In my experiments with 2.69 cycles render, I found that bright mesh lights don’t AA very well and cause massive jagged edges. Increasing AA samples doesn’t do anything and turning up the Gaussian makes the rest of the image look out of focus.
Anyone else having this problem?
Blend file link below:
Particle fluids.blend (555 KB)
It’s blowing out the alpha on the edges by increasing to the point that it’s => 1. The only workaround I know of for this is to render a duplicate scene without cycles enabled and only the “solid” check box enabled (solid only will reduce render time for scene 2). Also, on the passes panel, make sure that only the “Color” pass is enabled. Import this scene’s render layers into the current scene and use the Color pass with a “Separate RGBA” node into a “Set Alpha Node”. The color pass is the only pass other than the combined pass which contains an alpha channel. This will GREATLY reduce render time as opposed to using the combined pass to extract an alpha channel. Unfortunately you can’t choose your AA filter type or size with cycles so what you see is about as good as it gets barring rendering from two separate blend files (so you can get the filter type and size that you want for your alpha channel) and importing to recombine images. Check out the .blend attached below the image.
one thing you can do to avoid this isssue is making the camera see a different emission shader (via light paths, camera ray into mix shader), or hiding the emitter from the camera completely (Object Properties Button, Ray Visibility).
This isn’t just a Cycles issue, many path tracers have the same problem. There’s not a really elegant solution in terms of code as far as I know. Using different closures for camera visibility is probably the most common way to get around it.
I Copy the lights to a different layer and then hide the originals from the camera. The ones I copy are then toned down in strength and then composited back into the image. The originals just supply the light, and the copies are what is seen in the render.
As m9105826 (could have chosen a less crazy nick Matt 
) said, the best solution is to use the Is Camera Ray output from the light path node to use one emission shader for actual lighting, and a separate one to control what it looks like to the camera. Keep the camera’s one using a strength of 1.0 and you will have no AA issues afaik.
The blue node is the one the camera sees, the orange node is the one the rest of the scene “sees”.
This same technique can be used to make some funky effects:
