EVOLUTION-MANAGER

Edit File: zoom.py

from django.contrib.gis.geos import GEOSGeometry, LinearRing, Polygon, Point from django.contrib.gis.maps.google.gmap import GoogleMapException from django.utils.six.moves import xrange from math import pi, sin, log, exp, atan # Constants used for degree to radian conversion, and vice-versa. DTOR = pi / 180. RTOD = 180. / pi class GoogleZoom(object): """ GoogleZoom is a utility for performing operations related to the zoom levels on Google Maps. This class is inspired by the OpenStreetMap Mapnik tile generation routine `generate_tiles.py`, and the article "How Big Is the World" (Hack #16) in "Google Maps Hacks" by Rich Gibson and Schuyler Erle. `generate_tiles.py` may be found at: http://trac.openstreetmap.org/browser/applications/rendering/mapnik/generate_tiles.py "Google Maps Hacks" may be found at http://safari.oreilly.com/0596101619 """ def __init__(self, num_zoom=19, tilesize=256): "Initializes the Google Zoom object." # Google's tilesize is 256x256, square tiles are assumed. self._tilesize = tilesize # The number of zoom levels self._nzoom = num_zoom # Initializing arrays to hold the parameters for each one of the # zoom levels. self._degpp = [] # Degrees per pixel self._radpp = [] # Radians per pixel self._npix = [] # 1/2 the number of pixels for a tile at the given zoom level # Incrementing through the zoom levels and populating the parameter arrays. z = tilesize # The number of pixels per zoom level. for i in xrange(num_zoom): # Getting the degrees and radians per pixel, and the 1/2 the number of # for every zoom level. self._degpp.append(z / 360.) # degrees per pixel self._radpp.append(z / (2 * pi)) # radians per pixel self._npix.append(z / 2) # number of pixels to center of tile # Multiplying `z` by 2 for the next iteration. z *= 2 def __len__(self): "Returns the number of zoom levels." return self._nzoom def get_lon_lat(self, lonlat): "Unpacks longitude, latitude from GEOS Points and 2-tuples." if isinstance(lonlat, Point): lon, lat = lonlat.coords else: lon, lat = lonlat return lon, lat def lonlat_to_pixel(self, lonlat, zoom): "Converts a longitude, latitude coordinate pair for the given zoom level." # Setting up, unpacking the longitude, latitude values and getting the # number of pixels for the given zoom level. lon, lat = self.get_lon_lat(lonlat) npix = self._npix[zoom] # Calculating the pixel x coordinate by multiplying the longitude value # with with the number of degrees/pixel at the given zoom level. px_x = round(npix + (lon * self._degpp[zoom])) # Creating the factor, and ensuring that 1 or -1 is not passed in as the # base to the logarithm. Here's why: # if fac = -1, we'll get log(0) which is undefined; # if fac = 1, our logarithm base will be divided by 0, also undefined. fac = min(max(sin(DTOR * lat), -0.9999), 0.9999) # Calculating the pixel y coordinate. px_y = round(npix + (0.5 * log((1 + fac)/(1 - fac)) * (-1.0 * self._radpp[zoom]))) # Returning the pixel x, y to the caller of the function. return (px_x, px_y) def pixel_to_lonlat(self, px, zoom): "Converts a pixel to a longitude, latitude pair at the given zoom level." if len(px) != 2: raise TypeError('Pixel should be a sequence of two elements.') # Getting the number of pixels for the given zoom level. npix = self._npix[zoom] # Calculating the longitude value, using the degrees per pixel. lon = (px[0] - npix) / self._degpp[zoom] # Calculating the latitude value. lat = RTOD * ( 2 * atan(exp((px[1] - npix)/ (-1.0 * self._radpp[zoom]))) - 0.5 * pi) # Returning the longitude, latitude coordinate pair. return (lon, lat) def tile(self, lonlat, zoom): """ Returns a Polygon corresponding to the region represented by a fictional Google Tile for the given longitude/latitude pair and zoom level. This tile is used to determine the size of a tile at the given point. """ # The given lonlat is the center of the tile. delta = self._tilesize / 2 # Getting the pixel coordinates corresponding to the # the longitude/latitude. px = self.lonlat_to_pixel(lonlat, zoom) # Getting the lower-left and upper-right lat/lon coordinates # for the bounding box of the tile. ll = self.pixel_to_lonlat((px[0]-delta, px[1]-delta), zoom) ur = self.pixel_to_lonlat((px[0]+delta, px[1]+delta), zoom) # Constructing the Polygon, representing the tile and returning. return Polygon(LinearRing(ll, (ll[0], ur[1]), ur, (ur[0], ll[1]), ll), srid=4326) def get_zoom(self, geom): "Returns the optimal Zoom level for the given geometry." # Checking the input type. if not isinstance(geom, GEOSGeometry) or geom.srid != 4326: raise TypeError('get_zoom() expects a GEOS Geometry with an SRID of 4326.') # Getting the envelope for the geometry, and its associated width, height # and centroid. env = geom.envelope env_w, env_h = self.get_width_height(env.extent) center = env.centroid for z in xrange(self._nzoom): # Getting the tile at the zoom level. tile_w, tile_h = self.get_width_height(self.tile(center, z).extent) # When we span more than one tile, this is an approximately good # zoom level. if (env_w > tile_w) or (env_h > tile_h): if z == 0: raise GoogleMapException('Geometry width and height should not exceed that of the Earth.') return z-1 # Otherwise, we've zoomed in to the max. return self._nzoom-1 def get_width_height(self, extent): """ Returns the width and height for the given extent. """ # Getting the lower-left, upper-left, and upper-right # coordinates from the extent. ll = Point(extent[:2]) ul = Point(extent[0], extent[3]) ur = Point(extent[2:]) # Calculating the width and height. height = ll.distance(ul) width = ul.distance(ur) return width, height