  7. #7
    'the write stuff' SBowie's Avatar
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    Everyday fundamentals of alpha channels aside, I agree that it is desirable to permit the user to manually specify a variant in Clip Properties (as a backstop for errant 'guesses' by the software).
    --
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