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-- {
-- Return pixel resolution at the given zoom level
-- }{
CREATE OR REPLACE FUNCTION CDB_XYZ_Resolution(z INTEGER)
RETURNS FLOAT8
AS $$
-- circumference divided by 256 is z0 resolution, then divide by 2^z
SELECT 6378137.0*2.0*pi() / 256.0 / power(2.0, z);
$$ LANGUAGE SQL IMMUTABLE PARALLEL SAFE STRICT;
-- }
-- {
-- Returns a polygon representing the bounding box of a given XYZ tile
--
-- SRID of the returned polygon is forceably 3857
--
-- }{
CREATE OR REPLACE FUNCTION CDB_XYZ_Extent(x INTEGER, y INTEGER, z INTEGER)
RETURNS GEOMETRY
AS $$
DECLARE
origin_shift FLOAT8;
initial_resolution FLOAT8;
tile_geo_size FLOAT8;
pixres FLOAT8;
xmin FLOAT8;
ymin FLOAT8;
xmax FLOAT8;
ymax FLOAT8;
earth_circumference FLOAT8;
tile_size INTEGER;
BEGIN
-- Size of each tile in pixels (1:1 aspect ratio)
tile_size := 256;
initial_resolution := CDB_XYZ_Resolution(0);
--RAISE DEBUG 'Initial resolution: %', initial_resolution;
origin_shift := (initial_resolution * tile_size) / 2.0;
-- RAISE DEBUG 'Origin shift (after): %', origin_shift;
pixres := initial_resolution / (power(2,z));
--RAISE DEBUG 'Pixel resolution: %', pixres;
tile_geo_size = tile_size * pixres;
--RAISE DEBUG 'Tile_geo_size: %', tile_geo_size;
xmin := -origin_shift + x*tile_geo_size;
xmax := -origin_shift + (x+1)*tile_geo_size;
--RAISE DEBUG 'xmin: %', xmin;
--RAISE DEBUG 'xmax: %', xmax;
ymin := origin_shift - y*tile_geo_size;
ymax := origin_shift - (y+1)*tile_geo_size;
--RAISE DEBUG 'ymin: %', ymin;
--RAISE DEBUG 'ymax: %', ymax;
RETURN ST_MakeEnvelope(xmin, ymin, xmax, ymax, 3857);
END
$$ LANGUAGE 'plpgsql' IMMUTABLE STRICT PARALLEL SAFE;
-- }