I can't seem to find any documentation on pre/post_shaping_filter. I suspect that pre_shaping_filter is called very early on in both pre_linebreak_filter and hpack_filter, but this isn't immediately obvious by reading through the code, so I'm not too sure what the callbacks actually do. I don't expect these callbacks to be used very often, but adding something like this
luaotfload adds a new callback pre_shaping_filter. It is used in both pre_linebreak_filter and hpack_filter immediately before shaping occurs. luaotfload also adds a new callback post_shaping_filter, which is called immediately after shaping. (no idea if this is actually correct)
I can't seem to find any documentation on
pre/post_shaping_filter. I suspect thatpre_shaping_filteris called very early on in bothpre_linebreak_filterandhpack_filter, but this isn't immediately obvious by reading through the code, so I'm not too sure what the callbacks actually do. I don't expect these callbacks to be used very often, but adding something like thisto the manual would probably be enough.
(Based off of https://tex.stackexchange.com/questions/675932/using-lua-callback-to-substitute-some-arabic-characters-inserted-within-tex-comm/676977#comment1681782_676977)