# MBTiles

Starting with GDAL 1.10, the MBTiles driver allows reading rasters in the MBTiles format, which is a specification for storing tiled map data in SQLite databases.

Starting with GDAL 2.1, the MBTiles driver has creation and write support for MBTiles datasets.

GDAL/OGR must be compiled with OGR SQLite driver support, and JPEG and PNG drivers.

The SRS is always the Pseudo-Mercator (a.k.a Google Mercator) projection.

By default, the driver will try to determine the number of bands by probing the content of one tile. It is possible to alter this behaviour by defining the MBTILES_BAND_COUNT configuration option (or starting with GDAL 2.1, the BAND_COUNT open option) to the number of bands. The values supported are 1, 2, 3 or 4. Four band (Red,Green,Blue,Alpha) dataset gives the maximum compatibility with the various encodings of tiles that can be stored.

The driver will use the 'bounds' metadata in the metadata table and do necessary tile clipping, if needed, to respect that extent. However that information being optional, if omitted, the driver will use the extent of the tiles at the maximum zoom level. The user can also specify the USE_BOUNDS=NO open option to force the use of 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 driver can retrieve pixel attributes encoded according to the UTFGrid specification available in some MBTiles files. They can be obtained with the gdallocationinfo utility, or with a GetMetadataItem("Pixel_iCol_iLine", "LocationInfo") call on a band object.

## Opening options

Starting with GDAL 2.1, the following open options are available:

- ZOOM_LEVEL=value: Integer value between 0 and the maximum filled in the tiles table. By default, the driver will select the maximum zoom level, such as at least one tile at that zoom level is found in the raster table.
- BAND_COUNT=AUTO/1/2/3/4: Number of bands of the dataset exposed after opening. Some conversions will be done when possible and implemented, but this might fail in some cases, depending on the BAND_COUNT value and the number of bands of the tile. Defaults to AUTO.
- MINX=value: Minimum easting (in EPSG:3857) of the area of interest.
- MINY=value: Minimum northing (in EPSG:3857) of the area of interest.
- MAXX=value: Maximum easting (in EPSG:3857) of the area of interest.
- MAXY=value: Maximum northing (in EPSG:3857) of the area of interest.
- USE_BOUNDS=YES/NO: Whether to use the bounds metadata, when available, to determine the AOI. Defaults to YES.
- TILE_FORMAT=PNG/PNG8/JPEG: Format used to store tiles. See Tile formats section. Only used in update mode. Defaults to PNG.
- QUALITY=1-100: Quality setting for JPEG compression. Only used in update mode. Default to 75.
- ZLEVEL=1-9: DEFLATE compression level for PNG tiles. Only used in update mode. Default to 6.
- DITHER=YES/NO: Whether to use Floyd-Steinberg dithering (for TILE_FORMAT=PNG8). Only used in update mode. Defaults to NO.

## Creation issues

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. When using the CreateCopy() API (such as with gdal_translate), automatic reprojection of the input dataset to EPSG:3857 (WebMercator) will be done, with selection of the appropriate zoom level.

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 MBTiles 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: in the general case, GDAL blocks do not exactly match a single MBTiles tile. In which case, each GDAL block will overlap four MBTiles 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). To avoid that, the driver will create a temporary database next to the main MBTiles file to store partial MBTiles 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 MBTiles file with the appropriate compression. All of this is transparent to the user of GDAL API/utilities

### Tile formats

MBTiles can store tiles in PNG or JPEG. Support for those tile formats depend if the underlying drivers are available in GDAL. By default, GDAL will PNG tiles.

It is possible to select the tile format by setting the creation/open option TILE_FORMAT to one of PNG, PNG8 or JPEG. When using JPEG, the alpha channel will not be stored.

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.

### Creation options

The following creation options are available:

- NAME=string. Tileset name, used to set the 'name' metadata item. If not specified, the basename of the filename will be used.
- DESCRIPTION=string. A description of the layer, used to set the 'description' metadata item. If not specified, the basename of the filename will be used.
- TYPE=overlay/baselayer. The layer type, used to set the 'type' metadata item. Default to 'overlay'.
- VERSION=string. The version of the tileset, as a plain number, used to set the 'version' metadata item. Default to '1.1'.
- TILE_FORMAT=PNG/PNG8/JPEG: Format used to store tiles. See Tile formats section. Defaults to PNG.
- QUALITY=1-100: Quality setting for JPEG compression. Default to 75.
- ZLEVEL=1-9: DEFLATE compression level for PNG tiles. Default to 6.
- DITHER=YES/NO: Whether to use Floyd-Steinberg dithering (for TILE_FORMAT=PNG8). Defaults to NO.
- ZOOM_LEVEL_STRATEGY=AUTO/LOWER/UPPER. Strategy to determine zoom level. LOWER will select the zoom level immediately below the theoretical computed non-integral zoom level, leading to subsampling. On the contrary, UPPER will select the immediately above zoom level, leading to oversampling. Defaults to AUTO which selects the closest zoom level.
- RESAMPLING=NEAREST/BILINEAR/CUBIC/CUBICSPLINE/LANCZOS/MODE/AVERAGE. Resampling algorithm. Defaults to BILINEAR.
- WRITE_BOUNDS=YES/NO: Whether to write the bounds 'metadata' item. Defaults to YES.

