It doesn't draw circles. It does lighting calculations per tile vertex and then it interpolates the result between vertices.
See the other screenshots I've posted in earlier comments to this post.

[EDIT]: Well, the interpolation is all done beforehand, so it's kind of like autojoining. I just need to set the icon state for the shade after getting the luminosity values for each vertex for the tile. The icon state is presently in format "[TopLeft]-[TopRight]-[BottomLeft]-[BottomRight]"
It looks so cool =). I don't think I'mma even bother trying to make something like that for the time being :p
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