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7.5 strings

Internally the DWG consists of multiple different string formats, see Types. The most important are BITCODE_TV (i.e. char*) encoded according to dwg->header.codepage, and BITCODE_TU (i.e. wchar_t on Windows, UCS-2).

Externally most functions get and set strings as UTF-8, as in DXF or JSON.

Before r2007 DWG’s the TV and T strings are encoded in its codepage, and converted from and to their proper codepage to UTF-8 or \U+XXXX.

To encode unicode characters special \U+XXXX sequences are used, and pre-r2007 DXF MIF \M+nXXXX sequences, where n is one of the asian wide-character codepages 932 (Japanese), 950 (trad. Chinese), 949 (Korean Wansung), 1361 (Johab), and 936 (simplified Chinese).

On DWG’s r2007 and later most strings (T and TU) are encoded in the Microsoft specific two-byte UCS-2 Unicode encoding, without proper support for surrogate pairs and the upper planes (i.e. emojis).

Fixed TF strings are not encoded and have a length stored also. Normal strings are all zero-delimited. EED and XDATA strings do have a length though, but have length limitations.

Strings in DXF and JSON also have quoting rules for special characters, like \r, \n, \" and so on.

Transformations:

DWG to DWG: decode reads the T and TU strings in its natural format into the field. encode translate it to TV or TU. encode needs header.from_version and how it was read, from DWG or from an importer (in_dxf or in_json) or the add api (DWG_OPTS_IN).

DXF/JSON to DWG: in_dxf/json keeps the T and TU strings as TV. encode to <r2007 keeps it as TV, r2007+ translates it to TU. Unicode is encoded as \U+XXXX. It sets DWG_OPTS_IN.

DWG to DXF/JSON: decode keeps the T and TU strings as TV or TU. out_dxf/json translate them to TV or UTF-8 and quotes them via \U+XXXX.

add api to DWG/DXF: add reads strings as UTF-8, and encodes it from UTF-8 to TV or TU. (TU not yet, as we don’t encode r2004+ yet). add sets DWG_OPTS_IN.