Re: Misleading autobeaming in 3/4

[David Kastrup]

Carl Sorensen <address@hidden> writes:

> On 8/13/11 2:55 PM, "Bertrand Bordage" <address@hidden> wrote:
>
>> Hi everyone!
>> 
>> (I don't know if we can call this a defect, so I don't send it to 
>> bug-lilypond
>> for the moment)
>> 
>> I am working on a 3/4 score with this rhythmical pattern in the beginning :
>> "4. 8 8 8".
>> It is repeated five times, but the rest of the score is clearly in 3/4 and 
>> not
>> 6/8, contrary to what this pattern suggests.
>> 
>> It is therefore important to have a beaming that suggests 3/4, even if it's
>> already written at the beginning.
>> But here's what happens:
>> 
>> %%%
>> \markup "This:"
>> \relative c'' { \time 3/4 a4. b8 c d e d c b4. a8 b c d c b a2. }
>> \markup "looks like 6/8:"
>> \relative c'' { \time 6/8 a4. b8 c d e d c b4. a8 b c d c b a2. }
>> \markup "but should be like this:"
>> \relative c'' { \time 3/4 a4. b8\noBeam c d e d c\noBeam b4. a8 b c d c b a2.
>> }
>> %%%
>
> IMO this *is* a bug for default beaming.  The engraving sources (Stone,
> Read, Gould) are pretty united in saying that you should not beam 3/4 to
> look like 6/8.  
>
> I'm not sure what's going on with the Chopin and Brahms beaming that looks
> like LilyPond's current default.
>
> Please report to bug-lilypond.

The problem here is that I don't see us having the data structures
needed for getting proper beaming.  Where "I don't see" means "I don't
understand automatic beaming rules and exceptions enough in order to say
for sure".

Eighth notes in 3/4 measures are usually beamed in groups of 6.
However, if you don't get a complete group of 6 together (where a
subdivided beam is ok), the pattern falls apart into 2+2+2.

So the defaults should look like [8 8 8 8 8 8] and even [8. 16 8 8 8 8],
but 4. [8 8 8] is wrong and needs to be 4. 8 [8 8] instead.

6/8 would have [8 8 8] [8 8 8], [8. 16 8] [8 8 8] and 4. [8 8 8],
respectively.

-- 
David Kastrup