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Re: Contemporary music required feature #1: chromatic transposition

From: Joseph Wakeling
Subject: Re: Contemporary music required feature #1: chromatic transposition
Date: Tue, 22 Sep 2009 11:16:04 +0200
User-agent: Thunderbird (X11/20090817)

Hans Aberg wrote:
>> What you see is that
>>   (i) without naturalizeMusic, transposition fails: transposition alone
>>       leaves the final pitch being 'g+5/4' which has no accidental
> I think is just a bug. Somehow the sharp drops out.

It's not so much a bug as a notational impossibility.

Think of pitch as being staff position s plus alteration a.  The latter
is used to determine the accidental.  So in this case we're dealing with
a base pitch of g+1, which is displayed as g-double-sharp.

Now you transpose it up a quarter-tone, so all pitches are altered by
+1/4.  Your pitch is now g+5/4 and there is no accidental for +5/4 so of
course one can't be displayed.

So, if there _is_ a bug, it's that Lilypond doesn't recognise that an
alteration of >1 should change the staff position.

> Here, the most obvious thing for a machine to do, is to merely impose E12 
> enharmonic equivalents in order to minimize the number of accidentals. So the 
> problem is to figure out a rule for the changes.

Yup.  It's a not-entirely-trivial problem.  For example, you want to
preserve things like the notated g-three-quarters-flat which
naturalizeMusic destroys, but you want to avoid ridiculous notations
like E-three-quarters-sharp or C-three-quarters-flat.

But anyway, THAT I think I can do.  The question is, how to incorporate
a well-defined chromatic transposition rule as an option in Lilypond as
opposed to a function à la naturalizeMusic?

Thanks & best wishes,

    -- Joe

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