[Bf-cycles] problem compositing raytraced DOF

[Bassam Kurdali]

On Tue, 2014-03-04 at 04:57 +0100, Brecht Van Lommel wrote:
> C is right in general, however the scene setup is a bit strange here.
> The camera is close to the floor, and the aperture radius is big
> enough that part of the lens is below the floor. If you move the
> camera up a bit the problem is gone in this scene.
The camera in my real scene is riding on an escalator, exposing the
character's feet at the end; hence the camera plane and the ground being
level - and my need to keep it that way.
> 
> In general you indeed can't composite renders with alpha over
> accurately when there is motion blur or transparency, one of the
> layers needs to be clearly in front of the other for that to work
> (unless we supported deep compositing).
> 
> What you are trying to do with is some workaround for lack of light
> groups I guess? I think what you propose will work if the foreground
> is in front of the background everywhere.
indeed, I'm using renderlayers as a substitute for light groups.
> 
> If that's not the case it's still possible to add two layers together
> by letting both render layers mask out the objects on the other. Then
> you can simply add the layers together and it should be the same as
> the combined result. I did notice that the Mix node Add mode does not
> do this properly , it will take the alpha from only one of the images
> while it should be adding both in this case, so to get the correct
> results you need to add the alpha separately with its own Mix node
> with Add, and then use the Set Alpha node.

Thanks for the advice, I'll try the add node solution next :)
> 
> Brecht.
> 
> 
> On Tue, Mar 4, 2014 at 3:57 AM, Bassam Kurdali <bassam at urchn.org> wrote:
> > When compositing raytraced DOF I'm noticing that alpha is not where I
> > expect it to be, i.e. there is some transparency in opaque part,
> > resulting in light leaks, at least that's what I think is going on.
> >
> > Hi all, I'm not sure if this is:
> > a- a bug
> > b- not enough samples issue
> > c- just not possible to comp alpha blurs this way :)
> >
> > so I'm posting a .blend here:
> > http://www.pasteall.org/blend/27271
> >
> > If the answer is C am I correct in assuming I can still render in render
> > layers, and use the add node to add light in one layer on specific
> > objects?
> >
> > so instead of :
> >
> > background has light A
> >
> > foreground has light A + light B
> >
> > alpha over foreground to background
> >
> > I'd do:
> >
> > background + foreground has light B
> >
> > foreground has light A
> >
> > add foreground to background + foreground...
> >
> > cheers,
> > Bassam
> >
> >
> >
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