path: root/lv2/atom/atom.meta.ttl

@prefix atom: <http://lv2plug.in/ns/ext/atom#> .
@prefix dcs: <http://ontologi.es/doap-changeset#> .
@prefix doap: <http://usefulinc.com/ns/doap#> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix lv2: <http://lv2plug.in/ns/lv2core#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

<http://lv2plug.in/ns/ext/atom>
	a doap:Project ;
	doap:name "LV2 Atom" ;
	doap:shortdesc "A generic value container and several data types." ;
	doap:license <http://opensource.org/licenses/isc> ;
	doap:created "2007-00-00" ;
	doap:developer <http://drobilla.net/drobilla#me> ;
	doap:release [
		doap:revision "2.2" ;
		doap:created "2019-02-03" ;
		doap:file-release <http://lv2plug.in/spec/lv2-1.16.0.tar.bz2> ;
		dcs:blame <http://drobilla.net/drobilla#me> ;
		dcs:changeset [
			dcs:item [
				rdfs:label "Add lv2_atom_object_get_typed() for easy type-safe access to object properties."
			]
		]
	] , [
		doap:revision "2.0" ;
		doap:created "2014-08-08" ;
		doap:file-release <http://lv2plug.in/spec/lv2-1.10.0.tar.bz2> ;
		dcs:blame <http://drobilla.net/drobilla#me> ;
		dcs:changeset [
			dcs:item [
				rdfs:label "Deprecate Blank and Resource in favour of just Object."
			] , [
				rdfs:label "Add lv2_atom_forge_is_object_type() and lv2_atom_forge_is_blank() to ease backwards compatibility."
			] , [
				rdfs:label "Add lv2_atom_forge_key() for terser object writing."
			] , [
				rdfs:label "Add lv2_atom_sequence_clear() and lv2_atom_sequence_append_event() helper functions."
			]
		]
	] , [
		doap:revision "1.8" ;
		doap:created "2014-01-04" ;
		doap:file-release <http://lv2plug.in/spec/lv2-1.8.0.tar.bz2> ;
		dcs:blame <http://drobilla.net/drobilla#me> ;
		dcs:changeset [
			dcs:item [
				rdfs:label "Make lv2_atom_*_is_end() arguments const."
			]
		]
	] , [
		doap:revision "1.6" ;
		doap:created "2013-05-26" ;
		doap:file-release <http://lv2plug.in/spec/lv2-1.6.0.tar.bz2> ;
		dcs:blame <http://drobilla.net/drobilla#me> ;
		dcs:changeset [
			dcs:item [
				rdfs:label "Fix crash in forge.h when pushing atoms to a full buffer."
			]
		]
	] , [
		doap:revision "1.4" ;
		doap:created "2013-01-27" ;
		doap:file-release <http://lv2plug.in/spec/lv2-1.4.0.tar.bz2> ;
		dcs:blame <http://drobilla.net/drobilla#me> ;
		dcs:changeset [
			dcs:item [
				rdfs:label "Fix lv2_atom_sequence_end()."
			] , [
				rdfs:label "Remove atom:stringType in favour of owl:onDatatype so generic tools can understand and validate atom literals."
			] , [
				rdfs:label "Improve atom documentation."
			]
		]
	] , [
		doap:revision "1.2" ;
		doap:created "2012-10-14" ;
		doap:file-release <http://lv2plug.in/spec/lv2-1.2.0.tar.bz2> ;
		dcs:blame <http://drobilla.net/drobilla#me> ;
		dcs:changeset [
			dcs:item [
				rdfs:label "Fix implicit conversions in forge.h that are invalid in C++11."
			] , [
				rdfs:label "Fix lv2_atom_object_next() on 32-bit platforms."
			] , [
				rdfs:label "Add lv2_atom_object_body_get()."
			] , [
				rdfs:label "Fix outdated documentation in forge.h."
			] , [
				rdfs:label "Use consistent label style."
			] , [
				rdfs:label "Add LV2_ATOM_CONTENTS_CONST and LV2_ATOM_BODY_CONST."
			]
		]
	] , [
		doap:revision "1.0" ;
		doap:created "2012-04-17" ;
		doap:file-release <http://lv2plug.in/spec/lv2-1.0.0.tar.bz2> ;
		dcs:blame <http://drobilla.net/drobilla#me> ;
		dcs:changeset [
			dcs:item [
				rdfs:label "Initial release."
			]
		]
	] ;
	lv2:documentation """

An atom:Atom is a simple generic data container for holding any type of Plain
Old Data (POD).  An Atom can contain simple primitive types like integers,
floating point numbers, and strings; as well as structured data like lists and
dictionary-like <q>Objects</q>.  Since Atoms are POD, they can be easily copied
(for example, with `memcpy()`) anywhere and are suitable for use in real-time
code.

