[LV2] midi out example

Robin Gareus robin at gareus.org
Mon Mar 28 05:39:44 PDT 2016

On 03/28/2016 02:57 AM, Alex Norman wrote:
> Does someone have a simple example plugin that demonstrates how to send midi
> from LV2?

http://lv2plug.in/book/  - The Fifths plugin for example

> I've been looking into this:
> https://github.com/x42/midifilter.lv2/blob/master/midifilter.c
> But honestly I find the 'atom forge', event and atom documentation to be
> relatively confusing.

A less computer-sciencey way to describe it:

"LV2 Atom" is a "Word".

A "LV2 Atom Message" is a "meaningful sentence" (subject + predicate +
object) constructed of words.

To construct a LV2 sentence one uses the "Atom Forge". (The forge can do
this realtime safe and ensure proper formatting)

An "Event" is a "sentence" with a timestamp. e.g.

 1:47 Train leaves for Brooklyn

Last but not least, a "LV2 Event Sequence" is a sequence of Events: e.g.

 1:47 Train leaves to Brooklyn
 2:12 Last train [leaves] to Clarksville

LV2 provides some convenient macros and functions to handle LV2
sequences (both host and client). The two most common are:


For completeness:
LV2-Atoms are URIs which can be cumbersome to handle in C/C++ and long
strings can waste precious resources in realtime code.
So every URI is mapped to an integer. That's what the LV2 URI-MAP does
(the forge abstracts that part.

Just like a real language this can evolve. You could tell a LV2 host
    http://example.org/cool  http://example.org/ey!
but currently most LV2 Hosts will just frown at that message :)


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