# Spa Pod

`Pod` binds the WpSpaPod C API. Pods are SPA's serialization format for typed values; they are what PipeWire object params are made of. Scripts encounter them whenever they read or write a param — `Props`, `Format`, `Route`, `Profile`, `PortConfig` and so on.

    -- read the Props param of a node
    for p in node:iterate_params ("Props") do
      local props = p:parse ()
      log:info ("volume: " .. tostring (props.volume))
    end

Note

The shipped scripts usually go through `common-utils`' `parseParam (pod, "Props")` instead of calling `Pod.parse()` directly; it parses the pod and verifies that it really is of the expected object type, returning nil otherwise.

Building a pod and setting it as a param:

    local param = Pod.Object {
      "Spa:Pod:Object:Param:Route", "Route",
      index = route_index,
      device = device_id,
      props = Pod.Object {
        "Spa:Pod:Object:Param:Props", "Route",
        mute = false,
        channelVolumes = Pod.Array { "Spa:Float", 0.5, 0.5 },
      },
      save = false,
    }

    device:set_param ("Route", param)

## Methods

Pod.parse(self)

  Converts the pod into the equivalent Lua value. Primitives become Lua booleans, numbers or strings; containers become tables.

  For an object pod, the resulting table has the object's property names as keys, plus two extra entries: `pod_type` (the string `"Object"`) and `object_id`.

  Returns:

    the converted value

Pod.get_type_name(self)
  Returns:

    the name of the SPA type of this pod, e.g. `"Spa:Pod:Object:Param:Props"`

  Return type:

    string

Pod.fixate(self)

  Binds `wp_spa_pod_fixate()`

  Fixates a choice pod, i.e. reduces it to its default value.

  Returns:

    true if the pod was fixated

  Return type:

    boolean

Pod.filter(self, filter)

  Binds `wp_spa_pod_filter()`

  Returns the intersection of this pod and filter. Used to negotiate a format that satisfies both sides.

  Parameters:

    filter (Pod) -- the pod to intersect with; if nil, a copy of self is returned

  Returns:

    the intersection, or nil if it could not be made

  Return type:

    Pod

## Primitive constructors

Pod.None()

Pod.Boolean(value)

Pod.Id(value)

Pod.Int(value)

Pod.Long(value)

Pod.Float(value)

Pod.Double(value)

Pod.String(value)

Pod.Bytes(value)

Pod.Pointer(type_name, value)

Pod.Fd(value)

  Construct a pod holding a single value of the corresponding SPA type.

Pod.Rectangle(width, height)

  Constructs a rectangle pod.

  Parameters:

    - width (integer) -- the width

    - height (integer) -- the height

Pod.Fraction(num, denom)

  Constructs a fraction pod.

  Parameters:

    - num (integer) -- the numerator

    - denom (integer) -- the denominator

## Container constructors

Pod.Object(decl)

  Constructs an object pod. The table's first two array entries are the SPA type name of the object and the id name; the remaining string-keyed entries are the object's properties.

      Pod.Object {
        "Spa:Pod:Object:Param:Props", "Props",
        mute = false,
        volume = 0.5,
      }

  The property names and their types come from the SPA type information for the object type, so only names that the type actually defines can be used.

Pod.Struct(decl)

  Constructs a struct pod from the array entries of the table. Unlike arrays and choices, a struct is heterogeneous, so there is no type name entry; every entry is a value and they are stored in the order they appear. Entries may be Lua booleans, numbers, strings, or other pods — which is how a struct can nest other containers.

      Pod.Struct {
        "hello", 42, 0.5, true,
        Pod.Rectangle (1920, 1080),
        Pod.Struct { "inner", 1 },
      }

  Plain Lua numbers are stored as the widest matching type: integers become `Spa:Long` and non-integers become `Spa:Double`. If the receiving side expects a specific type, construct it explicitly:

      Pod.Struct { Pod.Int (42), Pod.Float (0.5) }

Pod.Array(decl)

  Constructs an array pod. Like the choice constructors below, the table's first entry is the type of the values and all the remaining entries are the values, which must all be of that type.

