[Bf-codereview] Alpha premul pipeline cleanup (issue 7018048)

[sergey.vfx at gmail.com]

> I think Gimp might not be reading the metadata then, since it seems we
are
> currently writing that the image as associated alpha (= premultiplied
alpha):
> int extraSampleTypes[1] = { EXTRASAMPLE_ASSOCALPHA };

> We should change that to EXTRASAMPLE_UNASSALPHA for byte images I
think.

Gimp does read metadata, but could be bug there yes. Not sure if
Gwenview reads metadata tho. Strangest thing last night was that it
seems setting alpha mode to TIFF didn't just wrote metadata, but also
affected on pixels. At least that was an expression of mine lats night.

I'm currently working on a slightly different task (cleaning PNG 16bit
write support), will look into TIFF again after that. Don't spend time
on your own with this. I only mentioned current issues with TIFF, just
to show what else remained to be solved before commit and if you'll have
time would rather ask for help with painting since i'm not so much up to
this area.

> This might actually just be a precision issue that we can't fix,
thought it
> depends what you are testing exactly.  2D painting in the image editor
currently
> always works as if using an Airbrush in Gimp. When using a low
strength this
> will give very clear rounding issues that you will also find in Gimp
using the
> same tool.

> There might be another issue here as well though, I didn't test yet.

I'm just doing stroke of white brush with strength=1 over a transparent
byte image with black color.

https://codereview.appspot.com/7018048/