This time GIMP doesn't complain about extrasamples .... sources I used today: https://it.programqa.com/question/3451806/ uint16 out[1]; out[0] = EXTRASAMPLE_ASSOCALPHA; TIFFSetField( outImage, TIFFTAG_EXTRASAMPLES, 1, &out ); and https://github.com/GNOME/gimp/blob/mainline/plug-ins/file-tiff/file-tiff-sav... But should it be associated or unassociated alpha ?