      Pod.Array { "Spa:Float", 0.5, 0.5 }
      Pod.Array { "Spa:Enum:AudioChannel", "FL", "FR" }

Pod.Sequence(decl)

  Constructs a sequence pod, i.e. a list of timed controls. Each entry is itself a table and, contrary to the other containers, its fields are named rather than positional:

  `offset`

    the media offset at which the control takes effect, relative to the start of the buffer that carries the sequence — for audio, this is a number of samples; defaults to 0 if omitted

  `typename`

    the kind of control, a short name of the `Spa:Enum:Control` id table, i.e. `"Properties"`, `"Midi"` or `"OSC"`

  `value`

    the payload of the control, whose expected type depends on `typename`; it may be a Lua boolean, number or string, or another pod. A `"Properties"` control carries a `Props` object pod, while `"Midi"` and `"OSC"` controls carry raw bytes.

      -- a volume ramp: full volume at the start of the buffer,
      -- half volume 4800 samples later
      Pod.Sequence {
        { offset = 0, typename = "Properties",
          value = Pod.Object { "Spa:Pod:Object:Param:Props", "Props",
                               volume = 1.0 } },
        { offset = 4800, typename = "Properties",
          value = Pod.Object { "Spa:Pod:Object:Param:Props", "Props",
                               volume = 0.5 } },
      }

  Entries that do not specify both a `typename` and a `value` are silently dropped from the resulting sequence.

## Choice constructors

Choice pods describe a set of acceptable values rather than a single one; they are what format negotiation is expressed in.

All the choice constructors take a single table, whose entries are purely positional. There are no string keys and the order is not interchangeable: the same values in a different order describe a different — or an invalid — choice. The general form is:

    Pod.Choice.<Kind> { <value type>, <default>, <more values...> }

1. The first entry is always a string with the type of the values, i.e. either a primitive SPA type name (`"Spa:Bool"`, `"Spa:Id"`, `"Spa:Int"`, `"Spa:Long"`, `"Spa:Float"`, `"Spa:Double"`, `"Spa:Fd"`) or the name of an id table, such as `"Spa:Enum:AudioFormat"`. In the latter case the values may be written as the short names of that table (`"S16LE"`, `"FL"`, ...). Strings and byte arrays cannot be used in a choice.

2. The second entry is always the default value, i.e. the value that `Pod.fixate()` reduces the choice to.

3. The remaining entries depend on the kind of choice and are described below. Their meaning is derived from their position, so none of them may be omitted.

Pod.Choice.None(decl)

  A choice with a single possible value: `{ <value type>, <value> }`

      Pod.Choice.None { "Spa:Bool", false }

Pod.Choice.Range(decl)

  A range of values, given as default, minimum and maximum, in this order: `{ <value type>, <default>, <min>, <max> }`

      Pod.Choice.Range { "Spa:Int", 44100, 8000, 192000 }

Pod.Choice.Step(decl)

  A range with a step, given as default, minimum, maximum and step, in this order: `{ <value type>, <default>, <min>, <max>, <step> }`

      Pod.Choice.Step { "Spa:Int", 1024, 256, 8192, 256 }

Pod.Choice.Enum(decl)

  An enumeration: the default, followed by the list of acceptable values: `{ <value type>, <default>, <value 1>, <value 2>, ... }`

      Pod.Choice.Enum { "Spa:Enum:AudioFormat", "S16LE", "S16LE", "F32LE" }

  The default is not implicitly part of the enumeration; it is only the preferred value. If it is also meant to be acceptable — which is almost always the case — it must be repeated as the first of the values, as in the example above, which reads "S16LE preferred, S16LE or F32LE acceptable".

  For `"Spa:Bool"`, this is enforced: the table must contain exactly three values, the first two being equal and the third one being the opposite, e.g. `Pod.Choice.Enum { "Spa:Bool", true, true, false }`.

Pod.Choice.Flags(decl)

  A set of flags that may be combined: the default combination, followed by the flags that may be set: `{ <value type>, <default>, <flag 1>, <flag 2>, ... }`

      Pod.Choice.Flags { "Spa:Int", 1 << 0, 1 << 0, 1 << 2, 1 << 3 }