Tiff writer does not specify ExtraSamples tag for alpha channel

[iger]

According to TIFF6 specification, if there are extra samples (values) in pixels, an ExtraSamples tag is required to specify their meaning.

ExtraSamples
...
The default is no extra samples. This field must be present if there are extra
samples.

I believe it should specify Unassociated alpha data (value 2) i.e. non-premultiplied alpha.

[Twinside]

Is there a use case where this is really problematic?

[Twinside]

Ah ok, make sense for YA* formats, I'll look into it

[iger]

Yes, it's for specifying transparency and it should only be present if there are extra values such as alpha in the data stream.

Sorry about not making myself clear.

I added the bug, because saving of RGBA images was already implemented and it (also) did not generate the tags. On programs I have tried (eye of gnome, gimp, Image Magick) the presence of the tag did not seem to make a difference on either RGBA or YA images. Programs seem to have good defaults as this is the most common use of an extra channel.

So no, it is not problematic, for now, for my use along with these programs.

Still, the generated RGBA and YA files are not proper tiffs and there are other valid uses of extra channels.