font availability test and font fallback?

[eroux]

Building very complex documents (like polyglossia documentation), I realize how useful it would be to be able to test if a font is available or not, this would allow fallbacks for non-available fonts.

Two usecases :

One for polyglossia : we want to write farsi, whatever the font, and not everyone has the same, so we could test a few fonts, and take the first available.

One for unicode-math-example: we build a table with all possible math symbols in all possible math fonts, but it would be useful to check if a math font is available before building its table...

These are very specific, but I'm sure others would come. This would be quite new in the TeX World I guess, but it sounds like an improvement... What do you think?

[phi-gamma]

Sounds useful. But:

• we have --find=<name> and --list=* in luaotfload-tool that assist in ascertaining whether the system has a font;
• font name fallbacks seems like a good idea to add another layer of unpredictability (fallbacks) on top of something that is already messy as hell (font names).

Apart from that, what you are pondering is what I’d express as lookup chaining: if the test for a font results in a negative, try the next one in the list of candidates etc. We’d need a special syntax (or csname) for that. It’s not impossible, though the current design doesn’t exactly lend itself for this kind of addition.

Personally I’d rather lobby for Latex to adopt the concept of typescripts :p.

[eroux]

Hmmm, I was not thinking about something interfering with the whole lookup thing, but just a small function returning a bool, true if the name or file resolves to something, and false otherwise. With this you could easily do something like

local resolves = luaotfload.aux.resolves
myfarsifont = "Farsi simple bold"
if not resolves(myfarsifont) then
  myfarsifont = "My fallback"
end
tex.sprint(myfarsifont)

it seems simple and clear... for sure, automatization would require something bigger... What do you think?

[phi-gamma]

It depends on what you intend to resolve -- a font name or a filename? For font names you can just do what luaotfload-tool does:

local query = "<font name here>"
local tmpspec = {
    name          = query,
    lookup        = "name",
    specification = "name:" .. query,
    optsize       = 0,
}

local foundname, subfont, success = fonts.names.resolve(nil, nil, tmpspec)

[eroux]

hmmm... yes, it sounds good! Though, for a public interface, having two nil arguments for a function is something really strange, what about a

function luaotfload.aux.resolve(s) = fonts.name.resolve(nil, nil, s)

?

[phi-gamma]

Better:

function luaotfload.aux.resolve_name (s) = return fonts.name.resolve(nil, nil, s) end

but still, only for font names.

[phi-gamma]

Just one question: you really only need raw fontnames, not the fancy /b, /i style modifiers?

[phi-gamma]

You now have two functions for that at your disposal: resolve_fontname() which resolves a single name, and resolve_fontlist() which returns the first font found in a list of font names.

\input luaotfload.sty
\directlua{
  print""
  local aux = luaotfload.aux

  %% hit: returns string and int
  local found, sub = aux.resolve_fontname("Iwona Regular")
  print(found, sub)

  %% miss: returns false, false (also triggers db update)
  local found, sub = aux.resolve_fontname("Expensive Adobe Stuff Pro")
  print(found, sub)

  %% or find the first resolvable in list
  local fontlist = {
    "Adobe Comic Sans Pro",
    "Zapf Klingon Bold Italic",
    "LM Roman 12 Regular",
  }
  local foundname, sub = aux.resolve_fontlist(fontlist)
  print(foundname, sub)
  print""
}

\bye

[eroux]

Sorry for the late answer, today is my day off... well, this is just wonderful, actually better than I asked, Thanks!