Continued from: 
http://groups.google.com/group/google-maps-js-api-v3/browse_thread/thread/b7d796cfa05a40de/48816cfff2d6807e
(can't bump the old thread for some reason).

A year on the project is very nearly complete, but I have a final
problem;

Each diagram uses about ~130 layers/overlays. To improve performance,
when large numbers of layers are visible, there need to be special
layers (which I’ll call “merged-layers”). These are basically just
flattened versions of several layers, designed to decrease load time
and bandwidth.

So in the example at: http://www.chiasm.co.uk  under the layers tab
you will see one called “Abdominal MERGED”. This is just a flattened
version of the 4 layers above it (plus one hidden layer).

But I’m having trouble finding a way to implement this smoothly. There
are several problematic areas;

1.) As far as the user is aware, these special layers don’t exist. So,
the checkboxes of those subordinate layers must remain ticked, even
though they aren’t really being displayed. If the user unchecks one of
them, the merged layer must be removed and the remaining ticked layers
added.

2.) There need to be several ‘tiers’ of merged layer. So for example
“Abdominal MERGED” is made up of those 4 muscles. But there might be a
‘higher’ (in zIndex) merged-layer which covers the entire thorax, and
maybe one above that that covers all muscles. The system must pick the
‘best/highest’ merged-layer to show.

3.) These merged layers will mess up any layer opacity settings, so if
layer opacity is < 100% they must not be used.

Some of these problems arise because currently layer visibility is
directly derived from the checkboxes. I wonder whether we could shift
control back to the layers array? So we make changes to the array
rather than checkboxes (which merely ‘emulate’ the parts of the array
we want them to).

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