GTiff: Use EXTRASAMPLE_UNASSALPHA by default (behaviour change w.r.t. previous GDAL versions)

[Even Rouault]

This was discussed in this email ​http://lists.osgeo.org/pipermail/gdal-dev/2012-May/032968.html and the subsequent thread.

The new behaviour is the following : when before, EXTRASAMPLE_ASSALPHA (=premultiplied) was used, we will now write EXTRASAMPLE_UNASSALPHA (=non-premultiplied), which better reflects the expected behaviour of writting applications.

This can however be altered by setting the ALPHA creation option to PREMULTIPLIED.

The ALPHA creation option now accepts :

• YES : EXTRASAMPLE_UNASSALPHA (was EXTRASAMPLE_ASSALPHA in GDAL < 2.0)
• PREMULTIPLIED: EXTRASAMPLE_ASSALPHA
• NON-PREMULTIPLIED: EXTRASAMPLE_UNASSALPHA
• UNSPECIFIED or NO : EXTRASAMPLE_UNSPECIFIED

This is only a change in writting mode. There is no change in reading mode (and in particular, there is no special processing of premultiplied)

[Even Rouault]

r24653 /trunk/gdal/frmts/gtiff/geotiff.cpp: GTiff: Use EXTRASAMPLE_UNASSALPHA by default (behaviour change w.r.t. previous GDAL versions) (#4733)

r24654 /trunk/autotest/gcore/data/ (3 files): Update test images to use EXTRASAMPLE_UNASSALPHA (#4733)

[Even Rouault]

r24655 /trunk/autotest/gcore/data/stefan_full_greyalpha.tif: Update test images to use EXTRASAMPLE_UNASSALPHA (#4733)

[Even Rouault]

Closing.

[Even Rouault]

r25411 /trunk/gdal/frmts/gtiff/frmt_gtiff.html: "GTiff: Document new accepted values for the ALPHA creation option (should have gone with r24653, #4733)"