So do you remember Euclidean and their "infinite detail" engine? If not 
there are plenty of videos on YouTube like this one:

https://www.youtube.com/watch?v=00gAbgBu8R4

Anyway, it seems they recently released a few more videos and in one of 
the comments I found a link to their patent:

http://tinyurl.com/eucli

It's quite interesting actually. They store their point cloud in an 
oct-tree structure and walk down the tree until you get to a single 
point in the data or a single pixel on the screen. But, the clever bit 
is that once they've done the perspective transform on an oct-tree node, 
they look at the "w" coordinate and use that to decide if an 
orthographic projection would be less than 1 pixel different from a full 
perspective projection. If yes, then all the child nodes' 
screen-positions can be computed very fast by just taking mid-points of 
the parent's 2D coordinates, rather than doing full perspective 
projections.

Still their claim of "unlimited detail" is misleading, but you can see 
how with the above system it would be straightforward to 
page-in/download nodes as you move around the scene. And with oct-trees, 
you only need a few levels to get huge numbers of points, eg 10 deep 
gets you a billion points.

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