[Bf-committers] Color space and alpha issues

Bartek Skorupa bartekskorupa at bartekskorupa.com
Fri Dec 16 08:50:44 CET 2011

Re wiki page: When explaining math behind mixing "Over" image with  
background (straight alpha) it should be: [(Over.RGB * Over.Alpha) +  
(Background.RGB * (1 - Over.Alpha))].
Formula at the page is: [(Over.RGB * Over.Alpha) + Background.RGB].
Shouldn't it be corrected?

In my opinion it should be as follows:
"over" image mixed with "normal" blending mode, or "mix" blending mode:
  [(Over.RGB * Over.Alpha) + (Background.RGB * (1 - Over.Alpha))]

"over" image mixed with "Add" blending mode:
  [((Over.RGB + Background.RGB) * Over.Alpha) + (Background.RGB * (1 -  

"over" image mixed with "Multiply" blending mode:
  [((Over.RGB * Background.RGB) * Over.Alpha) + (Background.RGB * (1 -  

Other blending modes - accordingly to their math.

Bartek Skorupa

On 2011-12-15, at 15:31, Brecht Van Lommel wrote:

> Hi,
> Color management and alpha issues are a recurring topic, I've written
> a wiki page explaining issues with the current code and what I think
> should be done to solve them:
> http://wiki.blender.org/index.php/Dev:Source/Image/Color
> You might notice I'm not really mentioning OIIO/OCIO. It's a nice
> project to integrate those, but it doesn't solve current issues and is
> fairly orthogonal to the proposed changes.
> I might not have time to tackle this for 2.62, but it doesn't
> necessarily need to be done all in one big change. Feedback welcome,
> about what you think should be done different, and which alpha format
> convention should be used?
> Thanks,
> Brecht.
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