使用 HarfBuzz 渲染器时，天城文 UTF-8 文本提取代码产生 lua 错误

Question 1

在节点模式下，通常无法恢复完整文本，因为您得到的是成形输出，而成形字形无法唯一地映射回输入文本。您只能使用 tounicode 值来近似它。这些映射到实际的 PDF 文件 ToUnicode CMap 条目，因此遵循其字形到 Unicode 映射的受限模型：每个字形都相当于一个固定的 unicode 代码点序列。这些映射按渲染顺序连接起来。如您所见，此模型不足以将 Devanagari 字形映射到输入文本。

您可以使用harf模式来避免此问题：harf模式不受此有限模型的影响，因为它不仅为您提供了形状的字形列表，而且还创建了 PDF 标记内容 ActualText 条目，这些条目覆盖了无法通过 ToUnicode 正确建模的序列中的 ToUnicode 映射。可以使用属性从 Lua 代码中查询此映射所需的数据glyph_data。（这是一个未记录的实现细节，将来可能会发生变化）

如果您想从任何文本中提取尽可能多的内容，您可以在 Lua 代码中结合此基于属性的方法和基于 ToUnicode 的方法：

extracttext.lua使用以下方式创建文件

local type = type
local char = utf8.char
local unpack = table.unpack
local getproperty = node.getproperty
local getfont = font.getfont
local is_glyph = node.is_glyph

-- tounicode id UTF-16 in hex, so we need to handle surrogate pairs...
local utf16hex_to_utf8 do -- Untested, but should more or less work
  local l = lpeg
  local tonumber = tonumber
  local hex = l.R('09', 'af', 'AF')
  local byte = hex * hex
  local simple = byte * byte / function(s) return char(tonumber(s, 16)) end
  local surrogate = l.S'Dd' * l.C(l.R('89', 'AB', 'ab') * byte)
                  * l.S'Dd' * l.C(l.R('CF', 'cf') * byte) / function(high, low)
                      return char(0x10000 + ((tonumber(high, 16) & 0x3FF) << 10 | (tonumber(low, 16) & 0x3FF)))
                    end
  utf16hex_to_utf8 = l.Cs((surrogate + simple)^0)
end

-- First the non-harf case

-- Standard caching setup
local identity_table = setmetatable({}, {__index = function(_, id) return char(id) end})
local cached_text = setmetatable({}, {__index = function(t, fid)
  local fontdir = getfont(fid)
  local characters = fontdir and fontdir.tounicode == 1 and fontdir.characters
  local font_cache = characters and setmetatable({}, {__index = function(tt, slot)
    local character = characters[slot]
    local text = character and character.tounicode or slot
    -- At this point we have the tounicode value in text. This can have different forms.
    -- The order in the if ... elseif chain is based on how likely it is to encounter them.
    -- This is a small performance optimization.
    local t = type(text)
    if t == 'string' then
      text = utf16hex_to_utf8:match(text)
    elseif t == 'number' then
      text = char(text)
    elseif t == 'table' then
      text = char(unpack(text)) -- I haven't tested this case, but it should work
    end
    tt[slot] = text
    return text
  end}) or identity_table
  t[fid] = font_cache
  return font_cache
end})

-- Now the tounicode case just has to look up the value
local function from_tounicode(n)
  local slot, fid = is_glyph(n)
  return cached_text[fid][slot]
end

-- Now the traversing stuff. Nothing interesting to see here except for the
-- glyph case
local traverse = node.traverse
local glyph, glue, disc, hlist, vlist = node.id'glyph', node.id'glue', node.id'disc', node.id'hlist', node.id'vlist'
local extract_text_vlist
-- We could replace i by #t+1 but this should be slightly faster
local function real_extract_text(head, t, i)
  for n, id in traverse(head) do
    if id == glyph then
      -- First handle harf mode: Look for a glyph_info property. If that does not exists
      -- use from_tounicode. glyph_info will sometimes/often be an empty string. That's
      -- intentional and it should *not* trigger a fallback. The actual mapping will be
      -- contained in surrounding chars.
      local props = getproperty(n)
      t[i] = props and props.glyph_info or from_tounicode(n)
      i = i + 1
    elseif id == glue then
      if n.width > 1001 then -- 1001 is arbitrary but sufficiently high to be bigger than most weird glue uses
        t[i] = ' '
        i = i + 1
      end
    elseif id == disc then
      i = real_extract_text(n.replace, t, i)
    elseif id == hlist then
      i = real_extract_text(n.head, t, i)
    elseif id == vlist then
      i = extract_text_vlist(n.head, t, i)
    end
  end
  return i
end
function extract_text_vlist(head, t, i) -- glue should not become a space here
  for n, id in traverse(head) do
    if id == hlist then
      i = real_extract_text(n.head, t, i)
    elseif id == vlist then
      i = extract_text_vlist(n.head, t, i)
    end
  end
  return i
end
return function(list)
  local t = {}
  real_extract_text(list.head, t, 1)
  return table.concat(t)
end

