Explore the table obtained from a TrueType font

Define the following macro in your TeX file:

\def\studyttffont#1{\directlua{
function printtable (ind,tableau,prof)
  local k,l
  texio.write_nl(string.rep("...",prof) .. ind .. " -> {")
  prof=prof+1
  for k,l in pairs(tableau) do 
    if (type(l)=="table") then
      printtable(k,l,prof)
    else
      texio.write_nl(string.rep("...",prof) .. k .. " -> " .. tostring(l))
    end
  end
  texio.write_nl(string.rep("...",prof-1) .. "}")
end

filename = kpse.find_file("#1","truetype fonts")
ttffont  = fontloader.to_table(fontloader.open(filename))
printtable("#1",ttffont,0)
}}

and call it giving the name of the font you want to explore (the font must be in some directory known by kpse). Here is what you get for the "Gentium" font (strongly truncated, otherwise it would be too long for wiki):

(
gentium -> {
...mark_class_cnt -> 0
...glyphs -> {
......1 -> {
.........boundingbox -> {
............1 -> 0
............2 -> 0
............3 -> 0
............4 -> 0
.........}
.........name -> .null
.........unicodeenc -> 0
.........width -> 0
......}
......2 -> {
.........boundingbox -> {
............1 -> 0
............2 -> 0
............3 -> 0
............4 -> 0
.........}
.........name -> nonmarkingreturn
.........unicodeenc -> 12
.........width -> 451
......}
......3 -> {
.........boundingbox -> {
............1 -> 0
............2 -> 0
............3 -> 0
............4 -> 0
.........}
.........name -> space
.........unicodeenc -> 32
.........width -> 451
......}
......4 -> {
.........boundingbox -> {
............1 -> 154
............2 -> -40
............3 -> 391
............4 -> 1480
.........}
.........name -> exclam
.........unicodeenc -> 33
.........width -> 557
......}
......5 -> {
.........boundingbox -> {
............1 -> 160
............2 -> 813
............3 -> 711
............4 -> 1480
.........}
.........name -> quotedbl
.........unicodeenc -> 34
.........width -> 870
......}

Explanation: the glyphs table is indexed by glyph indexes. Each glyph index maps to a bounding box, a potential (PostScript) name, a potential Unicode character, and to its width. Bounding box and width are given in font units.

......6 -> {
.........boundingbox -> {
............1 -> 87
............2 -> 127
............3 -> 982
............4 -> 1292
.........}
.........name -> numbersign
.........unicodeenc -> 35
.........width -> 961
......}
......7 -> {
.........boundingbox -> {
............1 -> 88
............2 -> -174
............3 -> 862
............4 -> 1384
.........}
.........name -> dollar
.........unicodeenc -> 36
.........width -> 961
......}
......8 -> {
.........boundingbox -> {
............1 -> 76
............2 -> -34
............3 -> 1337
............4 -> 1237
.........}
.........name -> percent
.........unicodeenc -> 37
.........width -> 1413
......}
......9 -> {
.........boundingbox -> {
............1 -> 80
............2 -> -30
............3 -> 1346
............4 -> 1450
.........}
.........name -> ampersand
.........unicodeenc -> 38
.........width -> 1366
......}
......10 -> {
.........boundingbox -> {
............1 -> 160
............2 -> 813
............3 -> 352
............4 -> 1480
.........}
.........name -> quotesingle
.........unicodeenc -> 39
.........width -> 512
......}
......11 -> {
.........boundingbox -> {
............1 -> 121
............2 -> -315
............3 -> 610
............4 -> 1600
.........}
.........name -> parenleft
.........unicodeenc -> 40
.........possub -> {
............1 -> {
...............flags -> 0
...............type -> substitution
...............subs -> {
..................variant -> parenright
...............}
...............script_lang_index -> 1
...............tag -> rtla
............}
.........}
.........width -> 649
......}

Here we have the special case of a glyph which has an attached OpenType transformation. In this case it is a (simple) substitution from the GSUB table. This substitution should happen when we are in the script+language combination indexed by number 1, and the OpenType which calls it is rtla, also known as �Right-to-left alternates�. The glyph �parenleft� is substituted by the glyph �parenright�.

