[Bf-committers] Problems with premultiplied alpha channel in 8 bit images

[Xavier Thomas]

A solution/workaround would be to add a "unpremultiply" option for image
input in the compositor. It is bad pratice to composite or color correct
premultiplied image anyway so it might be useful even if the bug is
corrected.

2011/2/9 Lukas Tönne <lukas.toenne at googlemail.com>

> The changes to the viewer node were only related to the gl blend mode
> when drawing image pixels. Those were rendered with unmodified
> background color, which leads to plain addition of colors and
> excessive brightness in opaque areas. I changed the blend mode from
> (src*alpha + dst) to (src*alpha + dst*(1-alpha)), which should be the
> correct blend mode for straight-alpha (!) images. For pre-multiplied
> alpha it should be (src + dst*(1-alpha)), but i don't know if there is
> any way to determine this from a given image in blender? If there is
> no flag or similar to automatically set the correct blend mode, this
> could be a fourth user option for image drawing, so users can at least
> get the correct display if they know what alpha mode the image uses:
>
> 1. color only
> 2. straight alpha (s*a + d*(1-a))
> 3. premultiplied (s + d*(1-a))
> 4. only alpha as b/w
>
> However, this problem is probably not related to the bug described
> here (it might just have hidden it from earlier notice). My first
> attempt at finding the cause was not successful, but this png issue
> sounds promising ...
>
> On Wed, Feb 9, 2011 at 10:20 PM, gespertino at gmail.com
> <gespertino at gmail.com> wrote:
> > I forgot to link the blend file where I tried to reproduce the bug
> > inside the compositor by gamma-correcting alpha channel.
> >
> > http://ubuntuone.com/p/ROq/
> > This forces the bright halo artifact inside compositor and the
> > resulting appearance is the same you get when you compose the
> > premulled PNGs from blender in other application (or Blender's VSE)
> > _______________________________________________
> > Bf-committers mailing list
> > Bf-committers at blender.org
> > http://lists.blender.org/mailman/listinfo/bf-committers
> >
> _______________________________________________
> Bf-committers mailing list
> Bf-committers at blender.org
> http://lists.blender.org/mailman/listinfo/bf-committers
>