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                                                     74
            CHAPTER 3                                                                        Syntax



            samples (a process called differencing), and encoding the differences rather than
            the raw sample values, can reduce the size of the output data. Furthermore, when
            the image data contains several color components (red-green-blue or cyan-
            magenta-yellow-black) per sample, taking the difference between the values of
            corresponding components in adjacent samples, rather than between different
            color components in the same sample, often reduces the output data size.

            Table 3.7 shows the parameters that can optionally be specified for LZWDecode
            and FlateDecode filters. Except where otherwise noted, all values supplied to the
            decoding filter for any optional parameters must match those used when the data
            was encoded.

                   TABLE 3.7 Optional parameters for LZWDecode and FlateDecode filters
KEY                   TYPE       VALUE

Predictor             integer    A code that selects the predictor algorithm, if any. If the value of this entry
                                 is 1, the filter assumes that the normal algorithm was used to encode the data,
                                 without prediction. If the value is greater than 1, the filter assumes that the
                                 data was differenced before being encoded, and Predictor selects the predic-
                                 tor algorithm. For more information regarding Predictor values greater
                                 than 1, see “LZW and Flate Predictor Functions,” below. Default value: 1.

Colors                integer    (Used only if Predictor is greater than 1) The number of interleaved color com-
                                 ponents per sample. Valid values are 1 to 4 in PDF 1.2 or earlier and 1 or
                                 greater in PDF 1.3 or later. Default value: 1.

BitsPerComponent      integer    (Used only if Predictor is greater than 1) The number of bits used to represent
                                 each color component in a sample. Valid values are 1, 2, 4, 8, and (in PDF 1.5)
                                 16. Default value: 8.

Columns               integer    (Used only if Predictor is greater than 1) The number of samples in each row.
                                 Default value: 1.

EarlyChange           integer    (LZWDecode only) An indication of when to increase the code length. If the
                                 value of this entry is 0, code length increases are postponed as long as pos-
                                 sible. If the value is 1, code length increases occur one code early. This pa-
                                 rameter is included because LZW sample code distributed by some vendors
                                 increases the code length one code earlier than necessary. Default value: 1.

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