How are non-unicode glyphs referenced in LuaTeX .dvi files?

Question

Dvi files reference glyphs by their unicode number. However, a document often contain some unencoded glyphs, such as large math symbols. By inspecting dvi files containing such glyphs, I conclude that these glyphs are referenced by some non-unicode (Private Use Area) character codes.

How can I know which character code LuaTeX will choose for a certain glyph, say, the display version of the integral sign? The same question goes for XeTeX, in case they are different.

Example:

\documentclass{standalone}
\usepackage{fontspec}
\usepackage{unicode-math}
\setmainfont{XITS}
\setmathfont{XITS Math}
\begin{document}
$\int\displaystyle\int$
\end{document}

When compiled with LuaLaTeX, using the --output-format=dvi option, the small integral is encoded as 0x222B (as expected) and the big integral is encoded as 0x0F001C (somewhat mystifying).

Update:

Some experimentation confirms that the encoding of the integral sign differs between fonts. The display version of the integral is 0x0F05C2 in Cambria Math, 0x10FF99 in Asana Math and 0x0F0314 in STIX Math, for instance.

Answer 1

LuaTeX uses a pretty simple algorithm to choose the DVI character codes: While reading the OTF file, it checks whether a character has a Unicode point assigned or not. If there's an entry in the Unicode mapping table, LuaTeX uses the Unicode point. Otherwise, it populates the Unicode range starting at 0xF0000. The first unmapped character is assigned to 0xF0000, the second one to 0xF0001 etc. Thus, you can only predict the DVI character codes if you evaluate the Unicode table of the OTF fonts yourself. Especially, there's no relation between the code and the semantics of an unmapped character, i.e. the big integral sign can get different DVI codes depending on the mapping tables and the characters present in the various font files.

XeTeX, on the other hand, simply uses the glyph indexes of the characters in an OTF file if the characters are set with XDV command XGlyphArray or XGlyphString. Each glyph in a font has a unique internal value with no relation to any encoding. The encoding itself is realized by the already mentioned encoding tables that, for example, map from Unicode or ISO-8859-1 to glyph indexes. So, XeTeX ignores the encodings present in the font file and uses the glyph indexes directly -- more precisely, those provided by the FreeType library (which may rearrange the values).