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The recode reference manual

13. All about surfaces

The trivial surface consists of using a fixed number of bits (often eight) for each character, the bits together hold the integer value of the index for the character in its charset table. There are many kinds of surfaces, beyond the trivial one, all having the purpose of increasing selected qualities for the storage or transmission. For example, surfaces might increase the resistance to channel limits (Base64), the transmission speed (gzip), the information privacy (DES), the conformance to operating system conventions (CR-LF), the blocking into records (VB), and surely other things as well(17). Many surfaces may be applied to a stream of characters from a charset, the order of application of surfaces is important, and surfaces should be removed in the reverse order of their application.

Even if surfaces may generally be applied to various charsets, some surfaces were specifically designed for a particular charset, and would not make much sense if applied to other charsets. In such cases, these conceptual surfaces have been implemented as recode charsets, instead of as surfaces. This choice yields to cleaner syntax and usage. See section 5. The universal charset.

Surfaces are implemented within recode as special charsets which may only transform to or from the data or tree special charsets. Clever users may use this knowledge for writing surface names in requests exactly as if they were pure charsets, when the only need is to change surfaces without any kind of recoding between real charsets. In such contexts, either data or tree may also be used as if it were some kind of generic, anonymous charset: the request `data..surface' merely adds the given surface, while the request `surface..data' removes it.

The recode library distinguishes between mere data surfaces, and structural surfaces, also called tree surfaces for short. Structural surfaces might allow, in the long run, transformations between a few specialised representations of structural information like MIME parts, Perl or Python initialisers, LISP S-expressions, XML, Emacs outlines, etc.

We are still experimenting with surfaces in recode. The concept opens the doors to many avenues; it is not clear yet which ones are worth pursuing, and which should be abandoned. In particular, implementation of structural surfaces is barely starting, there is not even a commitment that tree surfaces will stay in recode, if they do prove to be more cumbersome than useful. This chapter presents all surfaces currently available.

13.1 Permuting groups of bytes
13.2 Representation for end of lines
13.3 MIME contents encodings
13.4 Interpreted character dumps
13.5 Artificial data for testing