Metadata

This chapter explains how OTB deals with the metadata.

otbImageMetadata

OTB uses the classes otbImageMetadata and otbImageMetadataBase to store the metadata.

../_images/UmlImageMetadata.png

otbImageMetadataBase encapsulates seven std::map to store seven different kind of metadata:

  • Numeric metadata for the metadata that can be stored as a double
  • String metadata for the metadata that can be stored as a std::string
  • LUT 1D metadata the metadata that can be stored as a one dimension table
  • LUT 2D metadata for the metadata that can be stored as two dimensions table
  • Time metadata for the metadata that can be stored as a time object
  • Geometry metadata for the metadata that represent a model
  • Extra metadata for non generic metadata stored as std::string

The keys of the maps are described in the file otbMetaDataKey.h. This file also defines the time object.

For each map, the class provides 4 methods:

  • the [] operator for a read-only access the metadata from the key
  • the Add method to set a metadata value
  • the Remove method to delete a metadata va1ue
  • the Has method to test if a key has a value

An instance of otbImageMetadata is used to store the metadata. It inherits from otbImageMetadataBase, therefore it encapsulates seven maps used to store the metadata. It also contains a vector of otbImageMetadataBase to store band specific metadata (m_Bands).

Metadata workflow

OTB reads and writes metadata from and to products through the workflow described by this figure:

../_images/MetadataWorkflow.png

  • The GDAL input/output capabilities are encapsulated in the otbGDALImageIO class, which derivates from otbImageIO. This class is in charge of fetching the metadata from the product (supplier capabilities inherited from the class otbMetadataSupplierInterface) and storing them in memory as a keywordlist. It is also in charge of writing the metadata to the product (storage capabilities inherited from the class otbMetadataStorageInterface).

  • An ImageMetadataInterface (IMI) is then called to parse the metadata. There is one IMI per sensor. We use a classical Factory to find which one can parse the metadata of a product. The IMI’s parse method will pick the metadata from the ImageIO and fill an ImageMetadata object.

  • Some metadata are not read by GDAL. To parse those metadata, the IMI can call other suppliers, depending on the file format:

    • to parse XML files, otbXMLMetadataSupplier uses GDAL’s XML parsing mechanism (“ReadXMLToList” method from the “GDALMDReaderBase” class) to convert the XML file into a GDAL ListString, which is a succession of ‘key=value’ pairs.
    • to parse text files, TextMetadataSupplier tries to parse ‘key=value’ pairs.

    Other suppliers can be added if needed. Those classes (including otbGDALImageIO) all implement the method GetMetadataValue which returns the value of the metadata from a given key. The base class also implements the methods GetAs and GetAsVector which are used by the IMI.

  • The IMI finds the relevant metadata in the different Metadata Suppliers and use the Add method of the ImageMetadata object to store the metadata. If the parsing returns successfully, the generated ImageMetadata is given to the ImageCommon that propagate through the pipeline.