[OSM-dev] Proposal: Database accelerator and dirty tile marker based on simple algorithm.
Nick Hill
nick at nickhill.co.uk
Sat Sep 16 22:48:06 BST 2006
Lars Aronsson wrote:
> The way we currently render roads, by drawing every line segment
> that fits in a tile, makes it extremely time-consuming to draw
> tiles in smaller scales (zoom=10 thru 0). The way out of this
> must be to invent a smarter algorithm to decide what to draw at
> each zoom level. Instead of millions of individual simple line
> segments, they could be grouped as higher level objects, each
> having a bounding box. All the minor streets in Oxford could be
> grouped as a local street mesh, rendered as an area in grey on the
> tile that shows England in the map of Europe (zoom=3). But this
> grouping has to be done long before it is time to render that
> tile. It will always be too slow to do a "select line_segments
> from ..." that returns many thousand database records.
My suggestion is that different classes of objects have a different zoom
level associated or specifically set for them. If we select the whole of
Britain but only those objects with zoom >=x, ie nodes and segments
which constitute motorways, national rail and large lakes, it would be
practical to perform such a select and to render the result.
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