The history of luaTEX 2006–2009 / v 0.50 - Pragma ADE

when writing the result to le, TEX wants to insert the glyph data in the le. In both cases
we end up with a message in the log le and a result le with missing data.
In ConTEXt MkIV we intercept the node lists at several points and one of those is directly
after the construction. So let's consider the previous example again.
Because the font has no character 124 we need a way to substitute it with another character.
All we have to do is to change the font identier 32 into one that makes sense. Such
a replacement loop is kind of trivial.
for n in traverse_id(glyph,head) do
local v = vectors[n.font]
if v then
local id = v[n.char]
if id then
n.font = id
end
end
end
We have a table (vectors) that can have a subtable (v) for font with id (n.font) in which
there can be a reference from the current character (n.char) to another font (id) that we
use to replace the font reference (n.font).
Filling the table is relatively easy but an explanation is beyond this chapter. We only show
the high level interface, one that certainly will evolve.
\definefontfallback
[SerifFallback]
[Mono]
[0x000-0x3FF]
[check=yes,force=no]
This command registers an entry in the SerifFallback namespace. There can be multiple
replacement in row (by just using more of these commands), but here we have only
one. The range 0x000–0x3FF will be checked and if the main font lacks a glyph in that
range, it will be taken from the font with the symbolic name Mono. That name will be resolved
when the fallback is associated with a font. The check option tells the machinery
that we need to check for existence and because we don't force, we will only replace
missing glyphs. There is also an rscale option, that permits relative scaling of the fallback
font to the main font, something that may be needed when fonts come from different
sources.
234 Virtual Reality