[Lilypond-auto] Issue 3698 in lilypond: Enhancement: Allow capacity audio (MIDI) performers to interpret \repeat \alternative blocks intelligently

[lilypond]

Status: Accepted
Owner: ----
CC: address@hidden,  address@hidden,  address@hidden,   
address@hidden,  address@hidden
Labels: Type-Enhancement

New issue 3698 by address@hidden: Enhancement: Allow capacity audio  
(MIDI) performers to interpret \repeat \alternative blocks intelligently
http://code.google.com/p/lilypond/issues/detail?id=3698

This issue is a follow-on from Tracker Issue 3664  
http://code.google.com/p/lilypond/issues/detail?id=3664 and Rietveld Issue  
https://codereview.appspot.com/26470047.  Issue 3664 dealt with better  
playback for MIDI articulations etc. in MIDI output.

There was a lot of discussion during review of further MIDI enhancements  
regarding \repeat blocks.  This issue has been raised to capture that  
discussion and what conclusions were reached.

LilyPond audio/MIDI output currently only handles \repeat unfold blocks and  
plays back all \alternative blocks as if they were sequential bars.  In the  
past it was felt that since LilyPond's main purpose was to produce  
graphical notation output this was sufficient for the main purposes  
of 'proof-hearing' a score.  However, there is now interest in enhancing  
LP's to provide better interpretation of \repeat blocks.

Issue 3664 replaced much, but not all, of the functionality in  
articulate.ly.  This issue covers only functionality relating to \repeat  
blocks.  Other work to replace articulate.ly features should be covered by  
a separate issue.

How, and to what level, audio/MIDI output (I'll use MIDI from now on) will  
be determined by a property in a <tbd> context named, for example  
repeat-performance.  The default setting of the property would be one which  
specified the current behaviour for midi playback.

The suggested syntax for the property is a list of symbol values, to  
represent the various types of repeat
these could be
'volta      -  expand volta repeats
'volta-once -  don't expand voltas, but always use the last \alternative
                (if one is present)
'tremolo    -  expand tremolos to unfolded values
'unfold     -  always expanded
'percent    -  always expanded
'all        -  interpret all forms of repeat fully (logical OR  
of 'volta, 'tremolo, 'percent and 'unfold)
'minimal ( or #f or '() ?)
            - only observe unfold repeats as now and treat \alternative  
block bars sequentially as  (this is the default value)

Issue - how should \alternative be handled?
Use keywords as above in the main list, or
use a double list for the property
e.g. (('unfold 'percent 'volta-once) 'last-time)
where the second element of the list could be a specifier for handling  
\alternative - with values of
'last-time   - always perform the last \alternative
'select      - use the appropriate \alternative block corresponding to the
               volta iteration
'sequence    - as now, just hack through all the bars in all \alternative
               blocks in sequence.
It's possible the double list of (repeat-settings alternative-settings)  
could be derived from a single list specified in the API and this double  
list is the one the performers use to determine the behaviour.

Hope I didn't miss anything from the discussion.
Cheers, Ian

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