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What is a Tile Service?

We are starting to see implementations of servers that are "something" like a web map server. What is that something?

  • view of the world split up into tiles
  • tiles avaiable in a single projection
  • tiles available at known zoom levels

There are a number of these servers available:

  • Nasa World Wind
  • Virtual Earth
  • OSGeo Tile Map Service
  • WMS Tile Profile
Column

Servers

TileServer

So what does this client interface going to look like:

  • TileService - a good name, describes what we have
    • Configure with a single URL
    • deleage to TileServer strategy objects (one implementation per service type)
  • GeoResource
    • configured with a single URI
  • GeoResourceInfo describing the service
    • TileInfo extension descrbing zoom levels (and anything else needed)

TileServer API

Represents a single tile server, actual protocol negotiated by TileStrategy object.

Code Block
class TileServer {
    TileServer( URL server, ProgressMonitor monitor );
    TileServer( GridInfo info ); // incase info was created beforehand

    GridInfo getInfo();
    List<TileMap> getTileMapList();
}

Content is described and accessed through TileMap instances, these have enough information for rendering:

Code Block
class TileMap {
    TileMap( TileMapInfo info );

    // data access
    CoordinateReferenceSystem getCRS();
    TileMapInfo getInfo(); // describes tile sets

    TileSet getTileSet( ZoomLevel zoom );
}
class TileSet {
    TileSet( TileMap, ZoomLevel zoom );
    
    TileMap getTileMap();    
    ZoomLevel getZoomLevel();

    //GridCoverage getTile( Coordinate position );
    GridCoverage getTile( int row, int col );

    TileRange getTileRange( Envelope bbox ); // must match CRS
}

Tile Service/Resource Metadata

Metadata for the TileServer needs to capture enough information for discovery:

Code Block
class TileService extends Service {
    TileService ( URI, Memento ); // Memento holds associated state

    TileInfo info( ProgressMonitor monitor );
    List<TileResource> children( ProgressMonitor monitor );
}
interface TileServiceInfo extends ServiceInfo {
    int getTileWidth(); // often 256
    int getTileHeight(); // often 256
}

Metadata for each TileMap captures enough information to set up a display pipeline.

Code Block
class TileResource extends GeoResource {
    TileResource( URI, Memento );
    
    TileInfo getInfo( ProgressMonitor );
    TileMap getTileMap( ProgressMonitor );
}
class TileInfo extends GeoResourceInfo {
    CoordinateReferenceSystem getCRS();
    SortedSet<ZoomLevel> getZoomLevels();
}
class ZoomLevel(){
    int getScaleDenominator();    
    int getSize(); // number of tiles
    int getNumberOfRows();
    int getNumberOfColumns();
}

Extension Interface for Protocol Support

Individual protocol support is handled by TileStrategy objects. Once again this is similar to the WMS implementation.

TileStrategy Extention

We need a strategy object capturing the protocol specific behavior:

Code Block
interface TileStrategy {
    CRS getCoordinateReferenceSystem();
    ReferencedEnvelope getBounds(); // Bounds include CRS
    GridCoverage getTile( int x, int y, int zoom );
}

The factory has the difficult decision of deciding what URI it can support; factory negotiation determines the protocol used. Addition information may may be required if protocol version negotiation is also needed (as in the WMS case).

Code Block
interface TileStrategyFactory {
    boolean canTile( URL uri );
    TileInfo createInfo( URI uri ); // used to parse descriptors
    TileService createTileService( URI uri );
}

TileCache Extension

Part of the fun of having TileRange is the ability to interact smoothly with a Cache.

Code Block
interface TileCache {
     TileRange createRange( TileDraw draw, int row, int col, int rows, int cols );
}

TileDraw is used to isolate the TileCache; for reuse by those working with WMS or other renderers.

Code Block
interface TileDraw {
     GridCoverage drawPlaceholder( int row, int col );
     GridCoverage drawTile( int row, int col );
}

The following implementations are desired:

  • PassthroughTileCache - The initial implementaion can be virtual, and just create a standalone TileRange
  • MemoryTileCache - uses memory up to a specified capacity
  • DiskTileCache - uses diskspace up to a specified capacity

The most common compound request is handled with a special data structure, a progress monitor
is used to provide feedback on during the loadTile process, the getTiles method will return a set
of the correct size; although some GridCoverages may be transparent (or pixelated) during the loading
process.

Code Block
class TileRange {
    ReferencedEnvelope getBounds(); // bounds of tiles represented, w/ CRS

    load( ProgressMonitor ); // monitor advances as each tile is available
    boolean isLoaded();
    refresh( ProgressMonitor ); // leaves tiles as is, but redraws

    Set<GridCoverage> getTiles(); // tiles in range
}