Re: [NTG-context] GSUB LookupType 6 in fea file not working

9 Jan 2016


      That's really cool.
I ended up directly patching the font, but the patch itself would
probably be easier to produce with some scripting. I'll definitely
play around with your code.
Thanks Hans!

2016-01-07 13:43 GMT+01:00 Hans Hagen :
...
On 1/6/2016 5:28 PM, Paolo Brasolin wrote:
...
Hello.
I am trying to add some custom ligatures to a font using a feature file.
Everything works just fine, until I try what is called a chained
contextual substitution, i.e. a GSUB lookup of type 6.
I am studying the specs from
http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html#5....
A simple example.fea that does not work for me is:
languagesystem dflt dflt;
lookup FOOBAR {
   sub B by Q ;
} FOOBAR;
feature foba {
   sub [A] B' lookup FOOBAR;
} foba ;
I'd expect it to replace B with Q only when preceeded by A.
The same rule expressed with a lookup of type 8 works just fine:
languagesystem dflt dflt;
feature foba {
   sub [A] B' by Q;
} foba ;
Ultimately I would need to use multiple inline lookups, something like
   sub [A]' B' lookup FOO C' lookup BAR;
Are inline lookups unsupported by the engine?
That would seem strange to me.
I'm probably missing some point, but I'm unable to find more
documentation.
Am I using the wrong syntax?
As we have a way to define additional features in context i played a bit
with contextual lookups. As a result we now have an additional type
chainsubstitution. In due time I will document this better (once it's
settled). I still need to think of a way to kick this into place at a
specific location in the sequence.
Btw, contrary to fea extensions (not supported in the new font loader
anyway) these are runtime extensions i.e. not cached.
I'll upload a beta.
----------------------------------------------------------------------
\startluacode
    fonts.handlers.otf.addfeature {
        name = "stest",
        type = "substitution",
        data = {
            a = "X",
            b = "P",
        }
    }
\stopluacode
\startluacode
    fonts.handlers.otf.addfeature {
        name = "atest",
        type = "alternate",
        data = {
            a = { "X", "Y" },
            b = { "P", "Q" },
        }
    }
\stopluacode
\startluacode
    fonts.handlers.otf.addfeature {
        name = "mtest",
        type = "multiple",
        data = {
            a = { "X", "Y" },
            b = { "P", "Q" },
        }
    }
\stopluacode
\startluacode
    fonts.handlers.otf.addfeature {
        name = "ltest",
        type = "ligature",
        data = {
            ['1'] = { "a", "b" },
            ['2'] = { "d", "a" },
        }
    }
\stopluacode
\startluacode
    fonts.handlers.otf.addfeature {
        name = "ktest",
        type = "kern",
        data = {
            a = { b = -500 },
        }
    }
\stopluacode
\startluacode
    fonts.handlers.otf.addfeature {
        name    = "ytest",
        type    = "chainsubstitution",
     -- flags   = { false, false, false, false },
        lookups = {
            {
                type = "substitution",
                data = {
                    ["b"] = "B",
                    ["c"] = "C",
                },
            },
        },
     -- steps    = {
     --     {
     --         rules = {
     --             {
     --                 before  = { { "a" } },
     --                 current = { { "b", "c" } },
     --                 lookups = { 1 },
     --             },
     --         },
     --     },
     -- },
        data = {
            rules = {
                {
                    before  = { { "a" } },
                    current = { { "b", "c" } },
                    lookups = { 1 },
                },
            },
        },
    }
\stopluacode
\definefontfeature[stest][stest=yes]
\definefontfeature[atest][atest=yes]
\definefontfeature[mtest][mtest=yes]
\definefontfeature[ltest][ltest=yes]
\definefontfeature[ktest][ktest=yes]
\definefontfeature[ytest][ytest=yes]
\starttext
\startTEXpage
    \definedfont[file:dejavu-serif.ttf*default]%
                  abracadabra\par
    {\addff{stest}abracadabra\par}
    {\addff{atest}abracadabra\par}
    {\addff{mtest}abracadabra\par}
    {\addff{ltest}abracadabra\par}
    {\addff{ktest}abracadabra\par}
    \enabletrackers[otf.*]
    {\addff{ytest}abracadabra\par}
    \disabletrackers[otf.*]
\stopTEXpage
\stoptext
-----------------------------------------------------------------
                                          Hans Hagen | PRAGMA ADE
              Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
    tel: 038 477 53 69 | voip: 087 875 68 74 | www.pragma-ade.com
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