## Overviews

gdaladdo / BuildOverviews() can be used to compute overviews. Only power-of-two overview factors (2,4,8,16,...) are supported.

If more overview levels are specified than available, the extra ones are silently ignored.

Overviews can also be cleared with the -clean option of gdaladdo (or BuildOverviews() with nOverviews=0)

## Examples:

- Accessing a remote MBTiles raster :

      $ gdalinfo /vsicurl/http://a.tiles.mapbox.com/v3/kkaefer.iceland.mbtiles

  Output:

      Driver: MBTiles/MBTiles
      Files: /vsicurl/http://a.tiles.mapbox.com/v3/kkaefer.iceland.mbtiles
      Size is 16384, 16384
      Coordinate System is:
      PROJCS["WGS 84 / Pseudo-Mercator",
          GEOGCS["WGS 84",
              DATUM["WGS_1984",
                  SPHEROID["WGS 84",6378137,298.257223563,
                      AUTHORITY["EPSG","7030"]],
                  AUTHORITY["EPSG","6326"]],
              PRIMEM["Greenwich",0,
                  AUTHORITY["EPSG","8901"]],
              UNIT["degree",0.0174532925199433,
                  AUTHORITY["EPSG","9122"]],
              AUTHORITY["EPSG","4326"]],
          PROJECTION["Mercator_1SP"],
          PARAMETER["central_meridian",0],
          PARAMETER["scale_factor",1],
          PARAMETER["false_easting",0],
          PARAMETER["false_northing",0],
          UNIT["metre",1,
              AUTHORITY["EPSG","9001"]],
          AXIS["X",EAST],
          AXIS["Y",NORTH],
          EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"],
          AUTHORITY["EPSG","3857"]]
      Origin = (-3757031.250000000000000,11271093.750000000000000)
      Pixel Size = (152.873992919921875,-152.873992919921875)
      Image Structure Metadata:
        INTERLEAVE=PIXEL
      Corner Coordinates:
      Upper Left  (-3757031.250,11271093.750) ( 33d44'59.95"W, 70d36'45.36"N)
      Lower Left  (-3757031.250, 8766406.250) ( 33d44'59.95"W, 61d36'22.97"N)
      Upper Right (-1252343.750,11271093.750) ( 11d14'59.98"W, 70d36'45.36"N)
      Lower Right (-1252343.750, 8766406.250) ( 11d14'59.98"W, 61d36'22.97"N)
      Center      (-2504687.500,10018750.000) ( 22d29'59.97"W, 66d30'47.68"N)
      Band 1 Block=256x256 Type=Byte, ColorInterp=Red
        Overviews: 8192x8192, 4096x4096, 2048x2048, 1024x1024, 512x512
        Mask Flags: PER_DATASET ALPHA
        Overviews of mask band: 8192x8192, 4096x4096, 2048x2048, 1024x1024, 512x512
      Band 2 Block=256x256 Type=Byte, ColorInterp=Green
        Overviews: 8192x8192, 4096x4096, 2048x2048, 1024x1024, 512x512
        Mask Flags: PER_DATASET ALPHA
        Overviews of mask band: 8192x8192, 4096x4096, 2048x2048, 1024x1024, 512x512
      Band 3 Block=256x256 Type=Byte, ColorInterp=Blue
        Overviews: 8192x8192, 4096x4096, 2048x2048, 1024x1024, 512x512
        Mask Flags: PER_DATASET ALPHA
        Overviews of mask band: 8192x8192, 4096x4096, 2048x2048, 1024x1024, 512x512
      Band 4 Block=256x256 Type=Byte, ColorInterp=Alpha
        Overviews: 8192x8192, 4096x4096, 2048x2048, 1024x1024, 512x512

- Reading pixel attributes encoded according to the UTFGrid specification :

      $ gdallocationinfo /vsicurl/http://a.tiles.mapbox.com/v3/mapbox.geography-class.mbtiles -wgs84 2 49 -b 1 -xml

  Output:

      <Report pixel="33132" line="22506">
        <BandReport band="1">
          <LocationInfo>
            <Key>74</Key>
            <JSon>{"admin":"France","flag_png":" "}</JSon>
          </LocationInfo>
          <Value>238</Value>
        </BandReport>
      </Report>

- Converting a dataset to MBTiles and adding overviews :

      $ gdal_translate my_dataset.tif my_dataset.mbtiles -of MBTILES
      $ gdaladdo -r average my_dataset.mbtiles 2 4 8 16

See Also:

- MBTiles specification
- UTFGrid specification