......12 -> {
.........boundingbox -> {
............1 -> 41
............2 -> -315
............3 -> 530
............4 -> 1600
.........}
.........name -> parenright
.........unicodeenc -> 41
.........possub -> {
............1 -> {
...............flags -> 0
...............type -> substitution
...............subs -> {
..................variant -> parenleft
...............}
...............script_lang_index -> 1
...............tag -> rtla
............}
.........}
.........width -> 649
......}
......13 -> {
.........boundingbox -> {
............1 -> 70
............2 -> 709
............3 -> 846
............4 -> 1561
.........}
.........name -> asterisk
.........unicodeenc -> 42
.........width -> 915
......}
......14 -> {
.........boundingbox -> {
............1 -> 61
............2 -> 205
............3 -> 778
............4 -> 920
.........}
.........name -> plus
.........unicodeenc -> 43
.........width -> 840
......}
......15 -> {
.........boundingbox -> {
............1 -> 88
............2 -> -305
............3 -> 391
............4 -> 230
.........}
.........kerns -> {
............1 -> {
...............char -> quotedblright
...............off -> -220
...............flags -> 0
...............sli -> 0
............}
............2 -> {
...............char -> quoteright
...............off -> -220
...............flags -> 0
...............sli -> 0
............}
.........}
.........name -> comma
.........unicodeenc -> 44
.........width -> 469
......}

Here we have some kernings: the �comma� glyph gets kerned by -220 units when in front of �quoteright� or �quotedblright�.

