Monday, June 11, 2012

Writing Lv2 plugins : An Lv2 Overview


Although I'm relatively aware of what Lv2 is, and also relatively aware of how it works, and relatively know how to write a "host" class: what's inside an Lv2 plugin???

Starters: I'm assuming you know a bit of C/C++, you know what a plugin is, and you're well able to use a terminal. I'm not assuming you know much about plugin specs, Lv2 URI's, manifest files, or Turtle RDF. These things will become apparent as we go along.

For this tutorial, I'm going to talk us trough the "eg-amp" code, that comes with the lv2 source code. If you don't have it yet, grab it now. Extract, and /plugins/eg-amp.lv2/ shows you the sources.


Lv2 works by defining URI's. A URI means *nothing*. It is a string, that contains a unique identifyer for *stuff*. Usually it is in the form of a http:// address, and that web link will show you more info on the plugin. Don't fret about details, just remember this:

Lv2 plugins come in "bundles". A bundle contains one or more plugins. For simplicity, this example contains only one plugin. A bundle is just a directory, with three files inside:
  1. manifest.ttl
  2. <pluginName>.ttl
  3. <pluginName>.so
The other files (waf and wscript) are only to compile the source code into the binary plugin, and install them. Of course any other build system can be used to compile Lv2 plugins too!

Walk trough of the files:


This file lists all plugins that can be found in the bundle. For this example, there's only one. The file has various different bits of information about the bundle its contained in, and this information defines the plugin.

The eg-amp manifest contains this:
@prefix lv2:  <> .
@prefix rdfs: <> .

    a lv2:Plugin ;
    lv2:binary <>  ;
    rdfs:seeAlso <amp.ttl> .

First of all two "convenience" prefix' are defined. The core Lv2 URI, and the RDF scheme URI.

Next we declare a URI. It is the unique string, that when used always means exactly this object. Afterwards, some details about the "resource" are added:
The resource <> has the property lv2:Plugin, the property lv2:binary and for more info look at <amp.ttl>.


This file contains all the details about the plugin: think "Who what when how where"?. I won't paste the code here,the 85 lines would clog up this post. I'll talk you through it though, dont' worry!

First section after the license (lines 17-21):
One can declare some RDF data to show the "author" of a plugin. This can point to your own website, or else a company website.

The next section (lines 29-87) contain the implementation details of the plugin: things like the Maintainer of the plugin, the name of the plugin, the license, the plugins "features", and what "ports" it exposes.


Finally: the binary file, that a host will load, and use to do whatever the plugin does. Not much more to say really!

Lv2 Extensions

An extension is a definition of some extra functionality, that isn't contained in the core lv2 spec. These definitions have their own unique URI, and each has its own .ttl and C header files.

Since Lv2 is a "modular" or "extensible" format, anybody can write a Lv2 Feature, and then write plugins or hosts to use that feature. Note that not all hosts support every feature, so using "rare" features may mean you plugin can't be used in a host somebody else wrote.

Lv2 Ports

(If you are familiar with LADSPA ports, skip this section: they're pretty similar)
Ports are a way to transfer data between the plugin and the host. There are various types of ports, some built into the "lv2-core" spec (always available in every host) and some that are "feature" ports, (ie contained in a feature extension to lv2, and perhaps not every host supports it).

Most ports send float values around, like in the eg-amp:
lv2:port [
        a lv2:InputPort ,
        lv2:ControlPort ;
        lv2:index 0 ;
        lv2:symbol "gain" ;
        lv2:name "Gain";
        lv2:default 0.0 ;
        lv2:minimum -90.0 ;
        lv2:maximum 24.0 ;

The above RDF snipped defines a port, its an input port (to the plugin), its a control rate port (not audio), its index is 0, and it changes the "Gain" parameter of the plugin. Pretty readable right? I'll leave the rest to you! :D

Compiling eg-amp

So to get the eg-amp lv2 bundle compiled and running you've a couple of steps to take:
./waf configure           will setup Waf so it knows how to build
./waf                            will compile & link the files
./waf install                moves the .lv2 bundle to /usr/local/lib/lv2/ (by default)

Note that doing a
./waf configure --strict --debug --prefix=/usr
will enable strict compiler checks, a debug build, and install in /usr/lib/lv2/

Test the plugin using any Lv2 host, Ardour, QTractor, Jalv, lv2rack, zynjacku : they should all work, assuming they're up to date!


That's most of the main topics of Lv2 covered: you now have a decent understanding of what is necessary to write Lv2 plugins. In a future post I hope to cover the details of eg-amp.lv2, and the C code it consits of. Also intended is to post about how to implement extra features; both for hosts and plugins. But that'll remain for another day.

PS: Comments and changes welcomed: I'm new to writing Lv2 plugins myself, and I'm sure that improvements can be made on this post!