Font rendering: add some helpers for use by xtext

bump base for libkoreader-xtext.so:
- ffi/pic.lua: fix memory leak with some unsupported PNG files
- FreeType ffi: add methods for use with Harfbuzz shaping
- thirdparty: adds libunibreak
- Adds libkoreader-xtext.so: enhanced text shaping

Add a getFallbackFont(N) to each Lua font object to instantiate
(if not yet done) and return the Nth fallback font for the font.

Fallback fonts list: add NotoSansArabicUI-Regular.ttf

Add RenderText:getGlyphByIndex() to get a glyph bitmap by glyph
index, which is what we'll get from Harfbuzz shaping.
(RenderText:getGlyph() works with unicode charcode).

.luacheckrc

@@ -133,6 +133,7 @@ read_globals = {
     "cre",
     "lfs",
     "lipc",
+    "xtext",
 }
 
 exclude_files = {

base

@@ -1 +1 @@
-Subproject commit c140b752e16163ee8836214787caf7343bc4a579
+Subproject commit 6c480198b00ec7ae4e8af25216e8a12fcabda9e6

frontend/ui/font.lua

@@ -79,9 +79,10 @@ local Font = {
     fallbacks = {
         [1] = "NotoSans-Regular.ttf",
         [2] = "NotoSansCJKsc-Regular.otf",
-        [3] = "nerdfonts/symbols.ttf",
-        [4] = "freefont/FreeSans.ttf",
-        [5] = "freefont/FreeSerif.ttf",
+        [3] = "NotoSansArabicUI-Regular.ttf",
+        [4] = "nerdfonts/symbols.ttf",
+        [5] = "freefont/FreeSans.ttf",
+        [6] = "freefont/FreeSerif.ttf",
     },
 
     -- face table
@@ -92,6 +93,49 @@ if is_android then
     table.insert(Font.fallbacks, 3, "DroidSansFallback.ttf") -- for some ancient pre-4.4 Androids
 end
 
+-- Synthetized bold strength can be tuned:
+-- local bold_strength_factor = 1   -- really too bold
+-- local bold_strength_factor = 1/2 -- bold enough
+local bold_strength_factor = 3/8 -- as crengine, lighter
+
+-- Callback to be used by libkoreader-xtext.so to get Freetype
+-- instantiated fallback fonts when needed for shaping text
+local _getFallbackFont = function(face_obj, num)
+    if not num or num == 0 then -- return the main font
+        if not face_obj.embolden_half_strength then
+            -- cache this value in case we use bold, to avoid recomputation
+            face_obj.embolden_half_strength = face_obj.ftface:getEmboldenHalfStrength(bold_strength_factor)
+        end
+        return face_obj
+    end
+    if not face_obj.fallbacks then
+        face_obj.fallbacks = {}
+    end
+    if face_obj.fallbacks[num] ~= nil then
+        return face_obj.fallbacks[num]
+    end
+    local next_num = #face_obj.fallbacks + 1
+    local cur_num = 0
+    for index, fontname in pairs(Font.fallbacks) do
+        if fontname ~= face_obj.realname then -- Skip base one if among fallbacks
+            local fb_face = Font:getFace(fontname, face_obj.orig_size)
+            if fb_face ~= nil then -- valid font
+                cur_num = cur_num + 1
+                if cur_num == next_num then
+                    face_obj.fallbacks[next_num] = fb_face
+                    if not fb_face.embolden_half_strength then
+                        fb_face.embolden_half_strength = fb_face.ftface:getEmboldenHalfStrength(bold_strength_factor)
+                    end
+                    return fb_face
+                end
+            end
+        end
+    end
+    -- no more fallback font
+    face_obj.fallbacks[next_num] = false
+    return false
+end
+
 --- Gets font face object.
 -- @string font
 -- @int size optional size
@@ -144,12 +188,34 @@ function Font:getFace(font, size)
         -- @field hash hash key for this font face
         face_obj = {
             orig_font = font,
+            realname = realname,
             size = size,
             orig_size = orig_size,
             ftface = face,
-            hash = hash
+            hash = hash,
         }
         self.faces[hash] = face_obj
+
+        -- Callback to be used by libkoreader-xtext.so to get Freetype
+        -- instantiated fallback fonts when needed for shaping text
+        face_obj.getFallbackFont = function(num)
+            return _getFallbackFont(face_obj, num)
+        end
+        -- Font features, to be given by libkoreader-xtext.so to HarfBuzz.
+        -- (Could be tweaked by font if needed. Note that NotoSans does not
+        -- have common ligatures, like for "fi" or "fl", so we won't see
+        -- them in the UI.)
+        -- Use HB defaults, and be sure to enable kerning and ligatures
+        -- (which might be part of HB defaults, or not, not sure).
+        face_obj.hb_features = { "+kern", "+liga" }
+        -- If we'd wanted to disable all features that might be enabled
+        -- by HarfBuzz (see harfbuzz/src/hb-ot-shape.cc, quite unclear
+        -- what's enabled or not by default):
+        -- face_obj.hb_features = {
+        --      "-kern", "-mark", "-mkmk", "-curs", "-locl", "-liga",
+        --      "-rlig", "-clig", "-ccmp", "-calt", "-rclt", "-rvrn",
+        --      "-ltra", "-ltrm", "-rtla", "-rtlm", "-frac", "-numr",
+        --      "-dnom", "-rand", "-trak", "-vert", }
     end
     return face_obj
 end

