rendering of boundaries of touching objects

When objects touch on a boundary (both object have the same nodes on
the boundary) and it's rendered antialiased, with the objects filled
without stroke, the boundary line is visible, it's a bit transparent.
At least in most renderers it is (Adobe SVG viewer, Batik, KSVG,
inkscape).

The question is, is this behavior wanted? It seems to me that if
there's no space between regions there shouldn't be a transparenent
line.

This behavior is caused by the way transparency works, two shapes of
50% opacity don't make 100% opacity, so the antialiased edges keep
transparent.

I can't think of an easy way to get rid of those lines, but if there's
need for maybe we should think harder.

For example this is a problem with autotracers like autotrace or the
potrace based tracer of inkscape, where are transparent lines between
every shape.

see the attached file for an example.

boundary-lines.svg
Description: image/svg-xml