XML: Start supporting manual Beam

[uliska]

Export "first" beam.
This works only with regular eighth note beams. Anything else won't work.
Only beam nr 1 is created and ended.
But if more than one beam is involved they have to be encoded all.

That means
a) we have to check the number of beams at [ and ]
b) we have to check at each note if it is beamed, and if so
the current number of beams has to be calculated.
(seems challenging)

[uliska]

To get more specific:

{
  a'8 [ b'16 a' ]
}

will be exported to (excerpt)

    <measure number="1">
      <attributes>
        <divisions>8</divisions>
        <time symbol="common">
          <beats>4</beats>
          <beat-type>4</beat-type>
        </time>
        <clef>
          <sign>G</sign>
          <line>2</line>
        </clef>
      </attributes>
      <note>
        <pitch>
          <step>A</step>
          <octave>4</octave>
        </pitch>
        <duration>4</duration>
        <voice>1</voice>
        <type>eighth</type>
        <beam number="1">begin</beam>
      </note>
      <note>
        <pitch>
          <step>B</step>
          <octave>4</octave>
        </pitch>
        <duration>2</duration>
        <voice>1</voice>
        <type>16th</type>
      </note>
      <note>
        <pitch>
          <step>A</step>
          <octave>4</octave>
        </pitch>
        <duration>2</duration>
        <voice>1</voice>
        <type>16th</type>
        <beam number="1">end</beam>
      </note>
    </measure>

This correctly represents the beginning and end of the beam, and it would work if there is only one quaver beam. But MusicXML requires all intermediate states to be encoded in detail. The correct way to represent the music in XML is

      <note>
        <pitch>
          <step>A</step>
          <octave>4</octave>
        </pitch>
        <duration>4</duration>
        <voice>1</voice>
        <type>eighth</type>
        <beam number="1">begin</beam>
      </note>
      <note>
        <pitch>
          <step>B</step>
          <octave>4</octave>
        </pitch>
        <duration>2</duration>
        <voice>1</voice>
        <type>16th</type>
        <beam number="1">continue</beam>
        <beam number="2">begin</beam>
      </note>
      <note>
        <pitch>
          <step>A</step>
          <octave>4</octave>
        </pitch>
        <duration>2</duration>
        <voice>1</voice>
        <type>16th</type>
        <beam number="1">end</beam>
        <beam number="2">end</beam>
      </note>

In order to achieve this we'd have to keep track of

[uliska]

Tests for correctly handling the incorrect XML (encoding a single beam):

• MuseScore 2
• Verovio (converstion to MEI)
  (it seems MEI's representation is much closer to LilyPond than MusicxML: after Verovio converts the test file to MEI there's a single <beam> element enclosing the notes rather than attaching beam information to each note. The number of beam(let)s doesn't seem to be encoded in MEI, at least by default.)
• Finale (2014.5)
• Sibelius
• Dorico
• Mozart
• SCORE

[uliska]

OK, it seems all major programs properly handle the subpar code exported after this Pull Request. While it is definitely not satisfying not to have a proper translation of the beaming pattern to correct MusicXML representation I think this is definitely an improvement and should be merged (other coding issues notwithstanding ...)

[uliska]

@PeterBjuhr any chance to review these pull requests? I just stumbled over them again ...