DirectWrite 连字替换功能

Question

I am trying to get the proper glyphs for a given string with DirectWrite. I am trying to enable the OpenType features for testing, in this case font ligature substitutions. I am using a font that contains the OpenType 'liga' tag has a separate glyph for ll.

Here is my code:

        text = 'Hello'

        analyze = IDWriteTextAnalyzer()
        self.font.write_factory.CreateTextAnalyzer(byref(analyze))

        tags = [DWRITE_FONT_FEATURE_TAG_STANDARD_LIGATURES]

        num_feature = len(tags)
        if num_feature:
            typo_features = DWRITE_TYPOGRAPHIC_FEATURES()
            typo_features.featureCount = num_feature

            typo_features.features = (DWRITE_FONT_FEATURE * num_feature)()
            for i in range(num_feature):
                typo_features.features[i] = DWRITE_FONT_FEATURE(tags[i], 1)

            feature_range = (UINT32 * num_feature)(len(text))
        else:
            typo_features = None
            feature_range = None

        max_count = int(3 * len(text) / 2 + 16)

        length = len(text)
        clusters = (UINT16 * length)()
        text_props = (DWRITE_SHAPING_TEXT_PROPERTIES * length)()
        indices = (UINT16 * max_count)()
        glyph_props = (DWRITE_SHAPING_GLYPH_PROPERTIES * max_count)()
        actual_count = UINT32()

        analyze.GetGlyphs(text,
                          len(text),
                          self.font.font_face,
                          False,  # sideways
                          False,  # righttoleft
                          script,  # scriptAnalysis
                          None,  # localName
                          None,  # numberSub
                          typo_features,  # typo features
                          feature_range,  # feature range
                          1,  # count
                          max_count,
                          clusters, # cluster map
                          text_props, # text props
                          indices, # glyph indices
                          glyph_props, #glyph pops
                          byref(actual_count)#glyph count
                          )

However, upon checking the return values it has 5 glyphs total, and the ligature indice is not shown in the glyph indices, just the original two l's: [43, 72, 79, 79, 82, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

I am not sure if I am understanding it correctly. Am I incorrect in thinking it would substitute the two l's in the string for the ll glyph? Or is there another process where this indice becomes the true ligature? Any input is helpful as I am new to this. Thanks

Answer 1

There is a number of things to check:

• how 'liga' is defined by the font, which script/language pair is expected for it, what kind of lookup is used;
• try simple test program using IDirectWriteTextLayout with same font to see if you get this ligature by default. If you do, it means 'liga' is enabled by default and you don't need to specify it as a user feature;
• check you test code in more conventional environment first, like C/C++.