Le mercredi 30 mai 2012 22:26:18, Craig Bruce a écrit : > Even Rouault <[email protected]> wrote: > > There will be likely a compatibility problem if we do this, because > > GDAL up to now writes TIFF RGBA images with EXTRASAMPLE_ASSOCALPHA > > (pre-multiplied), but I think in most cases the data that people have > > feed in the GTiff driver was not pre-multiplied. So if we un-premultiply > > this would cause issues in case the alpha value is not 0 or 255. > > You only need to deal with altering the raw data on reading if the GTiff > driver were changed to always write unassociated alphas. Also, if people > have been writing non-pre-multipled samples to associated-alpha files in > the existing interface, then the TIFF data is damaged in any case. > > > The temporary conclusion of that is that there are likely images in > > the wild where the value of the flag does not reflect the reality > > of the imagery values... But, GDAL should likely be fixed to set > > EXTRASAMPLE_UNASSALPHA to better reflect the way it is generally used > > (presumably). > > There's not much you can do about damaged images in the wild. However, > we can try to prevent more from being created.
This is just what I've done in trunk. See http://trac.osgeo.org/gdal/ticket/4733 for the details. Best regards, Even _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
