** Changed in: unity (Ubuntu)
Status: New => Fix Released
--
You received this bug notification because you are a member of DX
Packages, which is subscribed to unity in Ubuntu.
Matching subscriptions: dx-packages
https://bugs.launchpad.net/bugs/706863
Title:
Switch to premultiplied alpha blending
Status in Unity:
Fix Released
Status in “unity” package in Ubuntu:
Fix Released
Bug description:
Premultiplied alpha blending is the standard way of compositing
digital images. Porter and Duff, who are mostly known for having fully
developed the process of compositing, have actually defined their
compositing operators in terms of premultiplied components [1].
Premultiplied alpha has a lot of advantages over the way people tend
to use blending nowadays in (hardware accelerated) real-time
rendering. One of them is that with non-premultiplied alpha, 5 of the
Porter-Duff operators can't be defined (DST_OVER, SRC_IN, SRC_ATOP,
DST_ATOP, SRC_XOR), even with a separate alpha function. Another is
that with non-premultiplied alpha, using bilinear filtering for
texture magnifications generates color bleeding issues [2].
Unity currently only uses the SRC_OVER operator for blending two types
of sources, the non-premultiplied ones coming from GdkPixbuf, and the
premultiplied ones coming from Cairo. The patch I propose
premultiplies the sources loaded by GdkPixbuf, unifying the way alpha
blending is done in Unity. It makes it easier to define the blend
operation by removing the current need of separating the color and
alpha functions.
Another important point for premultiplied alpha in Unity is that
rendertarget (FBO) composition is fundamentally impossible using non-
premultiplied alpha [3]. That could explain some of the blending
issues that have been seen so far (we are currently using
rendertargets). Note that for this exact same reason, the XComposite
extension specification requires premultiplied drawables.
For more informations on the subject, see this nice blog post [4].
Once the Unity code is adapted, it would also be nice to have a helper
function to simply define the blending state using a Porter-Duff
operator enumeration.
[1] http://graphics.pixar.com/library/Compositing/paper.pdf
[2] http://www.youtube.com/watch?v=dU9AXzCabiM
[3]
http://blogs.msdn.com/b/shawnhar/archive/2009/11/07/premultiplied-alpha-and-image-composition.aspx
[4]
http://blogs.msdn.com/b/shawnhar/archive/2010/04/09/how-shawn-learned-to-stop-worrying-and-love-premultiplied-alpha.aspx
To manage notifications about this bug go to:
https://bugs.launchpad.net/unity/+bug/706863/+subscriptions
--
Mailing list: https://launchpad.net/~dx-packages
Post to : dx-packages@lists.launchpad.net
Unsubscribe : https://launchpad.net/~dx-packages
More help : https://help.launchpad.net/ListHelp
