# LV2 Atom Extension # Copyright 2007-2012 David Robillard # # Permission to use, copy, modify, and/or distribute this software for any # purpose with or without fee is hereby granted, provided that the above # copyright notice and this permission notice appear in all copies. # # THIS SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES # WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF # MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR # ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES # WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN # ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF # OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. @prefix atom: . @prefix doap: . @prefix foaf: . @prefix lv2: . @prefix owl: . @prefix rdf: . @prefix rdfs: . @prefix xsd: . a lv2:Specification ; doap:name "LV2 Atom" ; doap:shortdesc "A generic value container and several data types." ; doap:license ; rdfs:seeAlso ; doap:release [ doap:revision "0.3" ; doap:created "2012-01-28" ] ; doap:maintainer [ a foaf:Person ; foaf:name "David Robillard" ; foaf:homepage ; rdfs:seeAlso ] ; lv2:documentation """

This extension defines a generic format for a typed piece of data, called an lv2:Atom (e.g. integers, strings, buffers, data structures, etc). Atoms allow LV2 plugins and hosts to communicate, process, serialise, and store values of any type via a generic mechanism (e.g. LV2 ports, events, disk, shared memory, network). Atoms are, with one exception, Plain Old Data (POD) and may be safely copied (e.g. with a simple call to memcpy).

Since Atom communication can be implemented generically, plugins that understand some type can be used together in a host that does not understand that type, and plugins (e.g. routers, delays) can process atoms of unknown type.

An Atom can be trivially constructed in-place from an Event as defined by the LV2 Event extension. In other words, an Event is simply an Atom with a time stamp header. Atoms SHOULD be used anywhere a "value" needs to be stored or communicated, to allow implementations to be polymorphic and extensible.

Atoms (the start of the LV2_Atom header) MUST be 32-bit aligned.

Atoms can be communicated in many ways. Since an Atom is the payload of an Event, an EventPort can be used for communicating Atoms in realtime with sub-sample time stamp accuracy. This extension also defines two port types for connecting directly to a single Atom: atom:ValuePort and atom:MessagePort, which both have the same buffer format but different semantics (with respect to how the run() callback interprets the Atom).

Implementing this extension requires a facility for mapping URIs to integers, such as the LV2 URID extension.

""" . atom:Atom a rdfs:Class ; rdfs:label "Atom" ; atom:cType "LV2_Atom" ; lv2:documentation """

Abstract base class for all atoms. An LV2_Atom has a 32-bit type and size followed by a body of size bytes.

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

The type field is the URI of a subclass of Atom mapped to an integer using the URI Map extension's LV2_URI_Map_Feature::uri_to_id() with map = "http://lv2plug.in/ns/ext/event". If a plugin or host does not understand type, that atom SHOULD be gracefully ignored (or copied if it does not have type 0).

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, actual specification of how references are used is left to another extension.

""" . atom:String a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "String" ; atom:cType "LV2_Atom_String" ; lv2:documentation """

A UTF-8 encoded string.

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 can be used for free-form strings, but in most cases it is better to use atom:Literal since this supports a language tag or datatype. Implementations SHOULD NOT use atom:String unless translating the string does not make sense and the string has no meaningful datatype.

""" . atom:Literal a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "String Literal" ; atom:cType "LV2_Atom_Literal" ; lv2:documentation """

A UTF-8 encoded string literal, with an optional datatype or language.

This type is compatible with rdf: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 <http://lexvo.org/id/term/LANG> where LANG is an ISO 693-2 or ISO 693-3 language code.

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

For example, a Literal can be "Hello" in English:

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

or a Turtle string:

void set_to_turtle_string(LV2_Atom_Literal* lit, const char* ttl) {
     lit->type     = map(expand("atom:Literal"));
     lit->size     = 64;
     lit->datatype = map("http://www.w3.org/2008/turtle#turtle");
     lit->lang     = 0;
     memcpy(lit->str, ttl, strlen(ttl) + 1);  // Assumes enough space
}
""" . atom:URID a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "Integer ID mapped from a URI" ; atom:cType "LV2_Atom_ID" ; lv2:documentation """

An unsigned 32-bit integer mapped from a URI using the URI Map extension's LV2_URI_Map_Feature::uri_to_id() with map = NULL.

