David Gowers schrieb:
> What happens when we want to save a 24bit png with no alpha from a
> single layered image? how is this detected?
> The current distinction between layers with and without alpha channel
> allows the user to make that clear..

A set of good rationales is IMO:

GIMP is an XCF editor. It can't be a perfect PNG, TIFF, GIF,.. editor
at the same time - at least not with a good UI.
That means, the complexities of export should not show up during editing.

Getting the the most out of a certain file format is an essential difficulty,
which deserves a dedicated UI of it's own. It is non-negotiable that
GIMP allows the user to export optimal files. And a lot of excellent work
has been done to provide this functionality.

Allowing access to all the nitty-gritty details doesn't rule out to
provide a bird's eye view on export which concentrates on the central
trade-offs (typically size vs. quality) for users who don't care.

Concretely, the export process must provide a way to remove the alpha
channels. The best way to present this i could think of yet, is to
do this as a chain of operations, resembling a GEGL branch on top of
the image. For the PNG example, this would be something like:

      write to bla.png
      convert to PNG
      fill transparency with image bg color
      merge layers

Such an 'export branch' can be autocreated with sensible defaults,
(many times as soon as on import time). Besides other benefits,
these defaults will just be "good enough" most of the time.


(of course the better part of the rationales is stolen
 from the UI team, any flaws are my own)

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