Changes between Initial Version and Version 1 of SoCTMSDriver

Timestamp:

Mar 26, 2008, 9:18:00 AM (16 years ago)

Author:

keo

Comment:

--

SoCTMSDriver

@@ +1 @@
+= Read/Write driver for TMS tiles = 
+
+This work is continuation of the SoC 2007 project [http://trac.osgeo.org/gdal/wiki/SoCTileDriver GDAL2Tiles].
+The goal is to create a new driver for reading and writing maps in TMS tiled format.
+
+== Data format ==
+
+=== TMS tiled maps ===
+
+The driver will operate on data in 
+the [http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification#TileMap_Resource OSGeo TMS TileMap Resource] format
+used already by GDAL2Tiles. In short, the map is described in one XML file. Image data is divided into
+several zooming levels represented by directories containing tiles as individual files. 
+
+=== Tiles ===
+
+The TMS specification doesn't prescribe the format of tiles but PNG and JPEG are
+the only useable formats in web setting. Both have existing drivers in GDAL.
+The TMS driver will use only PNG tiles at first. Extending the support to
+JPEG tiles as well is possible next step after implementing both reading and writing
+functionality for PNG. 
+
+== Work plan ==
+
+The work is divided into several steps. First two are essential, all others
+are optional.
+
+=== Read driver ===
+
+First step is to create a read-only driver. The `Open` method will be called on
+the main XML file describing the map. The `<TileSet>` URLs will be expected to be
+paths relative to the main file. This is the case with data produced by GDAL2Tiles.
+When this expectation is not met, the driver could try to guess the names of directories
+using the last part of the URL and the base directory of the file being opened.
+Another approach is to use auxiliary file describing the layout, this is similar
+to the way the WMS driver works.
+
+Although the tiled map has several zooming levels, only the lowest one will be used. 
+`IReadBlock` requests will be handled by loading data from the files representing 
+the selected area using existing PNG driver.
+
+=== Write driver ===
+
+Next on plan is the writing part. The driver will support the
+`CreateCopy` method returning the created dataset as `ReadOnly` (implemented in previous step). 
+This will allow to export existing datasets in tiled format. Creation options will include
+the dimensions of tiles, number of zooming levels and the zooming ratio between them.
+
+Implementing `Create` method and `Update` mode dataset is questionable.
+The PNG driver has this mode controlled by compile time option, so the
+TMS driver would have to do the same. Even then, the PNG driver imposes some 
+restrictions on the calling side: ''"Note: callers must provide blocks in increasing Y order."'', 
+from `pngdataset.cpp`. This would translate to the TMS driver itself and its users too.
+JPEG drivers have `CreateCopy` method only.
+
+=== Performance tuning ===
+
+The reading strategy at this point will be probably quite simple -- creating new
+datasets for appropriate tiles in the `IReadBlock` method and getting data from them. 
+This could prove to be inefficient. There are several ways to solve this issue:
+
+ * Holding all tiles opened -- could be wasting too much memory.
+ * Managing a cache of recently used tiles.
+ * Prefetching adjacent tiles into the cache.
+
+These and other should be explored and the final solution selected based on measured performance.
+
+=== HTTP Read driver ===
+
+The TMS tiled maps are mostly used on web, so the ability to read the map from
+http connection could be quite useful. This step is somewhat related to the
+preceding one because without intelligent caching, the performance could
+very well become unacceptable.
+
+== Student info ==
+
+My name is Václav Klusák and I am a graduate student of Applied Informatics at 
+[http://www.muni.cz Masaryk University] in Brno, Czech Republic. I can be 
+reached by email (`keo@keo.cz`) or ICQ (247 160 335).