COMPRESS BLOB

version 6.5.3 (Modified)

COMPRESS BLOB (blob{; compression})

The COMPRESS BLOB command compresses the BLOB blob using the internal 4th Dimension compression algorithm. This command only compresses BLOB whose size is over 255 bytes.

The optional compression parameter allows to set the way the BLOB will be compressed:

• If you pass 1, the BLOB is compressed as much as possible, at the expanse of the speed of compression and decompression operations.

• If you pass 2, the BLOB is compressed as fast as possible (and will be decompressed as fast as possible), at the expense of the compression ratio (the compressed BLOB will be bigger).

• If you pass another value or if you omit the parameter, the BLOB is compressed as much as possible, using the compression mode 1.

4th Dimension provides the following predefined constants:

After the call, the OK variable is set to 1 if the BLOB has been successfully compressed. If the compression could not be performed, the OK variable is set to 0. If the compression could not be performed because of a lack of memory or because the actual size of the blob is less than 255 bytes, no error is generated and the method resumes its execution.

In any other cases (i.e. the BLOB is damaged), the error -10600 is generated. This error can be trapped using the ON ERR CALL command.

After a BLOB has been compressed, you can expand it using the EXPAND BLOB command.

To detect if a BLOB has been compressed, use the BLOB PROPERTIES command.

WARNING: A compressed BLOB is still a BLOB, so there is nothing to stop you from modifying its contents. However, if you do so, the EXPAND BLOB command will not be able to decompress the BLOB properly.

1. This example tests if the BLOB vxMyBlob is compressed, and, if it is not, compresses it:

   BLOB PROPERTIES (vxMyBlob;$vlCompressed;$vlExpandedSize;$vlCurrentSize)
   If ($vlCompressed=Is not compressed)
      COMPRESS BLOB (vxMyBlob)
   End if

Note however, that if you apply COMPRESS BLOB to an already compressed BLOB, the command detects it and does nothing.

2. This example allows you to select a document and then compress it:

   $vhDocRef := Open document ("")
   If (OK=1)
      CLOSE DOCUMENT ($vhDocRef)
      DOCUMENT TO BLOB (Document;vxBlob)
      If (OK=1)
         COMPRESS BLOB (vxBlob)
         If (OK=1)
            BLOB TO DOCUMENT (Document;vxBlob)
         End if
      End if
   End if

BLOB PROPERTIES, EXPAND BLOB.

The OK variable is set to 1 if the BLOB has been successfully compressed; otherwise, it is set to 0.