这可以用作普通的 Lua 模块：

\documentclass{article}
\usepackage{fontspec}

\newfontfamily{\devharf}{Noto Sans Devanagari}[Script=Devanagari, Renderer=HarfBuzz]
\newfontfamily{\devnode}{Noto Sans Devanagari}[Script=Devanagari, Renderer=Node]

\begin{document}

% Devanagari text is at the right end of following line
% of code, you might have to scroll right to read it
\setbox0=\hbox{Příliš žluťoučký \textit{kůň} úpěl \hbox{ďábelské} ódy difference diffierence. \devharf एक गांव -- में मोहन नाम का लड़का रहता था। उसके पिताजी एक मामूली मजदूर थे।}
\setbox1=\hbox{Příliš žluťoučký \textit{kůň} úpěl \hbox{ďábelské} ódy difference diffierence. \devnode एक गांव -- में मोहन नाम का लड़का रहता था। उसके पिताजी एक मामूली मजदूर थे।}

\directlua{
  local extracttext = require'extracttext'
  local f = io.open("hello.harf.txt","w") % Can reproduce the full input text
  f:write(extracttext(tex.getbox(0)))
  f:close()

  f = io.open("hello.node.txt","w") % In node mode, we only get an approximation
  f:write(extracttext(tex.getbox(1)))
  f:close()
}

\box0
\box1
\end{document}

更一般的说明：如您所见，从形状列表中获取文本时需要做一些工作，尤其是在 ToUnicode 情况下，我们必须映射代理对等。这主要是因为形状文本是不是旨在用于此类用途。一旦字形节点受到保护（又称 subtype(n) >= 256 或not is_char(n)为true），.char条目就不再包含 Unicode 值，而是内部标识符，.font条目可能不再是您期望的值，并且某些字形可能根本不表示为字形。在大多数情况下，如果您想要实际访问框后面的文本而不仅仅是文本的视觉显示，您确实希望拦截列表前它首先被成形。

Answer

在节点模式下，通常无法恢复完整文本，因为您得到的是成形输出，而成形字形无法唯一地映射回输入文本。您只能使用 tounicode 值来近似它。这些映射到实际的 PDF 文件 ToUnicode CMap 条目，因此遵循其字形到 Unicode 映射的受限模型：每个字形都相当于一个固定的 unicode 代码点序列。这些映射按渲染顺序连接起来。如您所见，此模型不足以将 Devanagari 字形映射到输入文本。

您可以使用harf模式来避免此问题：harf模式不受此有限模型的影响，因为它不仅为您提供了形状的字形列表，而且还创建了 PDF 标记内容 ActualText 条目，这些条目覆盖了无法通过 ToUnicode 正确建模的序列中的 ToUnicode 映射。可以使用属性从 Lua 代码中查询此映射所需的数据glyph_data。（这是一个未记录的实现细节，将来可能会发生变化）

如果您想从任何文本中提取尽可能多的内容，您可以在 Lua 代码中结合此基于属性的方法和基于 ToUnicode 的方法：

extracttext.lua使用以下方式创建文件

local type = type
local char = utf8.char
local unpack = table.unpack
local getproperty = node.getproperty
local getfont = font.getfont
local is_glyph = node.is_glyph

-- tounicode id UTF-16 in hex, so we need to handle surrogate pairs...
local utf16hex_to_utf8 do -- Untested, but should more or less work
  local l = lpeg
  local tonumber = tonumber
  local hex = l.R('09', 'af', 'AF')
  local byte = hex * hex
  local simple = byte * byte / function(s) return char(tonumber(s, 16)) end
  local surrogate = l.S'Dd' * l.C(l.R('89', 'AB', 'ab') * byte)
                  * l.S'Dd' * l.C(l.R('CF', 'cf') * byte) / function(high, low)
                      return char(0x10000 + ((tonumber(high, 16) & 0x3FF) << 10 | (tonumber(low, 16) & 0x3FF)))
                    end
  utf16hex_to_utf8 = l.Cs((surrogate + simple)^0)
end

