Line around image plane in render

Hi everyone, I’ve created a nuclear explosion and have used an image imported as a plane to hide it behind the horizon. I know it’s quite a basic way of doing it, but it’s fine for what I need, although the only problem is that there’s a faint grey line around the image in the foreground when it’s rendered, as you can see here: http://i.imgur.com/Cw5BnTH.png

I’ve tried several things after looking online, with no success, such as checking premultiply, oversampling, alpha, shadeless, turning off filter/interpolation… Could anybody suggest how I can get rid of it? Any help would be greatly appreciated, since I’m quite happy with the nuke itself, and I’d rather not have to use an alternative method for the horizon at this stage.

Thanks!

Did not quite understand what exactly you hide behind where - screenshot would have helped, but anyways, here’s what i would have done. Not the state of the art, but hope helps.
http://www.pasteall.org/blend/19402

If it’s just 1 frame you’re after (and not an animation) you could bring that into gimp or photoshop and just blur the problem line area.

Thanks for the suggestion, but unfortunately it’s for a short animation :frowning:

Thank you for the file! I can import your foreground to replace the current one, so that solves the problem. The transparency of the top and the tree in the foreground is also really helpful. Thanks again for the great job!