Works out which tiles cover a viewport and where each one is drawn.
A source's tiles are either 256 or 512 pixels. A 512-pixel tile at zoom z - 1 covers
the same ground as four 256-pixel tiles at zoom z, so for a 512-pixel source the grid
asks for tiles one zoom lower than the viewport's.
Three rules keep the grid correct at the edges of the world:
- placements are computed from the unwrapped column, and only the tile key wraps, so a view across the antimeridian puts each tile on the correct side;
- rows above the north edge or below the south edge are left out rather than wrapped, so the area past the poles stays empty;
- a tile needed in several places, as happens when a wide image at zoom 0 shows the
world more than once, appears once per place but has one key, and
keys/1lists each key once.
Summary
Functions
Builds the grid for viewport from a source whose tiles are tile_size pixels.
The distinct tile keys in grid, in the order they first appear.
Types
@type key() :: {non_neg_integer(), non_neg_integer(), non_neg_integer()}
@type t() :: %StaticMaps.TileGrid{ tile_px: pos_integer(), tiles: [placement()], zoom: non_neg_integer() }
:zoom— the zoom of the tiles to fetch:tile_px— the width and height each tile is drawn at, in output pixels:tiles— every placement, row by row from the top left
Functions
@spec build(StaticMaps.Viewport.t(), 256 | 512) :: t()
Builds the grid for viewport from a source whose tiles are tile_size pixels.
Raises ArgumentError for a 512-pixel source at viewport zoom 0, which has no tiles
large enough to cover it.
iex> vp = StaticMaps.Viewport.new({0, 0}, 0, {256, 256}, 1)
iex> StaticMaps.TileGrid.build(vp, 256).tiles
[%{key: {0, 0, 0}, at: {0, 0}}]The distinct tile keys in grid, in the order they first appear.