-- First the non-harf case

-- Standard caching setup
local identity_table = setmetatable({}, {__index = function(_, id) return char(id) end})
local cached_text = setmetatable({}, {__index = function(t, fid)
  local fontdir = getfont(fid)
  local characters = fontdir and fontdir.tounicode == 1 and fontdir.characters
  local font_cache = characters and setmetatable({}, {__index = function(tt, slot)
    local character = characters[slot]
    local text = character and character.tounicode or slot
    -- At this point we have the tounicode value in text. This can have different forms.
    -- The order in the if ... elseif chain is based on how likely it is to encounter them.
    -- This is a small performance optimization.
    local t = type(text)
    if t == 'string' then
      text = utf16hex_to_utf8:match(text)
    elseif t == 'number' then
      text = char(text)
    elseif t == 'table' then
      text = char(unpack(text)) -- I haven't tested this case, but it should work
    end
    tt[slot] = text
    return text
  end}) or identity_table
  t[fid] = font_cache
  return font_cache
end})

-- Now the tounicode case just has to look up the value
local function from_tounicode(n)
  local slot, fid = is_glyph(n)
  return cached_text[fid][slot]
end

-- Now the traversing stuff. Nothing interesting to see here except for the
-- glyph case
local traverse = node.traverse
local glyph, glue, disc, hlist, vlist = node.id'glyph', node.id'glue', node.id'disc', node.id'hlist', node.id'vlist'
local extract_text_vlist
-- We could replace i by #t+1 but this should be slightly faster
local function real_extract_text(head, t, i)
  for n, id in traverse(head) do
    if id == glyph then
      -- First handle harf mode: Look for a glyph_info property. If that does not exists
      -- use from_tounicode. glyph_info will sometimes/often be an empty string. That's
      -- intentional and it should *not* trigger a fallback. The actual mapping will be
      -- contained in surrounding chars.
      local props = getproperty(n)
      t[i] = props and props.glyph_info or from_tounicode(n)
      i = i + 1
    elseif id == glue then
      if n.width > 1001 then -- 1001 is arbitrary but sufficiently high to be bigger than most weird glue uses
        t[i] = ' '
        i = i + 1
      end
    elseif id == disc then
      i = real_extract_text(n.replace, t, i)
    elseif id == hlist then
      i = real_extract_text(n.head, t, i)
    elseif id == vlist then
      i = extract_text_vlist(n.head, t, i)
    end
  end
  return i
end
function extract_text_vlist(head, t, i) -- glue should not become a space here
  for n, id in traverse(head) do
    if id == hlist then
      i = real_extract_text(n.head, t, i)
    elseif id == vlist then
      i = extract_text_vlist(n.head, t, i)
    end
  end
  return i
end
return function(list)
  local t = {}
  real_extract_text(list.head, t, 1)
  return table.concat(t)
end

这可以用作普通的 Lua 模块：

\documentclass{article}
\usepackage{fontspec}

\newfontfamily{\devharf}{Noto Sans Devanagari}[Script=Devanagari, Renderer=HarfBuzz]
\newfontfamily{\devnode}{Noto Sans Devanagari}[Script=Devanagari, Renderer=Node]

\begin{document}

% Devanagari text is at the right end of following line
% of code, you might have to scroll right to read it
\setbox0=\hbox{Příliš žluťoučký \textit{kůň} úpěl \hbox{ďábelské} ódy difference diffierence. \devharf एक गांव -- में मोहन नाम का लड़का रहता था। उसके पिताजी एक मामूली मजदूर थे।}
\setbox1=\hbox{Příliš žluťoučký \textit{kůň} úpěl \hbox{ďábelské} ódy difference diffierence. \devnode एक गांव -- में मोहन नाम का लड़का रहता था। उसके पिताजी एक मामूली मजदूर थे।}

\directlua{
  local extracttext = require'extracttext'
  local f = io.open("hello.harf.txt","w") % Can reproduce the full input text
  f:write(extracttext(tex.getbox(0)))
  f:close()

  f = io.open("hello.node.txt","w") % In node mode, we only get an approximation
  f:write(extracttext(tex.getbox(1)))
  f:close()
}