......16 -> {
.........boundingbox -> {
............1 -> 61
............2 -> 455
............3 -> 629
............4 -> 569
.........}
.........name -> hyphen
.........unicodeenc -> 45
.........width -> 690
......}
......17 -> {
.........boundingbox -> {
............1 -> 131
............2 -> -40
............3 -> 369
............4 -> 236
.........}
.........kerns -> {
............1 -> {
...............char -> quotedblright
...............off -> -220
...............flags -> 0
...............sli -> 0
............}
............2 -> {
...............char -> quoteright
...............off -> -220
...............flags -> 0
...............sli -> 0
............}
.........}
.........name -> period
.........unicodeenc -> 46
.........width -> 469
......}
......18 -> {
.........boundingbox -> {
............1 -> 49
............2 -> -335
............3 -> 909
............4 -> 1600
.........}
.........name -> slash
.........unicodeenc -> 47
.........width -> 961
......}
......19 -> {
.........boundingbox -> {
............1 -> 76
............2 -> -30
............3 -> 885
............4 -> 1235
.........}
.........name -> zero
.........unicodeenc -> 48
.........width -> 961
......}
......20 -> {
.........boundingbox -> {
............1 -> 139
............2 -> 0
............3 -> 840
............4 -> 1245
.........}
.........name -> one
.........unicodeenc -> 49
.........width -> 961
......}
......21 -> {
.........boundingbox -> {
............1 -> 98
............2 -> 0
............3 -> 836
............4 -> 1235
.........}
.........name -> two
.........unicodeenc -> 50
.........width -> 961
......}
......22 -> {
.........boundingbox -> {
............1 -> 68
............2 -> -30
............3 -> 825
............4 -> 1235
.........}
.........name -> three
.........unicodeenc -> 51
.........width -> 961
......}
......23 -> {
.........boundingbox -> {
............1 -> 55
............2 -> 0
............3 -> 874
............4 -> 1245
.........}
.........name -> four
.........unicodeenc -> 52
.........width -> 961
......}
......24 -> {
.........boundingbox -> {
............1 -> 78
............2 -> -30
............3 -> 836
............4 -> 1223
.........}
.........name -> five
.........unicodeenc -> 53
.........width -> 961
......}
......25 -> {
.........boundingbox -> {
............1 -> 111
............2 -> -30
............3 -> 881
............4 -> 1249
.........}
.........name -> six
.........unicodeenc -> 54
.........width -> 961
......}
......26 -> {
.........boundingbox -> {
............1 -> 104
............2 -> -30
............3 -> 885
............4 -> 1204
.........}
.........name -> seven
.........unicodeenc -> 55
.........width -> 961
......}
......27 -> {
.........boundingbox -> {
............1 -> 94
............2 -> -30
............3 -> 864
............4 -> 1235
.........}
.........name -> eight
.........unicodeenc -> 56
.........width -> 961
......}
......28 -> {
.........boundingbox -> {
............1 -> 100
............2 -> -43
............3 -> 870
............4 -> 1235
.........}
.........name -> nine
.........unicodeenc -> 57
.........width -> 961
......}
......29 -> {
.........boundingbox -> {
............1 -> 131
............2 -> -40
............3 -> 369
............4 -> 961
.........}
.........name -> colon
.........unicodeenc -> 58
.........width -> 469
......}
......30 -> {
.........boundingbox -> {
............1 -> 88
............2 -> -305
............3 -> 391
............4 -> 961
.........}
.........name -> semicolon
.........unicodeenc -> 59
.........width -> 469
......}
......31 -> {
.........boundingbox -> {
............1 -> 61
............2 -> 256
............3 -> 838
............4 -> 899
.........}
.........name -> less
.........unicodeenc -> 60
.........possub -> {
............1 -> {
...............flags -> 0
...............type -> substitution
...............subs -> {
..................variant -> greater
...............}
...............script_lang_index -> 1
...............tag -> rtla
............}
.........}
.........width -> 899
......}
......32 -> {
.........boundingbox -> {
............1 -> 61
............2 -> 354
............3 -> 838
............4 -> 768
.........}
.........name -> equal
.........unicodeenc -> 61
.........width -> 899
......}
......33 -> {
.........boundingbox -> {
............1 -> 61
............2 -> 256
............3 -> 838
............4 -> 899
.........}
.........name -> greater
.........unicodeenc -> 62
.........possub -> {
............1 -> {
...............flags -> 0
...............type -> substitution
...............subs -> {
..................variant -> less
...............}
...............script_lang_index -> 1
...............tag -> rtla
............}
.........}
.........width -> 899
......}
......34 -> {
.........boundingbox -> {
............1 -> 80
............2 -> -40
............3 -> 807
............4 -> 1480
.........}
.........name -> question
.........unicodeenc -> 63
.........width -> 887
......}
......35 -> {
.........boundingbox -> {
............1 -> 70
............2 -> -375
............3 -> 1583
............4 -> 1333
.........}
.........name -> at
.........unicodeenc -> 64
.........width -> 1653
......}
......36 -> {
.........boundingbox -> {
............1 -> 0
............2 -> 0
............3 -> 1198
............4 -> 1317
.........}
.........kerns -> {
............1 -> {
...............char -> quotedblright
...............off -> -200
...............flags -> 0
...............sli -> 0
............}
............2 -> {
...............char -> quoteright
...............off -> -200
...............flags -> 0
...............sli -> 0
............}
............3 -> {
...............char -> y
...............off -> -100
...............flags -> 0
...............sli -> 0
............}
............4 -> {
...............char -> w
...............off -> -80
...............flags -> 0
...............sli -> 0
............}
............5 -> {
...............char -> v
...............off -> -100
...............flags -> 0
...............sli -> 0
............}
............6 -> {
...............char -> Y
...............off -> -180
...............flags -> 0
...............sli -> 0
............}
............7 -> {
...............char -> W
...............off -> -120
...............flags -> 0
...............sli -> 0
............}
............8 -> {
...............char -> V
...............off -> -160
...............flags -> 0
...............sli -> 0
............}
............9 -> {
...............char -> U
...............off -> -80
...............flags -> 0
...............sli -> 0
............}
............10 -> {
...............char -> T
...............off -> -100
...............flags -> 0
...............sli -> 0
............}
............11 -> {
...............char -> Q
...............off -> -30
...............flags -> 0
...............sli -> 0
............}
............12 -> {
...............char -> O
...............off -> -30
...............flags -> 0
...............sli -> 0
............}
............13 -> {
...............char -> G
...............off -> -30
...............flags -> 0
...............sli -> 0
............}
............14 -> {
...............char -> C
...............off -> -30
...............flags -> 0
...............sli -> 0
............}
.........}
.........name -> A
.........unicodeenc -> 65
.........width -> 1219
......}
......37 -> {
.........boundingbox -> {
............1 -> 41
............2 -> -14
............3 -> 1016
............4 -> 1290
.........}
.........name -> B
.........unicodeenc -> 66
.........width -> 1126
......}
......38 -> {
.........boundingbox -> {
............1 -> 70
............2 -> -30
............3 -> 1018
............4 -> 1290
.........}
.........kerns -> {
............1 -> {
...............char -> A
...............off -> -60
...............flags -> 0
...............sli -> 0
............}
.........}
.........name -> C
.........unicodeenc -> 67
.........width -> 1098
......}
......39 -> {
.........boundingbox -> {
............1 -> 41
............2 -> 0
............3 -> 1124
............4 -> 1290
.........}
.........kerns -> {
............1 -> {
...............char -> Y
...............off -> -60
...............flags -> 0
...............sli -> 0
............}
............2 -> {
...............char -> W
...............off -> -20
...............flags -> 0
...............sli -> 0
............}
............3 -> {
...............char -> V
...............off -> 40
...............flags -> 0
...............sli -> 0
............}
............4 -> {
...............char -> A
...............off -> -60
...............flags -> 0
...............sli -> 0
............}
.........}
.........name -> D
.........unicodeenc -> 68
.........width -> 1214
......}

