[Bf-cycles] problem compositing raytraced DOF

Brecht Van Lommel brechtvanlommel at pandora.be
Tue Mar 4 04:57:59 CET 2014

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.

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.

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.


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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