""" . atom:BlankID a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "Integer ID for a blank node" ; atom:cType "LV2_Atom_ID" ; lv2:documentation """

An unsigned 32-bit integer identifier for a blank node. A BlankID is only meaningful within a limited scope (e.g. the Atom in which it appears), and MUST NOT be used as a global identifier. In particular, a BlankID is NOT a URID, and can not be mapped to/from a URI.

""" . atom:Vector a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "Vector" ; atom:cType "LV2_Atom_Vector" ; lv2:documentation """

A homogeneous sequence of atoms with equivalent type and size.

An LV2_Atom_Vector is a 32-bit elem_count and elem_type followed by elem_count atom bodies of type elem_type. The element type must be a fixed size atom:Atom type, i.e. the size of each element is the vector's size / elem_count.

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

struct VectorOf42Floats {
    uint32_t type;        // map(expand("atom:Vector"))
    uint32_t size;        // sizeof(LV2_Atom_Vector) + (42 * sizeof(float);
    uint32_t elem_count;  // 42
    uint32_t elem_type;   // map(expand("atom:Float"))
    float    elems[32];
};

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

""" . atom:Tuple a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "Tuple" ; lv2:documentation """

A sequence of lv2:Atom with varying type and size.

The body of a Tuple is simply a sequence of complete atoms, each aligned to 32 bits.

""" . atom:Thing a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "Thing" ; atom:cType "LV2_Thing" ; lv2:documentation """

Abstract base class for a "Thing", i.e. an atom:Atom with a number of properties. An LV2_Object is an unsigned 32-bit integer context and id followed by a sequence of LV2_Atom_Property .

The context is mapped using the URI Map extension's LV2_URI_Map_Feature::uri_to_id() with map = NULL, and may be 0 (the default context).

Note this is an abstract class, i.e. no Atom can exist with type = uri_to_id(atom:Thing). An Object is either an atom:Resource or an atom:Blank, but the body always has the same binary format, LV2_Object. Thus, both named and anonymous objects can be handled with common code using only a 64-bit header for both.

""" . atom:Resource a rdfs:Class ; rdfs:subClassOf atom:Thing ; atom:cType "LV2_Thing" ; lv2:documentation """

An atom:Thing where id is the URI of the resource mapped to an atom:URID.

""" . atom:Blank a rdfs:Class ; rdfs:subClassOf atom:Thing ; atom:cType "LV2_Thing" ; lv2:documentation """

An atom:Thing where id is the blank node ID of the object, which is only meaningful within a certain limited scope (e.g. the container of the Blank) and MUST NOT be used as a global ID. In particular, id is NOT a URID.

""" . atom:Message a rdfs:Class ; rdfs:subClassOf atom:Thing ; atom:cType "LV2_Thing" ; lv2:documentation """

A atom:Thing where id is a message type ID. Conceptually, a Message is identical to a Blank, but is a distinct type with a single type field to allow simple and fast dispatch by handling code.

A Message may be serialised as a Blank by adding an rdf:type property with the value id unmapped to a URI.

""" . atom:Event a rdfs:Class ; rdfs:label "Event" ; atom:cType "LV2_Atom_Event" ; lv2:documentation """

An atom with a time stamp header prepended, typically for sample accurate transmission via LV2 ports. See struct LV2_Atom_Event.

