Hi everyone!
When rendering the object on a transparent background around the edges are uneven, just like no anti-aliasing.
For comparison, the same piece but with the background.
How do I fix it?
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I’m not sure, but you might try changing the filter. ( maybe to gauss )
It could simply be a matter of interpreting the file correctly afterwards. The image with the black halo looks to be using a premultiplied matte, but is being interpreted as a straight matte.
Not quite sure what should I do?
Can not be changing filter, otherwise this will give a very different effect for render.
Post the blend file and I’ll take a look.
how do you render the object?
do you just use nodes(wrong).
or do you hide the other objects(right)
labobo
I do turn on “Only Cast” the material’s other objects. And render. 
benu
Here blend-file)
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o_o.blend (364 KB)
I think benu is completely right
I assume you overlay it over white background in Photoshop?
If so, then have a look at this thread: http://forums.cgsociety.org/showthread.php?t=496772
Especially post 5: http://forums.cgsociety.org/showpost.php?p=4394411&postcount=5
The problem has to do with premultiplied alpha, which photoshop seems to have problems with.
Thanks for posting the blend. In the render settings panel, change the alpha from Premultiply to Straight Alpha. As sebastian_k mentioned, photoshop isn’t the best with this sort of thing, and it seems to prefer straight mattes when doing composites. Good luck!
EDIT: Sorry, I forgot you’re using 2.49 – in your render panel, change it from “Premul” to “Sky” and it works like a Straight Alpha in 2.5 (or it has for me thus far; I just tested it with your blend and it seems to behave the same way). And, holy smokes, 2.54 rendered that image about 30 times faster than 2.49!
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benu and sebastian_k thank you very much!
Good luck you too!!!