Go Up to Defining BLOB Data Types
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
TO subtypes that correspond to a particular
BLOB filter. InterBase invokes the filter based on the
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.