Certainly sounds like an interesting idea. I suspect that the devil is in
the details since we have to manage many projections and data sets but your
approach seems like it should work.
Ian
On 6 Aug 2017 1:06 p.m., "Andrea Aime" <andrea.a...@geo-solutions.it> wrote:
> Hi,
> working on the OSM-bright clone I've stumbled into an interesting approach
> that couples
> avoiding loading polygons that are too small, along with eliminating the
> issue of repeated
> labels on very large polygons.
>
> Here is OSM-bright approach:
>
> - Precompute the area of the polygons (called way_area in the database)
> - Have variables providing the width/height of a pixel, in meters,
> pixel_width, pixel_height (in OSM the data is both stored and rendered in
> 3857)
> - Use these to avoid loading polygons whose area is too small to
> contribute anything significant, e.g.:
> - "... AND way_area > 100*!pixel_width!::real*!pixel_height!::real"
> (used to load polygons for labelling purposes, only if they are at least
> 100 square pixels)
> - "... AND way_area > 0.01*!pixel_width!::real*!pixel_height!::real"
> (used to avoid loading polygons that are just too small, less than 0.01
> square pixels)
> - Use these to avoid painting labels if the polygon they refer too
> became too big on screen (e.g., zooming on a country, at some point you
> don't want to add the country name anymore), combining:
> - "SELECT ...
> way_area/NULLIF(!pixel_width!::real*!pixel_height!::real,0)
> AS way_pixels" in the query
> - " [zoom >= 3][way_pixels > 1000][way_pixels < 360000] {" in the
> style (in other words, paint the label if the polygon is at least 100x10
> and no more than 600x600)
>
>
> The approach using way_pixels is neat in that a pure scale dependency does
> not account for the relative size of countries, e.g., while one wants to
> hide "Germany" soon enough, at that same zoom level "Liechtenstein" (pick
> any city state here) still needs to be visible.
>
> So, I want to have something similar. A pure rendering based approach
> would be to add a "max_polygon_area" vendor option, to be expressed in
> pixels ("goodness_of_fit" already takes care of the too small ones).
> However, this still loads everything to decide, later, if it's useful to
> draw or drop a certain label, with the amount of data in OSM it's smart to
> pass this information down to the database.
> However... this requires assuming that there is a pre-calculated area in
> the db with a given name (area calculation is not super-fast, especially
> for complex polygons).
>
> So instead, I'm thinking of adding a filter function that would take a
> target CRS and would return the area of a pixel in that CRS, which can then
> be used
> in a filter, for example:
>
> [(way_area / pixel_area('EPSG:3857')) between 1000 AND 360000] { label:
> .... }
>
> The data access code will recognize that pixel_area does not refer to
> attributes, and will optimize it out with a static value
> before running the query (making it go fully down in the dbms).
> Implementation wise, the pixel_area function will be in gs-wms and will
> leverage the existing wms_* env variables to do
> a rough job (no need to make it precise, but needs to be robus), like bbox
> area over pixel area, and then do a unit conversion
> if necessary (using tables to get a rough degree to meters conversion, if
> necessary).
>
> What do you think?
>
> Regards,
>
> Andrea Aime
>
>
> ==
> GeoServer Professional Services from the experts! Visit
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> ==
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> Ing. Andrea Aime
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> Technical Lead
>
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