This argument should actually say "Do not store premultiplied channel
values", which is what the TIFF spec calls "Unassociated alpha" (vs.
"associated alpha" when values are stored premultiplied by alpha).
Now I can see where the current description is coming from, which is
that any color with alpha 0 (totally masked) ends up as RGBA value (0,
0, 0, 0), in other words, the color information is completely lost. Yet
this label is not very helpful to understand what the checkbox really
does. I decided to not just change it altogether as people would have
gotten used to this for years, but at least adding completary
information in API and tooltip in GUI.
18a2f576bb