another 35,850 lines of code

...}
...fontstyle_id -> 0
...units_per_em -> 2048

2,048 units per em means that to obtain TeX quads we need to divide by that number.

...glyphcnt -> 1699
...gasp_cnt -> 3
...weight -> Book
...pfminfo -> {
......vlinegap -> 0
......os2_subyoff -> 1200
......hheadascent_add -> 1
......os2_strikeypos -> 512
......hhead_descent -> -565
......os2_typolinegap -> 90
......typodescent_add -> 1
......weight -> 400
......hhead_ascent -> 1759
......os2_strikeysize -> 102
......panose_set -> 1
......os2_breakchar -> 0
......pfmset -> 1
......os2_typoascent -> -88
......winascent_add -> 1
......firstchar -> 32
......os2_capheight -> 0
......windescent_add -> 1
......vheadset -> 0
......os2_subxsize -> 856
......os2_xheight -> 0
......os2_supxoff -> 0
......hheaddescent_add -> 1
......os2_defaultchar -> 0
......linegap -> 45
......os2_family_class -> 0
......os2_vendor -> SIL 
......os2_supyoff -> 1200
......os2_winascent -> 1759
......os2_subysize -> 856
......avgwidth -> 845
......os2_subxoff -> 0
......os2_supysize -> 856
......os2_windescent -> 565
......os2_typodescent -> -69
......hheadset -> 1
......fstype -> 0
......os2_supxsize -> 856
......lastchar -> 64260
......panose -> {
.........serifstyle -> Any
.........armstyle -> Any
.........contrast -> Medium
.........familytype -> Text and Display
.........proportion -> Modern
.........strokevariation -> Any
.........weight -> Book
.........xheight -> Constant/Large
.........midline -> Any
.........letterform -> Normal/Contact
......}
......width -> 5
......pfmfamily -> 17
......subsuper_set -> 1
......typoascent_add -> 1
...}
...fullname -> GentiumAlt
...map -> {
......map -> {
.........32 -> 3
.........33 -> 4
.........34 -> 5
.........35 -> 6
.........36 -> 7
.........37 -> 8
.........38 -> 9
.........39 -> 10
.........40 -> 11

another 1,679 lines

.........65764 -> 1688
.........65765 -> 1689
.........65766 -> 1690
.........65767 -> 1691
.........65768 -> 1692
.........65769 -> 1693
.........65770 -> 1694
.........65771 -> 1695
.........65772 -> 1696
.........65773 -> 1697
.........65774 -> 1698
......}
......enc -> {
.........1 -> {
............has_2byte -> 1
............char_max -> 0
............only_1byte -> 0
............is_unicodebmp -> 1
............char_cnt -> 65536
............high_page -> 0
............has_1byte -> 0
............low_page -> 0
............builtin -> 1
............enc_name -> UnicodeBmp
............hidden -> 1
.........}
.........2 -> {
............has_2byte -> 0
............char_max -> 0
............only_1byte -> 0
............is_original -> 1
............char_cnt -> 0
............high_page -> 0
............has_1byte -> 0
............low_page -> 0
............builtin -> 1
............enc_name -> Original
............hidden -> 1
.........}
.........3 -> {
............has_2byte -> 0
............char_max -> 0
............only_1byte -> 0
............char_cnt -> 0
............is_custom -> 1
............has_1byte -> 0
............low_page -> 0
............builtin -> 1
............enc_name -> Custom
............high_page -> 0
............hidden -> 1
.........}
.........4 -> {
............has_2byte -> 0
............char_max -> 0
............only_1byte -> 1
............char_cnt -> 256
............high_page -> 0
............has_1byte -> 1
............low_page -> 0
............builtin -> 1
............enc_name -> TeX-Base-Encoding
............unicode -> {
...............1 -> 729
...............2 -> 64257
...............3 -> 64258
...............4 -> 8260
...............5 -> 733
...............6 -> 321
...............7 -> 322
...............8 -> 731