Every atom starts with an LV2_Atom header, followed by the contents.  This
allows code to process atoms without requiring special code for every type of
data.  For example, plugins that mutually understand a type can be used
together in a host that does not understand that type, because the host is only
required to copy atoms, not interpret their contents.  Similarly, plugins (such
as routers, delays, or data structures) can meaningfully process atoms of a
type unknown to them.

Atoms should be used anywhere values of various types must be stored or
transmitted.  An atom:AtomPort can be used to transmit atoms via ports.  An
atom:AtomPort that contains a atom:Sequence can be used for sample accurate
communication of events, such as MIDI.

### Serialisation

Each Atom type defines a binary format for use at runtime, but also a
serialisation that is natural to express in Turtle format.  Thus, this
specification defines a powerful real-time appropriate data model, as well as a
portable way to serialise any data in that model.  This is particularly useful
for inter-process communication, saving/restoring state, and describing values
in plugin data files.

### Custom Atom Types

While it is possible to define new Atom types for any binary format, the
standard types defined here are powerful enough to describe almost anything.
Implementations SHOULD build structures out of the types provided here, rather
than define new binary formats (for example, using atom:Object rather than a
new C `struct` type).  Host and tool implementations have support for
serialising all standard types, so new binary formats are an implementation
burden which harms interoperabilty.  In particular, plugins SHOULD NOT expect
UI communication or state saving with custom binary types to work.  In general,
new Atom types should only be defined where absolutely necessary due to
performance reasons and serialisation is not a concern.

"""^^lv2:Markdown .

atom:Atom
	lv2:documentation """

An LV2_Atom has a 32-bit `size` and `type`, followed by a body of `size` bytes.
Atoms MUST be 64-bit aligned.

All concrete Atom types (subclasses of this class) MUST define a precise binary
layout for their body.

The `type` field is the URI of an Atom type mapped to an integer.
Implementations SHOULD gracefully pass through, or ignore, atoms with unknown
types.

All atoms are POD by definition except references, which as a special case have
`type` 0.  An Atom MUST NOT contain a Reference.  It is safe to copy any
non-reference Atom with a simple `memcpy`, even if the implementation does not
understand `type`.  Though this extension reserves the type 0 for references,
the details of reference handling are currently unspecified.  A future revision
of this extension, or a different extension, may define how to use non-POD data
and references.  Implementations MUST NOT send references to another
implementation unless the receiver is explicitly known to support references
(e.g. by supporting a feature).

The special case of a null atom with both `type` and `size` 0 is not considered
a reference.

"""^^lv2:Markdown .

atom:Chunk
	lv2:documentation """

This type is used to indicate a certain amount of space is available.  For
example, output ports with a variably sized type are connected to a Chunk so
the plugin knows the size of the buffer available for writing.

The use of a Chunk should be constrained to a local scope, since
interpreting it is impossible without context.  However, if serialised to RDF,
a Chunk may be represented directly as an xsd:base64Binary string, for example:

    :::turtle
    [] eg:someChunk "vu/erQ=="^^xsd:base64Binary .

"""^^lv2:Markdown .

atom:String
	lv2:documentation """

The body of an LV2_Atom_String is a C string in UTF-8 encoding, i.e. an array
of bytes (`uint8_t`) terminated with a NULL byte (`'\\0'`).

This type is for free-form strings, but SHOULD NOT be used for typed data or
text in any language.  Use atom:Literal unless translating the string does not
make sense and the string has no meaningful datatype.

"""^^lv2:Markdown .

atom:Literal
	lv2:documentation """

This type is compatible with rdfs:Literal and is capable of expressing a
string in any language or a value of any type.  A Literal has a
`datatype` and `lang` followed by string data in UTF-8
encoding.  The length of the string data in bytes is `size -
sizeof(LV2_Atom_Literal)`, including the terminating NULL character.  The
`lang` field SHOULD be a URI of the form
&lt;http://lexvo.org/id/iso639-3/LANG&gt; or
&lt;http://lexvo.org/id/iso639-1/LANG&gt; where LANG is a 3-character ISO 693-3
language code, or a 2-character ISO 693-1 language code, respectively.

A Literal may have a `datatype` or a `lang`, but never both.

