Ubuntu Manpage: ascii85 - Encoding "ascii85"

Provided by: tcl-trf-doc_2.1.4-dfsg3-3_all

NAME

       ascii85 - Encoding "ascii85"

SYNOPSIS

       package require Tcl  ?8.2?

       package require Trf  ?2.1.3?

       ascii85 ?options...? ?data?

DESCRIPTION

The command ascii85 is one of several data encodings provided by the package trf. See trf-
intro for an overview of the whole package.

This encoding transforms every block of four bytes into the Ascii85 representation as
defined in the 'Postscript Reference Manual' (2nd Edition, section 3.13, page 129).

ascii85 ?options...? ?data?

-mode encode|decode
This option has to be present and is always understood by the encoding.

For immediate mode the argument value specifies the operation to use. For
an attached encoding it specifies the operation to use for writing. Reading
will automatically use the reverse operation. See section IMMEDIATE versus
ATTACHED for explanations of these two terms.

Beyond the argument values listed above all unique abbreviations are
recognized too.

Encode converts from arbitrary (most likely binary) data into the described
representation, decode does the reverse .

-attach channel
The presence/absence of this option determines the main operation mode of
the transformation.

If present the transformation will be stacked onto the channel whose handle
was given to the option and run in attached mode. More about this in section
IMMEDIATE versus ATTACHED.

If the option is absent the transformation is used in immediate mode and the
options listed below are recognized. More about this in section IMMEDIATE
versus ATTACHED.

-in channel
This options is legal if and only if the transformation is used in immediate
mode. It provides the handle of the channel the data to transform has to be
read from.

If the transformation is in immediate mode and this option is absent the
data to transform is expected as the last argument to the transformation.

-out channel
This options is legal if and only if the transformation is used in immediate
mode. It provides the handle of the channel the generated transformation
result is written to.

If the transformation is in immediate mode and this option is absent the
generated data is returned as the result of the command itself.

IMMEDIATE VERSUS ATTACHED

The transformation distinguishes between two main ways of using it. These are the
immediate and attached operation modes.

For the attached mode the option -attach is used to associate the transformation with an
existing channel. During the execution of the command no transformation is performed,
instead the channel is changed in such a way, that from then on all data written to or
read from it passes through the transformation and is modified by it according to the
definition above. This attachment can be revoked by executing the command unstack for the
chosen channel. This is the only way to do this at the Tcl level.

In the second mode, which can be detected by the absence of option -attach, the
transformation immediately takes data from either its commandline or a channel, transforms
it, and returns the result either as result of the command, or writes it into a channel.
The mode is named after the immediate nature of its execution.

Where the data is taken from, and delivered to, is governed by the presence and absence of
the options -in and -out. It should be noted that this ability to immediately read from
and/or write to a channel is an historic artifact which was introduced at the beginning of
Trf's life when Tcl version 7.6 was current as this and earlier versions have trouble to
deal with \0 characters embedded into either input or output.

KEYWORDS

       ascii85, encoding, postscript

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