[Qgis-user] Density dependent rendering?

[fork]

Hi all,

This may be a wishlist/ plugin thing, but is there a way have a layer render or
not depending on how many features there are in the frame?  So if there are 100
points in the frame the layer renders, but if there are 101 the layer is
invisible?

My use case is this: I look at census tracts overlaid on aerial imagery (a lot),
panning and zooming and identifying to verify population attributes. In urban
dense areas, the tracts are really small (because the census aims for more or
less constant population), but in rural areas they can cover an entire county.
I use scale dependent rendering, but I have to change it all the time depending
on what I am looking at. About a dozen tracts is usually the perfect amount to
look at in a frame.

It would be easier to write an algorithm for points than polys, but maybe it is
an interesting enough problem someone (hehe) should implement a naive approach
and see if it works.  I can imagine there being problems when the frame has both
high and low density pieces (is non-homogeneous), but maybe it would work
anyway.

I can also imagine that chaining several layers together so that they switch on
and off depending on the density might be useful.  You could use hierarchical
geographies (e.g. blocks, block groups, tracts, counties, states), but also more
abstract features too.

I have absolutely no idea how to implement this, but I would be happy to try/
report bugs/ whatever.

Thanks for the attention!