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Re: Stream event structure

From: Ralph Little
Subject: Re: Stream event structure
Date: Thu, 06 Sep 2007 13:58:09 -0700
User-agent: Thunderbird (X11/20070824)

Erik Sandberg wrote:
According to the thesis, the stream event contains a "context" field
which contains the id of the outlet context for the event. I can't seem
to find this in the event structure at all and I can't think of a good
reason for it to be there anyway for the implementation as it stands.

I'm playing with exporting streams and being able to identify a context
id for the Voice would be really useful to me. Does anyone know how
reproduce the listing that Erik has in his thesis including the outlet
context id?

Any help would be really cool.

There is no official external representation format for music streams; in the internal representation the context information is represented by appearance (if the event appears in voice V, then it belongs to voice V). A music stream dumper will typically consist of one listener per context, which exports the events to some textual format. This listener will typically associate each context object with some unique human-readable ID, and tag each heard event with this ID (when exporting you could e.g. use a context -> id hash table, and when importing a stream, use a hash table for id -> context).

Note that you will need to listen in each context for the creation of new child contexts, so that you recursively can add listeners to them as well.

FWIW, I've attached the code I used for import + export in my thesis (beware that they are written for an old lily version, and that they are not cleaned up, and that context ids were hardcoded in C++ code back then).

In that particular version of lily (2.6), I could override the translation procedure by setting the global function translator-proc; this is the entry point of my dumper/undumper. I don't know if this still is the way to go.

IIRC, I have written some similar code for the official implementation of streams, but it was lost when my laptop was stolen. If you're lucky, I might have published some code on lilypond-devel (but I don't remember).

Hi Erik,
Many thanks for this - your comments below confirm what I suspected about ids - the context that gets the event is whatever happens to be the bottom listening context at the time.

I will look through what you have sent and see if I can get something working.

I'm really trying to get this Braille export thing off the ground and it seemed like a good place to start. I know others are looking at Midi also so perhaps I will contact them and collaborate as the framework is likely to be very similar.


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