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                                                   69
      SECTION 3.3                                                                Filters



        T* ( for both data and images , while others are \
        suitable only ) Tj
        T* ( for continuous−tone images . ) Tj
        ET
        endstream
        endobj


3.3.1 ASCIIHexDecode Filter

      The ASCIIHexDecode filter decodes data that has been encoded in ASCII
      hexadecimal form. ASCII hexadecimal encoding and ASCII base-85 encoding
      (described in the next section) convert binary data, such as image data, to 7-bit
      ASCII characters. In general, ASCII base-85 encoding is preferred to ASCII
      hexadecimal encoding because it is more compact: it expands the data by a factor
      of 4 : 5, compared with 1 : 2 for ASCII hexadecimal encoding.

      The ASCIIHexDecode filter produces one byte of binary data for each pair of
      ASCII hexadecimal digits (0–9 and A–F or a–f ). All white-space characters (see
      Section 3.1, “Lexical Conventions”) are ignored. A right angle bracket character
      (>) indicates EOD. Any other characters cause an error. If the filter encounters
      the EOD marker after reading an odd number of hexadecimal digits, it behaves as
      if a 0 followed the last digit.


3.3.2 ASCII85Decode Filter

      The ASCII85Decode filter decodes data that has been encoded in ASCII base-85
      encoding and produces binary data. The following paragraphs describe the
      process for encoding binary data in ASCII base-85; the ASCII85Decode filter
      reverses this process.

      The ASCII base-85 encoding uses the characters ! through u and the character z,
      with the 2-character sequence ~> as its EOD marker. The ASCII85Decode filter
      ignores all white-space characters (see Section 3.1, “Lexical Conventions”). Any
      other characters, and any character sequences that represent impossible
      combinations in the ASCII base-85 encoding, cause an error.

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