another 238 lines

...............247 -> 247
...............248 -> 248
...............249 -> 249
...............250 -> 250
...............251 -> 251
...............252 -> 252
...............253 -> 253
...............254 -> 254
...............255 -> 255
...............0 -> 0
............}
............hidden -> 1
.........}
......}
......backmax -> 1699
......enccount -> 65775
......encmax -> 65775
......backmap -> {
.........1 -> 65537
.........2 -> 65538
.........3 -> 32
.........4 -> 33
.........5 -> 34
.........6 -> 35
.........7 -> 36
.........8 -> 37
.........9 -> 38
.........10 -> 39
.........11 -> 40
.........12 -> 41
.........13 -> 42

another 1,679 lines

.........1693 -> 65769
.........1694 -> 65770
.........1695 -> 65771
.........1696 -> 65772
.........1697 -> 65773
.........1698 -> 65774
.........0 -> 65536
......}
...}
...italicangle -> 0
...strokedfont -> 0
...cidinfo -> {
......supplement -> 0
......version -> 0
...}
...familyname -> GentiumAlt
...script_lang -> {
......1 -> {
.........script -> latn
.........langs -> {
............1 -> dflt
.........}
......}
...}

The table script_lang contains scripts and languages. Here we have script latn (Latin) with default language.

...upos -> -48
...version -> 1.01; 2003; initial SIL release
...glyphmax -> 1699
...names -> {
......1 -> {
.........lang -> English (US)
.........names -> {
............designerurl -> http://www.sil.org/~gaultney
............venderurl -> http://www.sil.org/
............compatfull -> GentiumAlt
............trademark -> Gentium is a trademark of SIL International.
............license -> This font is the property of SIL International. It is distributed as copyrighted freeware. You may use this software without any charge and may distribute it, as is, to others. Commercial distribution of this software is restricted 
without prior written permission. If you wish to distribute this software commercially, contact SIL for details on obtaining a license. You may not rent or lease the software, nor may you modify, adapt, translate, reverse engineer, decompile, or disassemb
le the software. You may not make derivative fonts from this software. THE SOFTWARE AND RELATED FILES ARE PROVIDED "AS IS" AND WITHOUT WARRANTY OF ANY KIND.
............fullname -> GentiumAlt
............preffamilyname -> GentiumAlt
............designer -> J. Victor Gaultney
............copyright -> Copyright (c) SIL International 2003. All rights reserved.
............version -> Version 1.01; 2003; initial SIL release
............prefmodifiers -> Regular
............family -> GentiumAlt
............postscriptname -> GentiumAlt
............subfamily -> Regular
............uniqueid -> SIL: GentiumAlt: 2003
.........}
......}
...}
...gentags -> {
......tt_cur -> 0
...}
...modificationtime -> 1063146346
...fontname -> GentiumAlt
...changed -> 0
...vertical_origin -> 0
...table_version -> 0.1
...hasvmetrics -> 0
...creationtime -> 1032220800
...order2 -> 1
...head_optimized_for_cleartype -> 0
...strokewidth -> 0
...design_range_bottom -> 0
...design_size -> 0
...design_range_top -> 0
...sli_cnt -> 1
...macstyle -> -1
...ttf_tables -> {
......1 -> {
.........len -> 32
.........data -> 
.........maxlen -> 0
.........tag -> maxp
......}
...}
...copyright -> Copyright (c) SIL International 2003. All rights reserved.
...os2_version -> 1
...gasp_version -> 0
...uni_interp -> none
...uwidth -> 14
...weight_width_slope_only -> 0
...descent -> 410
...ascent -> 1638
...uniqueid -> 0
} )