""" . atom:Bang a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "Bang (activity) (size = 0)" . atom:Number a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "Number (abstract class)." . atom:Int32 a rdfs:Class ; rdfs:subClassOf atom:Number ; rdfs:label "Signed 32-bit integer" ; atom:cType "LV2_Atom_Int32" . atom:Int64 a rdfs:Class ; rdfs:subClassOf atom:Number ; rdfs:label "Signed 64-bit integer" ; atom:cType "LV2_Atom_Int64" . atom:Bool a rdfs:Class ; rdfs:subClassOf atom:Atom ; rdfs:label "An atom:Int32 where 0 is false and all other values true" ; atom:cType "LV2_Atom_Int32" . atom:Float a rdfs:Class ; rdfs:subClassOf atom:Number ; rdfs:label "32-bit IEEE-754 floating point number" ; atom:cType "LV2_Atom_Float" . atom:Double a rdfs:Class ; rdfs:subClassOf atom:Number ; rdfs:label "64-bit IEEE-754 floating point number" ; atom:cType "LV2_Atom_Double" . atom:blobSupport a lv2:Feature ; rdfs:label "Blob support" ; lv2:documentation """

Support for dynamically allocated blobs. If a host supports this feature, it MUST pass a LV2_Feature with URI http://lv2plug.in/ns/ext/atom#blobSupport and data pointing to a LV2_Blob_Support.

""" . atom:AtomPort a rdfs:Class ; rdfs:subClassOf lv2:Port ; rdfs:label "Atom Port" ; lv2:documentation """

A port which contains an lv2:Atom. Ports of this type will be connected to a 32-bit aligned LV2_Atom immediately followed by size bytes of data.

This is an abstract port type, i.e. a port MUST NOT only be an AtomPort, but must be a more descriptive type that is a subclass of AtomPort which defines the port's semantics (typically atom:ValuePort or atom:MessagePort).

Before calling a method on the plugin that writes to an AtomPort output, the host MUST set the size of the Atom in that output to the amount of available memory immediately following the Atom header. The plugin MUST write a valid Atom to that port (leaving it untouched is illegal). If there is no reasonable value to write to the port, the plugin MUST write NULL (the Atom with both type = 0 and size = 0).

""" . atom:ValuePort a rdfs:Class ; rdfs:subClassOf atom:AtomPort ; rdfs:label "Value Port" ; lv2:documentation """

An AtomPort that interprets its data as a persistent and time-independent "value".

  • If a plugin has fixed input values for all ports, all ValuePort outputs are also fixed regardless of the number of times the plugin is run.
  • If a plugin has fixed input values for all ports except a ValuePort, each value V of that ValuePort corresponds to a single set of outputs for all ports.
  • If a ValuePort contains a reference then the blob it refers to is constant; plugin MUST NOT modify the blob in any way.

Value ports can be thought of as purely functional ports: if a plugin callback has only value ports, then the plugin callback is a pure function.

""" . atom:MessagePort a rdfs:Class ; rdfs:subClassOf atom:AtomPort ; rdfs:label "Message Port" ; rdfs:comment """ An AtomPort that "receives", "consumes", "executes", or "sends" its value. The Atom contained in a MessagePort is considered transient and/or time-dependent, and is only valid for a single run invocation. Unlike a ValuePort, a MessagePort may be used to manipulate internal plugin state. Intuitively, a MessagePort contains a "message" or "command" or "event" which is reacted to, NOT a "value" or "signal" (which is computed with). """ . atom:cType a rdf:Property , owl:DatatypeProperty ; rdfs:label "C type" ; rdfs:domain rdfs:Class ; rdfs:range xsd:string ; rdfs:comment """ The identifier for a C type describing the in-memory representation of an instance of this class. """ . atom:EventPort a rdfs:Class ; rdfs:label "Event port" ; rdfs:subClassOf lv2:Port ; lv2:documentation """

A port used for communicating time-stamped atoms in the audio context. Ports of this type are connected to an LV2_Atom_Buffer, which contains a flat time-stamped sequence of atom:Event.

This port type is intended as a simpler and atom compatible successor to ev:EventPort.

""" . atom:supports a rdf:Property ; rdfs:domain lv2:Port ; rdfs:range atom:Atom ; rdfs:label "supports" ; lv2:documentation """

Indicates that a Port supports a certain atom:Atom type. This is distinct from the port type - e.g. the port type ValuePort can hold atoms with many different types. This property is used to describe which Atom types a Port expects to receive or send.

""" .