Handle ASSOCALPHA and UNASSALPHA (associated and unassociated transparency values)

[GenevieveBuckley]

Handle ASSOCALPHA and UNASSALPHA (associated and unassociated transparency values)

https://github.com/GenevieveBuckley/napari-tifffile/blob/155ea1318b0f5156900f6cda898042776b984fc4/napari_tifffile/napari_tifffile.py#L153

From the docs at hyperspy tifffile

extrasamples (tuple of {'UNSPECIFIED', 'ASSOCALPHA', 'UNASSALPHA'}) – Defines the interpretation of extra components in pixels.

• ‘UNSPECIFIED’: no transparency information (default).
• ‘ASSOCALPHA’: single, true transparency with pre-multiplied color.
• ‘UNASSALPHA’: independent transparency masks.