Documentation: migrate "Wide Font Support" from wiki

link: https://cwiki.apache.org/confluence/display/NUTTX/Wide+Font+Support

Documentation/components/nxgraphics/nxfonts.rst

@@ -124,3 +124,87 @@ NXFONTS types
 
   :return: ``OK`` on success; ``ERROR`` on failure with ``errno`` set appropriately.
 
+Wide Font Support
+=================
+
+Question::
+
+  > My team is trying the nuttx graphics with chinese fonts, but nx seems not
+  > support fonts quantity more than 256 chars, right?
+
+Answer::
+
+  NuttX currently only uses fonts with 7-bit and 8-bit character sets. But
+  I believe that that limitation is mostly arbitrary. It should be a simple
+  extension to the font subsystem to use 16-bit fonts.
+
+Adding 16-Bit Font support
+--------------------------
+
+Current 7/8-bit Font Implementation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+All of critical font interfaces allow for 16-bit character sets:
+
+.. code-block:: C
+
+   FAR const struct nx_fontbitmap_s *nxf_getbitmap(NXHANDLE handle, uint16_t ch)
+
+The character code is only used to look-up of a glyph in a table. There is a
+definition that controls the width of the character set: CONFIG_NXFONTS_CHARBITS.
+This currently defaults to 7 but all existing fonts support 8-bits.
+
+My first guess is that the only thing that would have to change is that single
+file nxfonts_bitmaps.c (and the function nxf_getglyphset() in the file
+nxfonts_getfont.c) . nxfonts_bitmaps.c is used to auto-generate 7/8-bit font
+data sents. Here is how that works:
+
+* Each 7-8 bit file is described by a header file like, for example,
+  nxfonts_sans17x22.h.
+
+* At build time each of these header files is used to create a C file,
+  like, nxfonts_bitmaps_sans17x22.c.
+
+* It creates the C file (like nxfonts_bitmaps_sans17x22.c) by compiling
+  nxfonts_bitmaps.c and including nxfonts_sans17x22.h to create the font
+  dataset at build time.
+
+The function nxf_getglyphset() in the file nxfonts_getfont.c selects the 7-bit
+font range (codes < 128) or the 8-bit range (code >= 128 > 256). The fonts are
+kept in simple arrays splitting the data up into ranges of values lets you above
+the non-printable codes at the beginning and end of each range. There is even a
+comment in the code there "Someday, perhaps 16-bit fonts will go here".
+
+Adding Wide Fonts
+~~~~~~~~~~~~~~~~~
+
+To add a single wide font, the easiest way would be to simply add the final
+.C file without going through the C auto-generation step. That should be VERY
+easy. (But since it has never been used with larger character sets, I am sure
+that there are bugs and things that need to be fixed).
+
+If you want to add many wide fonts, then perhaps you would have to create a new
+version of the C auto-generation logic. That would require more effort.
+
+I am willing to help and advise. Having good wide character support in the NuttX
+graphics would be an important improvement to NuttX. This is not a lot of code
+nor is it very difficult code so you should not let it be an obstacle for you.
+
+Font Storage Issues
+-------------------
+
+One potential problem may be the amount of memory required by fonts with
+thousands of characters. If you have a lot of flash, it may not be a problem,
+but on many microcontrollers it will be quite limiting.
+
+Options are:
+
+* **Font Compression** Addition of some font compression algorithm in NuttX.
+  However, Chinese character bitmaps do not compress well: Many of them contain
+  so much data that there is not much of anything to compress. Some actually
+  expand under certain compression algorithms.
+
+* **Mass Storage** A better option would be put the wide the fonts in file
+  system, in NAND or serial FLASH or on an SD card. In this case, additional
+  logic would be required to (1) format a font binary file and to (2) access
+  the font binary from the file system as needed.