Particle alpha woes 2005-05-15 - By Andy Jones
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Hey,
Has anyone had problems getting proper alpha channels on particles? I
just inherited a bunch of scenes from some artists we contracted some
particle work to, and the renders don't give proper alpha channels.
Instead of giving nice gradients as the particle effect fades off at the
edges, I get mostly white balls, with a short little gradient on the
outside edges. The straight color channels also don't behave correctly,
as there are traces of black left in them from the background, as you
would expect from unmultiplying an incorrect alpha. I've actually
encountered this problem before myself, and haven't been able to find a
good workaround. I did just find a really crazy work-around, by
sticking a color math basic node between the particle renderer node and
the cloud material volume input. The color math node is set to multiply
the alpha channel by .1. The result is that everything works
correctly. Also strange is that if I multiply by a value less than .1,
nothing changes, but a value like .2 produces an intermediate result.
And the result isn't a grey alpha like you might think -- instead it's
just a normal black to white alpha, but with a much better gradient. I
rendered this out and compared the result composited onto a black
background an got the exact same result as if I had rendered the
particles premultiplied on black. It's as though the particle renderer
accidentally multiplies its alphas by a factor of 10.
The only drawback to my workaround is that using the color math node
seems to disallow having particles from multiple clouds in the same
particle volume. So, I have two basic smoke trails being emitted from
rocket engines, and without the color math node, they both render (but
with a screwy alpha channel). Then, when I add the color math node,
particles don't render in a box-shaped area between the two particles
clouds.
Does anyone have an explanation for this and/or a better workaround?
-Andy
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