\box0
\box1
\end{document}

更一般的说明：如您所见，从形状列表中获取文本时需要做一些工作，尤其是在 ToUnicode 情况下，我们必须映射代理对等。这主要是因为形状文本是不是旨在用于此类用途。一旦字形节点受到保护（又称 subtype(n) >= 256 或not is_char(n)为true），.char条目就不再包含 Unicode 值，而是内部标识符，.font条目可能不再是您期望的值，并且某些字形可能根本不表示为字形。在大多数情况下，如果您想要实际访问框后面的文本而不仅仅是文本的视觉显示，您确实希望拦截列表前它首先被成形。

Question 2

我不太了解 Luaotfload 如何处理 HarfBuzz 字体，但tounicode多亏了，我找到了获取字段的方法table.serialize。所以我为 Harfbuzz 改编的原始代码如下所示：

\documentclass{article}
\usepackage[lmargin=0.5in,tmargin=0.5in,rmargin=0.5in,bmargin=0.5in]{geometry}
\usepackage{fontspec}
\usepackage{microtype}
\usepackage{luacode}

%\newfontscript{Devanagari}{deva,dev2}
\newfontfamily{\devanagarifam}{Noto Sans Devanagari}[Script=Devanagari, Scale=1, Renderer=HarfBuzz]
\newfontfamily{\arabicfam}{Amiri}[Script=Arabic, Scale=1, Renderer=HarfBuzz]

\begin{document}

\setbox0=\hbox{Příliš žluťoučký \textit{kůň} úpěl \hbox{ďábelské} ódy difference diffierence. \devanagarifam एक गांव -- में मोहन नाम का लड़का रहता था। उसके पिताजी एक मामूली मजदूर थे।}
\setbox1=\hbox{\arabicfam \textdir TRT  هذه المقالة عن براغ. لتصفح عناوين مشابهة، انظر براغ (توضيح).}

\begin{luacode*}
  -- local fontstyles = require "l4fontstyles"
  local char = unicode.utf8.char
  local glyph_id = node.id("glyph")
  local glue_id  = node.id("glue")
  local hlist_id = node.id("hlist")
  local vlist_id = node.id("vlist")
  local disc_id  = node.id("disc")
  local minglue  = tex.sp("0.2em")
  local usedcharacters = {}
  local identifiers = fonts.hashes.identifiers
  local fontcache = {}

  local function to_unicode_chars(uchar)
    local uchar = uchar or ""
    -- put characters into a table
    local current = {}
    -- each codepoint is 4 bytes long, we loop over tounicode entry and cut it into 4 bytes chunks
    for i= 1, string.len(uchar), 4 do
      local cchar = string.sub(uchar, i, i + 3)
      -- codepoint is hex string, we need to convert it to number ad then to UTF8 char
      table.insert(current,char(tonumber(cchar,16)))
    end
    return current
  end
  -- cache character lookup, to speed up things
  local function get_character_from_cache(xchar, font_id)
    local current_font = fontcache[font_id] or {characters = {}}
    fontcache[font_id] = current_font -- initialize font cache for the current font if it doesn't exist
    return current_font.characters[xchar]
  end

  -- save characters to cache for faster lookup
  local function save_character_to_cache(xchar, font_id, replace)
    fontcache[font_id][xchar] = replace
    -- return value
    return replace
  end

  local function initialize_harfbuzz_cache(font_id, hb)
    -- save some harfbuzz tables for faster lookup
    local current_font = fontcache[font_id]
    -- the unicode data can be in two places
    -- 1. hb.shared.glyphs[glyphid].backmap
    current_font.glyphs = current_font.glyphs or hb.shared.glyphs
    -- 2. hb.shared.unicodes 
    -- it contains mapping between Unicode and glyph id
    -- we must create new table that contains reverse mapping
    if not current_font.backmap then 
      current_font.backmap = {} 
      for k,v in pairs(hb.shared.unicodes) do
        current_font.backmap[v] = k
      end
    end
    -- save it back to the font cache
    fontcache[font_id] = current_font
    return current_font.glyphs, current_font.backmap
  end

