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

Xavier Thomas xavier.thomas.1980 at gmail.com
Thu Feb 10 00:19:40 CET 2011

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

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)
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