examples: Add audiomixer example with mixing 4 stereo tracks into 8ch output

Will be useful to show how to create and set the mix-matrix via our bindings.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1408>

examples/Cargo.toml

@@ -204,3 +204,6 @@ required-features = ["cairo-rs", "gst-video/v1_18"]
 [[bin]]
 name = "d3d11videosink"
 required-features = ["windows"]
+
+[[bin]]
+name = "audio_multichannel_interleave"

examples/src/bin/audio_multichannel_interleave.rs

@@ -0,0 +1,153 @@
+// This example demonstrates how to mix multiple audio
+// streams into a single output using the audiomixer element.
+// In this case, we're mixing 4 stereo streams into a single 8 channel output.
+
+use gst::prelude::*;
+use std::env;
+
+#[path = "../examples-common.rs"]
+mod examples_common;
+
+const TRACKS: i32 = 4;
+
+fn create_source_and_link(pipeline: &gst::Pipeline, mixer: &gst::Element, track_number: i32) {
+    let freq = ((track_number + 1) * 1000) as f64;
+    let audiosrc = gst::ElementFactory::make("audiotestsrc")
+        .property("freq", freq)
+        .property("num-buffers", 2000)
+        .build()
+        .unwrap();
+    let caps = gst_audio::AudioCapsBuilder::new().channels(2).build();
+    let capsfilter = gst::ElementFactory::make("capsfilter")
+        .property("caps", &caps)
+        .build()
+        .unwrap();
+
+    pipeline.add_many([&audiosrc, &capsfilter]).unwrap();
+    gst::Element::link_many([&audiosrc, &capsfilter]).unwrap();
+
+    let src_pad = capsfilter.static_pad("src").unwrap();
+    let mixer_pad = mixer.request_pad_simple("sink_%u").unwrap();
+
+    // audiomixer expects a mix-matrix set on each input pad,
+    // indicating which output channels our input should appear in.
+    // Rows => input channels, columns => output channels.
+    // Here each input channel will appear in exactly one output channel.
+    let mut mix_matrix: Vec<Vec<f32>> = vec![];
+    for i in 0..TRACKS {
+        if i == track_number {
+            mix_matrix.push(vec![1.0, 0.0]);
+            mix_matrix.push(vec![0.0, 1.0]);
+        } else {
+            mix_matrix.push(vec![0.0, 0.0]);
+            mix_matrix.push(vec![0.0, 0.0]);
+        }
+    }
+    let mut audiomixer_config = gst_audio::AudioConverterConfig::new();
+    audiomixer_config.set_mix_matrix(&mix_matrix);
+    mixer_pad.set_property("converter-config", audiomixer_config);
+
+    src_pad.link(&mixer_pad).unwrap();
+}
+
+fn example_main() {
+    gst::init().unwrap();
+
+    let args: Vec<_> = env::args().collect();
+    let output_file = if args.len() == 2 {
+        &args[1]
+    } else {
+        println!("Usage: audiomixer <output file>");
+        std::process::exit(-1);
+    };
+
+    let pipeline = gst::Pipeline::new();
+    let audiomixer = gst::ElementFactory::make("audiomixer").build().unwrap();
+
+    // Using an arbitrary layout of 4 stereo pairs.
+    let positions = [
+        gst_audio::AudioChannelPosition::FrontLeft,
+        gst_audio::AudioChannelPosition::FrontRight,
+        gst_audio::AudioChannelPosition::RearLeft,
+        gst_audio::AudioChannelPosition::RearRight,
+        gst_audio::AudioChannelPosition::SideLeft,
+        gst_audio::AudioChannelPosition::SideRight,
+        gst_audio::AudioChannelPosition::TopFrontLeft,
+        gst_audio::AudioChannelPosition::TopFrontRight,
+    ];
+
+    let mask = gst_audio::AudioChannelPosition::positions_to_mask(&positions, true).unwrap();
+    let caps = gst_audio::AudioCapsBuilder::new()
+        .channels(positions.len() as i32)
+        .channel_mask(mask)
+        .build();
+    let capsfilter = gst::ElementFactory::make("capsfilter")
+        .property("caps", &caps)
+        .build()
+        .unwrap();
+
+    let audioconvert = gst::ElementFactory::make("audioconvert").build().unwrap();
+    let audioresample = gst::ElementFactory::make("audioresample").build().unwrap();
+    let wavenc = gst::ElementFactory::make("wavenc").build().unwrap();
+    let sink = gst::ElementFactory::make("filesink")
+        .property("location", output_file)
+        .build()
+        .unwrap();
+
+    pipeline
+        .add_many([
+            &audiomixer,
+            &capsfilter,
+            &audioconvert,
+            &audioresample,
+            &wavenc,
+            &sink,
+        ])
+        .unwrap();
+    gst::Element::link_many([
+        &audiomixer,
+        &capsfilter,
+        &audioconvert,
+        &audioresample,
+        &wavenc,
+        &sink,
+    ])
+    .unwrap();
+
+    for i in 0..TRACKS {
+        create_source_and_link(&pipeline, &audiomixer, i);
+    }
+
+    let bus = pipeline.bus().expect("Pipeline without bus");
+
+    pipeline
+        .set_state(gst::State::Playing)
+        .expect("Unable to start pipeline");
+
+    for msg in bus.iter_timed(gst::ClockTime::NONE) {
+        use gst::MessageView;
+        match msg.view() {
+            MessageView::Eos(..) => break,
+            MessageView::Error(err) => {
+                eprintln!(
+                    "Error from {:?}: {} ({:?})",
+                    msg.src().map(|s| s.path_string()),
+                    err.error(),
+                    err.debug()
+                );
+                break;
+            }
+            _ => (),
+        }
+    }
+
+    pipeline
+        .set_state(gst::State::Null)
+        .expect("Unable to change pipeline state to NULL");
+}
+
+fn main() {
+    // tutorials_common::run is only required to set up the application environment on macOS
+    // (but not necessary in normal Cocoa applications where this is set up automatically)
+    examples_common::run(example_main);
+}