  local function get_unicode(xchar,font_id)
    -- try to load character from cache first
    local current_char = get_character_from_cache(xchar, font_id) 
    if current_char then return current_char end
    -- get tounicode for non HarfBuzz fonts
    local characters = identifiers[font_id].characters
    local uchar = characters[xchar].tounicode
    -- stop processing if tounicode exists
    if uchar then return save_character_to_cache(xchar, font_id, to_unicode_chars(uchar)) end
    -- detect if font is processed by Harfbuzz
    local hb = identifiers[font_id].hb
    -- try HarfBuzz data
    if not uchar and hb then 
      -- get glyph index of the character
      local index = characters[xchar].index
      -- load HarfBuzz tables from cache
      local glyphs, backmap = initialize_harfbuzz_cache(font_id, hb)
      -- get tounicode field from HarfBuzz glyph info
      local tounicode = glyphs[index].tounicode
      if tounicode then
        return save_character_to_cache(xchar, font_id, to_unicode_chars(tounicode))
      end
      -- if this fails, try backmap, which contains mapping between glyph index and Unicode
      local backuni = backmap[index]
      if backuni then 
        return save_character_to_cache(xchar, font_id, {char(backuni)})
      end
      -- if this fails too, discard this character
      return save_character_to_cache(xchar, font_id, {})
    end
    -- return just the original char if everything else fails
    return save_character_to_cache(xchar, font_id, {char(xchar)})
  end

  local function nodeText(n)
    -- output buffer
    local t =  {}
    for x in node.traverse(n) do
      -- glyph node
      if x.id == glyph_id then
        -- get table with characters for current node.char
        local chars = get_unicode(x.char,x.font)
        for _, current_char in ipairs(chars) do
          -- save characters to the output buffer
          table.insert(t,current_char)
        end
      -- glue node
      elseif x.id == glue_id and  node.getglue(x) > minglue then
        table.insert(t," ")
      -- discretionaries
      elseif x.id == disc_id then
        table.insert(t, nodeText(x.replace))
      -- recursivelly process hlist and vlist nodes
      elseif x.id == hlist_id or x.id == vlist_id then
        table.insert(t,nodeText(x.head))
      end
    end
    return table.concat(t)
  end
  local n = tex.getbox(0)
  local n1 = tex.getbox(1)
  print(nodeText(n.head))
  local f = io.open("hello.txt","w")
  f:write(nodeText(n.head))
  f:write(nodeText(n1.head))
  f:close()
\end{luacode*}

\box0

\box1
\end{document}

我还添加了来自维基百科。以下是的内容hello.txt：

Příliš žluťoučký kůň úpěl ďábelské ódy difference diffierence. एक गांव -- में मोहन नाम का लड़का रहता था। उसके पताजी एक मामूली मजदूर थे।هذه المقالة عن براغ. لتصفح عناوين مشابهة، انظر براغ (توضيح).

这两个重要功能是

  local function to_unicode_chars(uchar)
    local uchar = uchar or ""
    local current = {}
    for i= 1, string.len(uchar), 4 do
      local cchar = string.sub(uchar, i, i + 3)
      table.insert(current,char(tonumber(cchar,16)))
    end
    return current
  end

to_unicode_chars函数将 to_unicode 条目拆分为四字节块，然后将其转换为 UTF 8 字符。它还可以处理没有条目的字形tounicode，在这种情况下它只返回空字符串。

  local function get_unicode(xchar,font_id)
    -- try to load character from cache first
    local current_char = get_character_from_cache(xchar, font_id) 
    if current_char then return current_char end
    -- get tounicode for non HarfBuzz fonts
    local characters = identifiers[font_id].characters
    local uchar = characters[xchar].tounicode
    -- stop processing if tounicode exists
    if uchar then return save_character_to_cache(xchar, font_id, to_unicode_chars(uchar)) end
    -- detect if font is processed by Harfbuzz
    local hb = identifiers[font_id].hb
    -- try HarfBuzz data
    if not uchar and hb then 
      -- get glyph index of the character
      local index = characters[xchar].index
      -- load HarfBuzz tables from cache
      local glyphs, backmap = initialize_harfbuzz_cache(font_id, hb)
      -- get tounicode field from HarfBuzz glyph info
      local tounicode = glyphs[index].tounicode
      if tounicode then
        return save_character_to_cache(xchar, font_id, to_unicode_chars(tounicode))
      end
      -- if this fails, try backmap, which contains mapping between glyph index and Unicode
      local backuni = backmap[index]
      if backuni then 
        return save_character_to_cache(xchar, font_id, {char(backuni)})
      end
      -- if this fails too, discard this character
      return save_character_to_cache(xchar, font_id, {})
    end
    -- return just the original char if everything else fails
    return save_character_to_cache(xchar, font_id, {char(xchar)})
  end

此函数首先尝试从当前字体信息加载 Uniocode 数据。如果失败，它会尝试在 Harfbuzz 表中查找。大多数字符在表tounicode中都有映射glyphs。如果不可用，它会尝试unicodes包含字形索引和 Unicode 之间映射的表。如果这都失败了，那么我们会丢弃这个字符。

Answer