[Bug 783591] openjpegdec: doesn't support grayscale with alpha

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=783591

--- Comment #8 from Aaron Boxer <boxerab at gmail.com> ---
Another issue to consider: there are two types of alpha - pre-multiplied and
non-premultiplied. With pre-multiplied, the alpha has already been applied to
the channel, and stored in the alpha channel. With non-pre-multiplied, alpha
represents a floating point number between 0 and 1, which must then be
multiplied with the colour channel to give the alpha representation.

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