frontend/ui/rendertext.lua

@@ -172,7 +172,7 @@ function RenderText:sizeUtf8Text(x, width, face, text, kerning, bold)
     local pen_y_bottom = 0
     local prevcharcode = 0
     for _, charcode, uchar in utf8Chars(text) do
-        if pen_x < (width - x) then
+        if not width or pen_x < (width - x) then
             local glyph = self:getGlyph(face, charcode, bold)
             if kerning and (prevcharcode ~= 0) then
                 pen_x = pen_x + (face.ftface):getKerning(prevcharcode, charcode)
@@ -264,6 +264,11 @@ function RenderText:renderUtf8Text(dest_bb, x, baseline, face, text, kerning, bo
 end
 
 local ellipsis = "…"
+
+function RenderText:getEllipsisWidth(face, bold)
+    return self:sizeUtf8Text(0, false, face, ellipsis, false, bold).x
+end
+
 --- Returns a substring of a given text that meets the maximum width (in pixels)
 -- restriction with ellipses (…) at the end if required.
 --
@@ -275,10 +280,36 @@ local ellipsis = "…"
 -- @treturn string
 -- @see getSubTextByWidth
 function RenderText:truncateTextByWidth(text, face, max_width, kerning, bold)
-    local ellipsis_width = self:sizeUtf8Text(0, max_width, face, ellipsis, false, bold).x
+    local ellipsis_width = self:getEllipsisWidth(face, bold)
     local new_txt_width = max_width - ellipsis_width
     local sub_txt = self:getSubTextByWidth(text, face, new_txt_width, kerning, bold)
     return sub_txt .. ellipsis
 end
 
+--- Returns a rendered glyph by glyph index
+-- xtext/Harfbuzz, after shaping, gives glyph indexes in the font, which
+-- is usually different from the unicode codepoint of the original char)
+--
+-- @tparam ui.font.FontFaceObj face font face for the text
+-- @int glyph index
+-- @bool[opt=false] bold whether the glyph should be artificially boldened
+-- @treturn glyph
+function RenderText:getGlyphByIndex(face, glyphindex, bold)
+    local hash = "xglyph|"..face.hash.."|"..glyphindex.."|"..(bold and 1 or 0)
+    local glyph = GlyphCache:check(hash)
+    if glyph then
+        -- cache hit
+        return glyph[1]
+    end
+    local rendered_glyph = face.ftface:renderGlyphByIndex(glyphindex, bold and face.embolden_half_strength)
+    if not rendered_glyph then
+        logger.warn("error rendering glyph (glyphindex=", glyphindex, ") for face", face)
+        return
+    end
+    glyph = CacheItem:new{rendered_glyph}
+    glyph.size = glyph[1].bb:getWidth() * glyph[1].bb:getHeight() / 2 + 32
+    GlyphCache:insert(hash, glyph)
+    return rendered_glyph
+end
+
 return RenderText