The DB2 driver is based largely on the source code for the OGR/DB2 GeoPackage (GPKG) driver, replaceing SQLITE functionality with corresponding DB2 functionality. Appreciation is expressed for the major development required to produce the GeoPackage driver which dramatically reduced the effort to produce the DB2 driver. At some point it may be worthwhile to explore whether refactoring of the class structure to share common functionality is practical.
The DB2 driver largely implements the GeoPackage standard, with the main difference that the "gpkg_" prefix is replaced with "gpkg." in the DB2 table name, assigning them to a distinct database schema.
The documentation below is largely copied from the drv_geopackage_raster.html documentation. References to the GeoPackage standard have been left as such. References to the implementation have been changed to DB2. In some cases it isn't clear whether we should refer to "DB2 tiles" or "GeoPackage tiles".
Starting with GDAL 2.0, this driver implements full read/creation/update of tables containing raster tiles in the OGC GeoPackage format standard. The GeoPackage standard uses a SQLite database file as a generic container, and the standard defines:
gpkg.contents,
gpkg.spatial_ref_sys, gpkg.tile_matrix,
gpkg.tile_matrix_set, ...)This driver reads and writes DB2 tables in the DB2 database, so it must be run by a user with create authority on the database it is working with.
The driver can also handle DB2 vectors. See DB2 vector documentation page
Various kind of input datasets can be converted to DB2 raster :
All raster extensions standardized by the GeoPackage specification are supported in read and creation :
By default, the driver will expose a DB2 dataset as a four band (Red,Green, Blue,Alpha) dataset, which gives the maximum compatibility with the various encodings of tiles that can be stored. It is possible to specify an explicit number of bands with the BAND_COUNT opening option.
The driver will use the geographic/projected extent indicated in the gpkg.contents table, and do necessary clipping, if needed, to respect that extent. However that information being optional, if omitted, the driver will use the extent provided by the gpkg.tile_matrix_set, which covers the extent at all zoom levels. The user can also specify the USE_TILE_EXTENT=YES open option to use the actual extent of tiles at the maximum zoom level. Or it can specify any of MINX/MINY/MAXX/MAXY to have a custom extent.
The following open options are available:
Depending of the number of bands of the input dataset and the tile format selected, the driver will do the necessary conversions to be compatible with the tile format.
To add several tile tables to a DB2 dataset (seen as GDAL subdatasets), or to add a tile table to an existing vector-only DB2, the generic APPEND_SUBDATASET=YES creation option must be provided.
Fully transparent tiles will not be written to the database, as allowed by the format.
The driver implements the Create() and IWriteBlock() methods, so that arbitrary writing of raster blocks is possible, enabling the direct use of DB2 as the output dataset of utilities such as gdalwarp.
On creation, raster blocks can be written only if the geotransformation matrix has been set with SetGeoTransform() This is effectively needed to determine the zoom level of the full resolution dataset based on the pixel resolution, dataset and tile dimensions.
Technical/implementation note: when a dataset is opened with a non-default area of interest (i.e. use of MINX,MINY,MAXX,MAXY or USE_TILE_EXTENT open option), or when creating/ opening a dataset with a non-custom tiling scheme, it is possible that GDAL blocks do not exactly match a single DB2 tile. In which case, each GDAL block will overlap four DB2 tiles. This is easily handled on the read side, but on creation/update side, such configuration could cause numerous decompression/ recompression of tiles to be done, which might cause unnecessary quality loss when using lossy compression (JPEG, WebP). To avoid that, the driver will create a temporary database next to the main DB2 table to store partial DB2 tiles in a lossless (and uncompressed) way. Once a tile has received data for its four quadrants and for all the bands (or the dataset is closed or explicitly flushed with FlushCache()), those uncompressed tiles are definitely transferred to the DB2 table with the appropriate compression. All of this is transparent to the user of GDAL API/utilities
DB2 can store tiles in different formats, PNG and/or JPEG for the baseline specification, and WebP for extended DB2. Support for those tile formats depend if the underlying drivers are available in GDAL, which is generally the case for PNG and JPEG, but not necessarily for WebP since it requires GDAL to be compiled against the optional libwebp.
By default, GDAL will use a mix of PNG and JPEG tiles. PNG tiles will be used to store tiles that are not completely opaque, either because input dataset has an alpha channel with non fully opaque content, or because tiles are partial due to clipping at the right or bottom edges of the raster, or when a dataset is opened with a non-default area of interest, or with a non-custom tiling scheme. On the contrary, for fully opaque tiles, JPEG format will be used.