For example, a Literal can be <q>Hello</q> in English:

    :::c
    void set_to_hello_in_english(LV2_Atom_Literal* lit) {
         lit->atom.type     = map(expand("atom:Literal"));
         lit->atom.size     = 14;
         lit->body.datatype = 0;
         lit->body.lang     = map("http://lexvo.org/id/iso639-1/en");
         memcpy(LV2_ATOM_CONTENTS(LV2_Atom_Literal, lit),
                "Hello",
                sizeof("Hello"));  // Assumes enough space
    }

or a Turtle string:

    :::c
    void set_to_turtle_string(LV2_Atom_Literal* lit, const char* ttl) {
         lit->atom.type     = map(expand("atom:Literal"));
         lit->atom.size     = 64;
         lit->body.datatype = map("http://www.w3.org/2008/turtle#turtle");
         lit->body.lang     = 0;
         memcpy(LV2_ATOM_CONTENTS(LV2_Atom_Literal, lit),
                ttl,
                strlen(ttl) + 1);  // Assumes enough space
    }

"""^^lv2:Markdown .

atom:Path
	lv2:documentation """

A Path is a URI reference with only a path component: no scheme, authority,
query, or fragment.  In particular, paths to files in the same bundle may be
cleanly written in Turtle files as a relative URI.  However, implementations
may assume any binary Path (e.g. in an event payload) is a valid file path
which can passed to system functions like fopen() directly, without any
character encoding or escape expansion required.

Any implemenation that creates a Path atom to transmit to another is
responsible for ensuring it is valid.  A Path SHOULD always be absolute, unless
there is some mechanism in place that defines a base path.  Since this is not
the case for plugin instances, effectively any Path sent to or received from a
plugin instance MUST be absolute.

"""^^lv2:Markdown .

atom:URI
	lv2:documentation """

This is useful when a URI is needed but mapping is inappropriate, for example
with temporary or relative URIs.  Since the ability to distinguish URIs from
plain strings is often necessary, URIs MUST NOT be transmitted as atom:String.

This is not strictly a URI, since UTF-8 is allowed.  Escaping and related
issues are the host's responsibility.

"""^^lv2:Markdown .

atom:URID
	lv2:documentation """

A URID is typically generated with the LV2_URID_Map provided by the host .

"""^^lv2:Markdown .

atom:Vector
	lv2:documentation """

A homogeneous series of atom bodies with equivalent type and size.

An LV2_Atom_Vector is a 32-bit `child_size` and `child_type` followed by `size
/ child_size` atom bodies.

For example, an atom:Vector containing 42 elements of type atom:Float:

    :::c
    struct VectorOf42Floats {
        uint32_t size;        // sizeof(LV2_Atom_Vector_Body) + (42 * sizeof(float);
        uint32_t type;        // map(expand("atom:Vector"))
        uint32_t child_size;  // sizeof(float)
        uint32_t child_type;  // map(expand("atom:Float"))
        float    elems[42];
    };

Note that it is possible to construct a valid Atom for each element of the
vector, even by an implementation which does not understand `child_type`.

If serialised to RDF, a Vector SHOULD have the form:

    :::turtle
    eg:someVector
         a atom:Vector ;
         atom:childType atom:Int ;
         rdf:value (
             "1"^^xsd:int
             "2"^^xsd:int
             "3"^^xsd:int
             "4"^^xsd:int
         ) .

"""^^lv2:Markdown .

atom:Tuple
	lv2:documentation """

The body of a Tuple is simply a series of complete atoms, each aligned to
64 bits.

If serialised to RDF, a Tuple SHOULD have the form:

    :::turtle
    eg:someVector
         a atom:Tuple ;
         rdf:value (
             "1"^^xsd:int
             "3.5"^^xsd:float
             "etc"
         ) .

"""^^lv2:Markdown .

atom:Property
	lv2:documentation """

An LV2_Atom_Property has a URID `key` and `context`, and an Atom `value`.  This
corresponds to an RDF Property, where the <q>key</q> is the <q>predicate</q>
and the <q>value</q> is the object.

The `context` field can be used to specify a different context for each
property, where this is useful.  Otherwise, it may be 0.

Properties generally only exist as part of an atom:Object.  Accordingly,
they will typically be represented directly as properties in RDF (see
atom:Object).  If this is not possible, they may be expressed as partial
reified statements, for example:

    :::turtle
    eg:someProperty
        rdf:predicate eg:theKey ;
        rdf:object eg:theValue .

"""^^lv2:Markdown .

atom:Object
	lv2:documentation """

An <q>Object</q> is an atom with a set of properties.  This corresponds to an
RDF Resource, and can be thought of as a dictionary with URID keys.

An LV2_Atom_Object body has a uint32_t `id` and `type`, followed by a series of
atom:Property bodies (LV2_Atom_Property_Body).  The LV2_Atom_Object_Body::otype
field is equivalent to a property with key rdf:type, but is included in the
structure to allow for fast dispatching.

Code SHOULD check for objects using lv2_atom_forge_is_object() or
lv2_atom_forge_is_blank() if a forge is available, rather than checking the
atom type directly.  This will correctly handle the deprecated atom:Resource
and atom:Blank types.

