SECTION 3.3
69
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
(
>
) 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.