examples: add a lua example showing how to load filter-chain from a script

tests/examples/filter-chain.lua

@@ -0,0 +1,96 @@
+#!/usr/bin/wpexec
+--
+-- WirePlumber
+--
+-- Copyright © 2022 Collabora Ltd.
+--    @author George Kiagiadakis <george.kiagiadakis@collabora.com>
+--
+-- SPDX-License-Identifier: MIT
+--
+-- This is an example of how to load a pipewire module that takes a JSON
+-- string as arguments.
+--
+-- For illustration purposes, this loads module-filter-chain with a 6-band
+-- equalizer. This is the same configuration found in the sink-eq6.conf
+-- file that can be found in pipewire's source tree.
+-----------------------------------------------------------------------------
+
+-- For illustration purposes, we declare 'args' here as a native lua table,
+-- we populate it in multiple steps and we transform it later to a json object
+local args = {
+  ["node.description"] = "Equalizer Sink",
+  ["media.name"] = "Equalizer Sink",
+  ["audio.channels"] = 2,
+  ["audio.position"] = Json.Array { "FL", "FR" },
+}
+
+args["filter.graph"] = Json.Object {
+  nodes = Json.Array {
+    Json.Object {
+        type  = "builtin",
+        name  = "eq_band_1",
+        label = "bq_lowshelf",
+        control = Json.Object { Freq = 100.0, Q = 1.0, Gain = 0.0 },
+    },
+    Json.Object {
+        type  = "builtin",
+        name  = "eq_band_2",
+        label = "bq_peaking",
+        control = Json.Object { Freq = 100.0, Q = 1.0, Gain = 0.0 },
+    },
+    Json.Object {
+        type  = "builtin",
+        name  = "eq_band_3",
+        label = "bq_peaking",
+        control = Json.Object { Freq = 500.0, Q = 1.0, Gain = 0.0 },
+    },
+    Json.Object {
+        type  = "builtin",
+        name  = "eq_band_4",
+        label = "bq_peaking",
+        control = Json.Object { Freq = 2000.0, Q = 1.0, Gain = 0.0 },
+    },
+    Json.Object {
+        type  = "builtin",
+        name  = "eq_band_5",
+        label = "bq_peaking",
+        control = Json.Object { Freq = 5000.0, Q = 1.0, Gain = 0.0 },
+    },
+    Json.Object {
+        type  = "builtin",
+        name  = "eq_band_6",
+        label = "bq_highshelf",
+        control = Json.Object { Freq = 5000.0, Q = 1.0, Gain = 0.0 },
+    },
+  },
+  links = Json.Array {
+    Json.Object { output = "eq_band_1:Out", input = "eq_band_2:In" },
+    Json.Object { output = "eq_band_2:Out", input = "eq_band_3:In" },
+    Json.Object { output = "eq_band_3:Out", input = "eq_band_4:In" },
+    Json.Object { output = "eq_band_4:Out", input = "eq_band_5:In" },
+    Json.Object { output = "eq_band_5:Out", input = "eq_band_6:In" },
+  },
+}
+
+args["capture.props"] = Json.Object {
+  ["node.name"]   = "effect_input.eq6",
+  ["media.class"] = "Audio/Sink",
+}
+
+args["playback.props"] = Json.Object {
+  ["node.name"]   = "effect_output.eq6",
+  ["node.passive"] = true,
+}
+
+-- Transform 'args' to a json object here
+local args_json = Json.Object(args)
+
+-- and get the final JSON as a string from the json object
+local args_string = args_json:get_data()
+
+local properties = {}
+
+print("Loading module-filter-chain with arguments = ")
+print(args_string)
+
+filter_chain = LocalModule("libpipewire-module-filter-chain", args_string, properties)