It is possible to select one unique tile format by setting the creation/open option TILE_FORMAT to one of PNG, JPEG or WEBP. When using JPEG, the alpha channel will not be stored. When using WebP, the gpkg.webp extension will be registered. The lossy compression of WebP is used. Note that a recent enough libwebp (>=0.1.4) must be used to support alpha channel in WebP tiles.
PNG8 can be selected to use 8-bit PNG with a color table up to 256 colors. On creation, an optimized color table is computed for each tile. The DITHER option can be set to YES to use Floyd/Steinberg dithering algorithm, which spreads the quantization error on neighbouring pixels for better rendering (note however than when zooming in, this can cause non desirable visual artifacts). Setting it to YES will generally cause less effective compression. Note that at that time, such an 8-bit PNG formulation is only used for fully opaque tiles, as the median-cut algorithm currently implemented to compute the optimal color table does not support alpha channel (even if PNG8 format would potentially allow color table with transparency). So when selecting PNG8, non fully opaque tiles will be stored as 32-bit PNG.
By default, conversion to DB2 will create a custom tiling scheme, such that the input dataset can be losslessly converted, both at the pixel and georeferencing level (if using a lossless tile format such as PNG). That tiling scheme is such that its origin (min_x, max_y) in the gpkg.tile_matrix_set table perfectly matches the top left corner of the dataset, and the selected resolution (pixel_x_size, pixel_y_size) at the computed maximum zoom_level of the gpkg.tile_matrix table will match the pixel width and height of the raster.
However to ease interoperability with other implementations, and enable use of DB2 with tile servicing software, it is possible to select a predefined tiling scheme that has world coverage. The available tiling schemes are :
In all the above tiling schemes, consecutive zoom levels defer by a resolution of a factor of two.
The following creation options are available:
gdaladdo / BuildOverviews() can be used to compute overviews. Power-of-two overview factors (2,4,8,16,...) should be favored to be conformant with the baseline GeoPackage specification. Use of other overview factors will work with the GDAL driver, and cause the gpkg.zoom_other extension to be registered, but that could potentially cause interoperability problems with other implementations that do not support that extension.
Overviews can also be cleared with the -clean option of gdaladdo (or BuildOverviews() with nOverviews=0)
GDAL uses the standardized
gpkg.metadata and
gpkg.metadata_reference tables to read and write metadata.
GDAL metadata, from the default metadata domain and possibly other metadata domains, is serialized in a single XML document, conformant with the format used in GDAL PAM (Persistent Auxiliary Metadata) .aux.xml files, and registered with md_scope=dataset and md_standard_uri=http://gdal.org in gpkg.metadata. In gpkg.metadata_reference, this entry is referenced with a reference_scope=table and table_name={name of the raster table}
It is possible to read and write metadata that applies to the global DB2, and not only to the raster table, by using the GEOPACKAGE metadata domain.
Metadata not originating from GDAL can be read by the driver and will be exposed as metadata items with keys of the form gpkg.METADATA_ITEM_XXX and values the content of the metadata columns of the gpkg.metadata table. Update of such metadata is not currently supported through GDAL interfaces ( although it can be through direct SQL commands).
The specific DESCRIPTION and IDENTIFIER metadata item of the default metadata domain can be used in read/write to read from/update the corresponding columns of the gpkg.contents table.
% gdal_translate -of DB2ODBC byte.tif DB2ODBC:database=sample;DSN=SAMPLE
% gdal_translate -of DB2ODBC byte.tif DB2ODBC:database=sample;DSN=SAMPLE -co TILE_FORMAT=WEBP
% gdal_translate -of DB2ODBC byte.tif DB2ODBC:database=sample;DSN=SAMPLE -co TILING_SCHEME=GoogleMapsCompatible
% gdaladdo -oo RASTER_TABLE=world -r cubic DB2ODBC:database=sample;DSN=SAMPLE 2 4 8 16 32 64
% gdal_translate -of DB2ODBC new.tif DB2ODBC:database=sample;DSN=SAMPLE -co APPEND_SUBDATASET=YES -co RASTER_TABLE=new_table
% gdalwarp -of DB2ODBC -co RASTER_TABLE=new_table in.tif DB2ODBC:database=sample;DSN=SAMPLE -t_srs EPSG:3857
% gdalinfo DB2ODBC:database=sample;DSN=SAMPLE -oo TABLE=a_table