Transparency of a white object dimming objects behind it.

[Leo]

Hey there, I need some help figuring out this shading problem I have here.

As you can see in the picture I have created some seed parachutes from a dandelion. Now while it looks fine from a little further away, I unfortunately get these weird black seems around the individual parachutes.
The object causing this is just a simple sphere with a the rendertree seen below on it. The alpha at the sides of these spheres is as far as I know 0 and they therefore shouldn't turn the white constant to this nasty black.

Nothing in the scene casts or accepts any shadows, so that can't be it.

What should happen here, is the outer edges of the spheres being completely see-through, just revealing the other parachutes behind them, but instead they are dimming the ones behind them.

I would very much appreciate any help you guys might be able to give.

Thanks in advance.

[Leo]

Here is another picture with the input we used for the transparency plugged into a gradient.
Red = 0 and pink = ~0.2
So the values coming out of there are exactly what they should be.

[Chris_TC]

Instead of using a Constant you should try using a Sprite node. As far as I know, sprite nodes can have unlimited amounts of alpha overlap without casting any transparency rays, i.e. they render much faster.
If you want to use a Constant you will probably have to increase the number of transparency rays in the renderer options.

[Leo]

Thanks, I do feel an improvement in rendertime using the sprite node. Weirdly enough a restart of xsi without any other changes made, completely fixed the problem.
I don't really understand how a restart of the program could rectify a problem like this and am kinda curious what might be responsible, but for now I'm happy that this strange behaviour is gone.