Antialiasing part of a picture

I am trying to create some web graphics where the top of the image needs to be antialiased to the background (white) and the bottom needs to be antialiased to a table which is blue. I cant work out how to do this, I would have thought if i create a blue square and put half the image over it then export the image that would have worked, but no joy.
Hope that makes sense
Any ideas?

I am trying to create some web graphics where the top of the image needs to be antialiased to the background (white) and the bottom needs to be antialiased to a table which is blue. I cant work out how to do this, I would have thought if i create a blue square and put half the image over it then export the image that would have worked, but no joy.
Hope that makes sense
Any ideas?

Just make two large rectangles the colors you need to anti-alias to: top one white, bottom one blue, and align them to match your web page layout. Put these behind all the other objects.

When you export, do NOT select those two rectangles as part of the export. This will make a fringe of anti-alias pixels the color of the background rectangles, without including the rectangles in the export. You need to export as a transparent gif or in another transparent format to do this.

I haven't tried this with two different anti-alias colors, but I don't see why it wouldn't work.

Let me know how it turns out if you try it.

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Thanks, that worked nicely