LibTiff Mailing List
TIFF and LibTiff Mailing List Archive
April 2009
Previous Thread
Next Thread
Previous by Thread
Next by Thread
Previous by Date
Next by Date
Contact
The TIFF Mailing List Homepage
This list is run by Frank Warmerdam
Archive maintained by AWare Systems
Thread
2009.04.27 20:28 "Re: Question regarding unassociated alpha", by Edward Lam
PS. I meant to say "Porter & Duff"'s paper. Edward Lam wrote: > > Probably a bug. > > This code only gets used in the high-level libtiff TIFFReadRGBAImage() > interfaces. Given that it's supposed to do all sorts of conversions for > you, I think it's semantically supposed to always return pre-multiplied > alpha images. Reading the code, it leaves the RGB components alone when > reading an associated alpha tiff, and multiplies in the alpha for > unassociated alpha tiff files. > > Given that this code was originally developed on SGIs, the norm back > then was probably to use premultiplied alpha. ie. influenced by Duff's > Compositing Digital Images paper. > > http://keithp.com/~keithp/porterduff/p253-porter.pdf > > Regards, > -Edward > > Chris Cox wrote: >> Probably a bug. Lots of people get alpha channels confused with >> transparency (associated alpha), especially in the 3D and video >> world. They forget that you can have alpha channels that are not >> transparency(associated) and you can have more than 1 alpha channel in >> a single file. >> >> Chris >> >> ________________________________________ >> From: Amir Ebrahimi [amir@unity3d.com] >> Sent: Monday, April 27, 2009 1:03 PM >> To: Chris Cox >> Cc: tiff@lists.maptools.org >> Subject: Re: [Tiff] Question regarding unassociated alpha >> >> On Apr 27, 2009, at 1:00 PM, Chris Cox wrote: >> >>> Unassociated alpha should never be premultiplied - it is not >>> associated with the color channels. >>> Only assoicated alpha should be premultiplied. >>> >>> Chris >> >> >> I agree. Why is libtiff doing this? >> >> :: Amir Ebrahimi :: >> Developer @ Unity Technologies >> www.unity3d.com >> >> >> >> >> _______________________________________________ >> Tiff mailing list: Tiff@lists.maptools.org >> http://lists.maptools.org/mailman/listinfo/tiff >> http://www.remotesensing.org/libtiff/ > >