gstreamer-rs/examples/src/bin/appsink.rs

// This example demonstrates the use of the appsink element.
// It operates the following pipeline:

// {audiotestsrc} - {appsink}

// The application specifies what format it wants to handle. This format
// is applied by calling set_caps on the appsink. Now it's the audiotestsrc's
// task to provide this data format. If the element connected to the appsink's
// sink-pad were not able to provide what we ask them to, this would fail.
// This is the format we request:
// Audio / Signed 16bit / 1 channel / arbitrary sample rate

use std::i16;

use anyhow::Error;
use byte_slice_cast::*;
use derive_more::{Display, Error};
use gst::{element_error, prelude::*};

#[path = "../examples-common.rs"]
mod examples_common;

#[derive(Debug, Display, Error)]
#[display(fmt = "Received error from {src}: {error} (debug: {debug:?})")]
struct ErrorMessage {
    src: glib::GString,
    error: glib::Error,
    debug: Option<glib::GString>,
}

fn create_pipeline() -> Result<gst::Pipeline, Error> {
    gst::init()?;

    let pipeline = gst::Pipeline::default();
    let src = gst::ElementFactory::make("audiotestsrc").build()?;
    let appsink = gst_app::AppSink::builder()
        // Tell the appsink what format we want. It will then be the audiotestsrc's job to
        // provide the format we request.
        // This can be set after linking the two objects, because format negotiation between
        // both elements will happen during pre-rolling of the pipeline.
        .caps(
            &gst_audio::AudioCapsBuilder::new_interleaved()
                .format(gst_audio::AUDIO_FORMAT_S16)
                .channels(1)
                .build(),
        )
        .build();

    pipeline.add_many(&[&src, appsink.upcast_ref()])?;
    src.link(&appsink)?;

    // Getting data out of the appsink is done by setting callbacks on it.
    // The appsink will then call those handlers, as soon as data is available.
    appsink.set_callbacks(
        gst_app::AppSinkCallbacks::builder()
            // Add a handler to the "new-sample" signal.
            .new_sample(|appsink| {
                // Pull the sample in question out of the appsink's buffer.
                let sample = appsink.pull_sample().map_err(|_| gst::FlowError::Eos)?;
                let buffer = sample.buffer().ok_or_else(|| {
                    element_error!(
                        appsink,
                        gst::ResourceError::Failed,
                        ("Failed to get buffer from appsink")
                    );

                    gst::FlowError::Error
                })?;

                // At this point, buffer is only a reference to an existing memory region somewhere.
                // When we want to access its content, we have to map it while requesting the required
                // mode of access (read, read/write).
                // This type of abstraction is necessary, because the buffer in question might not be
                // on the machine's main memory itself, but rather in the GPU's memory.
                // So mapping the buffer makes the underlying memory region accessible to us.
                // See: https://gstreamer.freedesktop.org/documentation/plugin-development/advanced/allocation.html
                let map = buffer.map_readable().map_err(|_| {
                    element_error!(
                        appsink,
                        gst::ResourceError::Failed,
                        ("Failed to map buffer readable")
                    );

                    gst::FlowError::Error
                })?;

                // We know what format the data in the memory region has, since we requested
                // it by setting the appsink's caps. So what we do here is interpret the
                // memory region we mapped as an array of signed 16 bit integers.
                let samples = map.as_slice_of::<i16>().map_err(|_| {
                    element_error!(
                        appsink,
                        gst::ResourceError::Failed,
                        ("Failed to interprete buffer as S16 PCM")
                    );

                    gst::FlowError::Error
                })?;

                // For buffer (= chunk of samples), we calculate the root mean square:
                // (https://en.wikipedia.org/wiki/Root_mean_square)
                let sum: f64 = samples
                    .iter()
                    .map(|sample| {
                        let f = f64::from(*sample) / f64::from(i16::MAX);
                        f * f
                    })
                    .sum();
                let rms = (sum / (samples.len() as f64)).sqrt();
                println!("rms: {rms}");

                Ok(gst::FlowSuccess::Ok)
            })
            .build(),
    );

    Ok(pipeline)
}

fn main_loop(pipeline: gst::Pipeline) -> Result<(), Error> {
    pipeline.set_state(gst::State::Playing)?;

    let bus = pipeline
        .bus()
        .expect("Pipeline without bus. Shouldn't happen!");

    for msg in bus.iter_timed(gst::ClockTime::NONE) {
        use gst::MessageView;

        match msg.view() {
            MessageView::Eos(..) => break,
            MessageView::Error(err) => {
                pipeline.set_state(gst::State::Null)?;
                return Err(ErrorMessage {
                    src: msg
                        .src()
                        .map(|s| s.path_string())
                        .unwrap_or_else(|| glib::GString::from("UNKNOWN")),
                    error: err.error(),
                    debug: err.debug(),
                }
                .into());
            }
            _ => (),
        }
    }

    pipeline.set_state(gst::State::Null)?;

    Ok(())
}

fn example_main() {
    match create_pipeline().and_then(main_loop) {
        Ok(r) => r,
        Err(e) => eprintln!("Error! {e}"),
    }
}

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);
}