font-get-glyphs

font-get-glyphs is a built-in function in `C source code'.

(font-get-glyphs FONT-OBJECT FROM TO &optional OBJECT)

Return a vector of FONT-OBJECT's glyphs for the specified characters.
FROM and TO are positions (integers or markers) specifying a region
of the current buffer, and can be in either order. If the optional
fourth arg OBJECT is not nil, it is a string or a vector containing
the target characters between indices FROM and TO, which are treated
as in `substring'.

Each element is a vector containing information of a glyph in this format:
[FROM-IDX TO-IDX C CODE WIDTH LBEARING RBEARING ASCENT DESCENT ADJUSTMENT]
where
FROM is an index numbers of a character the glyph corresponds to.
TO is the same as FROM.
C is the character of the glyph.
CODE is the glyph-code of C in FONT-OBJECT.
WIDTH thru DESCENT are the metrics (in pixels) of the glyph.
ADJUSTMENT is always nil.
If FONT-OBJECT doesn't have a glyph for a character,
the corresponding element is nil.