When serialised to RDF, an Object is represented as a resource, for example:

    :::turtle
    eg:someObject
        eg:firstPropertyKey "first property value" ;
        eg:secondPropertyKey "first loser" ;
        eg:andSoOn "and so on" .

"""^^lv2:Markdown .

atom:Resource
	lv2:documentation """

This class is deprecated.  Use atom:Object directly instead.

An atom:Object where the <code>id</code> field is a URID, that is, an Object
with a URI.

"""^^lv2:Markdown .

atom:Blank
	lv2:documentation """

This class is deprecated.  Use atom:Object with ID 0 instead.

An atom:Object where the LV2_Atom_Object::id is a blank node ID (NOT a URI).
The ID of a Blank is valid only within the context the Blank appears in.  For
ports this is the context of the associated run() call, i.e. all ports share
the same context so outputs can contain IDs that correspond to IDs of blanks in
the input.

"""^^lv2:Markdown .

atom:Sound
	lv2:documentation """

The format of a atom:Sound is the same as the buffer format for lv2:AudioPort
(except the size may be arbitrary).  An atom:Sound inherently depends on the
sample rate, which is assumed to be known from context.  Because of this,
directly serialising an atom:Sound is probably a bad idea, use a standard
format like WAV instead.

"""^^lv2:Markdown .

atom:Event
	lv2:documentation """

An Event is typically an element of an atom:Sequence.  Note that this is not an Atom type since it begins with a timestamp, not an atom header.

"""^^lv2:Markdown .

atom:Sequence
	lv2:documentation """

A flat sequence of atom:Event, that is, a series of time-stamped Atoms.

LV2_Atom_Sequence_Body.unit describes the time unit for the contained atoms.
If the unit is known from context (e.g. run() stamps are always audio frames),
this field may be zero.  Otherwise, it SHOULD be either units:frame or
units:beat, in which case ev.time.frames or ev.time.beats is valid,
respectively.

If serialised to RDF, a Sequence has a similar form to atom:Vector, but for
brevity the elements may be assumed to be atom:Event, for example:

    :::turtle
    eg:someSequence
        a atom:Sequence ;
        rdf:value (
            [
                atom:frameTime 1 ;
                rdf:value "901A01"^^midi:MidiEvent
            ] [
                atom:frameTime 3 ;
                rdf:value "902B02"^^midi:MidiEvent
            ]
        ) .

"""^^lv2:Markdown .

atom:AtomPort
	lv2:documentation """

Ports of this type are connected to an LV2_Atom with a type specified by
atom:bufferType.

Output ports with a variably sized type MUST be initialised by the host before
every run() to an atom:Chunk with size set to the available space.  The plugin
reads this size to know how much space is available for writing.  In all cases,
the plugin MUST write a complete atom (including header) to outputs.  However,
to be robust, hosts SHOULD initialise output ports to a safe sentinel (e.g. the
null Atom) before calling run().

"""^^lv2:Markdown .

atom:bufferType
	lv2:documentation """

Indicates that an AtomPort may be connected to a certain Atom type.  A port MAY
support several buffer types.  The host MUST NOT connect a port to an Atom with
a type not explicitly listed with this property.  The value of this property
MUST be a sub-class of atom:Atom.  For example, an input port that is connected
directly to an LV2_Atom_Double value is described like so:

    :::turtle
    &lt;plugin&gt;
        lv2:port [
            a lv2:InputPort , atom:AtomPort ;
            atom:bufferType atom:Double ;
        ] .

This property only describes the types a port may be directly connected to.  It
says nothing about the expected contents of containers.  For that, use
atom:supports.

"""^^lv2:Markdown .

atom:supports
	lv2:documentation """

This property is defined loosely, it may be used to indicate that anything
<q>supports</q> an Atom type, wherever that may be useful.  It applies
<q>recursively</q> where collections are involved.

In particular, this property can be used to describe which event types are
expected by a port.  For example, a port that receives MIDI events is described
like so:

    :::turtle
    &lt;plugin&gt;
        lv2:port [
            a lv2:InputPort , atom:AtomPort ;
            atom:bufferType atom:Sequence ;
            atom:supports midi:MidiEvent ;
        ] .

"""^^lv2:Markdown .

atom:eventTransfer
	lv2:documentation """

Transfer of individual events in a port buffer.  Useful as the `format` for a
LV2UI_Write_Function.

This protocol applies to ports which contain events, usually in an
atom:Sequence.  The host must transfer each individual event to the recipient.
The format of the received data is an LV2_Atom, there is no timestamp header.

"""^^lv2:Markdown .

atom:atomTransfer
	lv2:documentation """

Transfer of the complete atom in a port buffer.  Useful as the `format` for a
LV2UI_Write_Function.

This protocol applies to atom ports.  The host must transfer the complete atom
contained in the port, including header.

"""^^lv2:Markdown .