BLOB Filters

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BLOB subtypes are used in conjunction with BLOB filters. A BLOB filter is a routine that translates BLOB data from one subtype to another. InterBase includes a set of special internal BLOB filters that convert from subtype 0 to subtype 1 (TEXT), and from InterBase system subtypes to subtype 1 (TEXT). In addition to using the internal text filters, programmers can write their own external filters to provide special data translation. For example, an external filter might automatically translate from one bit-mapped image format to another.

Associated with every filter is an integer pair that specifies the input subtype and the output subtype. When declaring a cursor to read or write BLOB data, specify FROM and TO subtypes that correspond to a particular BLOB filter. InterBase invokes the filter based on the FROM and TO subtype specified by the read or write cursor declaration.

The display of BLOB subtypes in isql can be specified with SET BLOBDISPLAY in command-line isql or with the Session | Advanced Settings command in Windows ISQL.

For more information about Windows ISQL and command-line isql, see the Operations Guide. For more information about creating external BLOB filters, see the Embedded SQL Guide.

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