gstreamer-rs/examples/src/bin/pango-cairo.rs

// This example demonstrates how to overlay a video using the cairo
// library. For this, the cairooverlay element is used on a video stream.
// Additionally, this example uses functionality of the pango library, which handles
// text layouting. The pangocairo crate is a nice wrapper combining both libraries
// into a nice interface.
// The drawing surface which the cairooverlay element creates internally can then
// normally be drawn on using the cairo library.
// The operated pipeline looks like this:

// {videotestsrc} - {cairooverlay} - {capsfilter} - {videoconvert} - {autovideosink}
// The capsfilter element allows us to dictate the video resolution we want for the
// videotestsrc and the cairooverlay element.

extern crate gstreamer as gst;
use gst::prelude::*;

extern crate gstreamer_video as gst_video;
use pango::prelude::*;

use std::ops;
use std::sync::{Arc, Mutex};

use anyhow::Error;
use derive_more::{Display, Error};

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

#[derive(Debug, Display, Error)]
#[display(fmt = "Missing element {}", _0)]
struct MissingElement(#[error(not(source))] &'static str);

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

struct DrawingContext {
    layout: glib::SendUniqueCell<LayoutWrapper>,
    info: Option<gst_video::VideoInfo>,
}

#[derive(Debug)]
struct LayoutWrapper(pango::Layout);

impl ops::Deref for LayoutWrapper {
    type Target = pango::Layout;

    fn deref(&self) -> &pango::Layout {
        &self.0
    }
}

unsafe impl glib::SendUnique for LayoutWrapper {
    fn is_unique(&self) -> bool {
        self.0.ref_count() == 1
    }
}

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

    let pipeline = gst::Pipeline::new(None);
    let src = gst::ElementFactory::make("videotestsrc", None)
        .map_err(|_| MissingElement("videotestsrc"))?;
    let overlay = gst::ElementFactory::make("cairooverlay", None)
        .map_err(|_| MissingElement("cairooverlay"))?;
    let capsfilter =
        gst::ElementFactory::make("capsfilter", None).map_err(|_| MissingElement("capsfilter"))?;
    let videoconvert = gst::ElementFactory::make("videoconvert", None)
        .map_err(|_| MissingElement("videoconvert"))?;
    let sink = gst::ElementFactory::make("autovideosink", None)
        .map_err(|_| MissingElement("autovideosink"))?;

    pipeline.add_many(&[&src, &overlay, &capsfilter, &videoconvert, &sink])?;
    gst::Element::link_many(&[&src, &overlay, &capsfilter, &videoconvert, &sink])?;

    // Plug in a capsfilter element that will force the videotestsrc and the cairooverlay to work
    // with images of the size 800x800.
    let caps = gst::Caps::builder("video/x-raw")
        .field("width", &800i32)
        .field("height", &800i32)
        .build();
    capsfilter.set_property("caps", &caps).unwrap();

    // The videotestsrc supports multiple test patterns. In this example, we will use the
    // pattern with a white ball moving around the video's center point.
    src.set_property_from_str("pattern", "ball");

    // The PangoFontMap represents the set of fonts available for a particular rendering system.
    let fontmap = pangocairo::FontMap::new().unwrap();
    // Create a new pango layouting context for the fontmap.
    let context = fontmap.create_context().unwrap();
    // Create a pango layout object. This object is a string of text we want to layout.
    // It is wrapped in a LayoutWrapper (defined above) to be able to send it across threads.
    let layout = LayoutWrapper(pango::Layout::new(&context));

    // Select the text content and the font we want to use for the piece of text.
    let font_desc = pango::FontDescription::from_string("Sans Bold 26");
    layout.set_font_description(Some(&font_desc));
    layout.set_text("GStreamer");

    // The following is a context struct (containing the pango layout and the configured video info).
    // We have to wrap it in an Arc (or Rc) to get reference counting, that is: to be able to have
    // shared ownership of it in multiple different places (the two signal handlers here).
    // We have to wrap it in a Mutex because Rust's type-system can't know that both signals are
    // only ever called from a single thread (the streaming thread). It would be enough to have
    // something that is Send in theory but that's not how signal handlers are generated unfortunately.
    // The Mutex (or otherwise if we didn't need the Sync bound we could use a RefCell) is to implement
    // interior mutability (see Rust docs). Via this we can get a mutable reference to the contained
    // data which is checked at runtime for uniqueness (blocking in case of mutex, panic in case
    // of refcell) instead of compile-time (like with normal references).
    let drawer = Arc::new(Mutex::new(DrawingContext {
        layout: glib::SendUniqueCell::new(layout).unwrap(),
        info: None,
    }));

    let drawer_clone = drawer.clone();
    // Connect to the cairooverlay element's "draw" signal, which is emitted for
    // each videoframe piped through the element. Here we have the possibility to
    // draw on top of the frame (overlay it), using the cairo render api.
    // Signals connected with the connect(<name>, ...) API get their arguments
    // passed as array of glib::Value. For a documentation about the actual arguments
    // it is always a good idea to either check the element's signals using either
    // gst-inspect, or the online documentation.
    overlay
        .connect("draw", false, move |args| {
            use std::f64::consts::PI;

            let drawer = &drawer_clone;
            let drawer = drawer.lock().unwrap();

            // Get the signal's arguments
            let _overlay = args[0].get::<gst::Element>().unwrap().unwrap();
            // This is the cairo context. This is the root of all of cairo's
            // drawing functionality.
            let cr = args[1].get::<cairo::Context>().unwrap().unwrap();
            let timestamp = args[2].get_some::<gst::ClockTime>().unwrap();
            let _duration = args[3].get_some::<gst::ClockTime>().unwrap();

            let info = drawer.info.as_ref().unwrap();
            let layout = drawer.layout.borrow();

            let angle = 2.0
                * PI
                * ((timestamp % (10 * gst::SECOND)).unwrap() as f64
                    / (10.0 * gst::SECOND_VAL as f64));

            // The image we draw (the text) will be static, but we will change the
            // transformation on the drawing context, which rotates and shifts everything
            // that we draw afterwards. Like this, we have no complicated calulations
            // in the actual drawing below.
            // Calling multiple transformation methods after each other will apply the
            // new transformation on top. If you repeat the cr.rotate(angle) line below
            // this a second time, everything in the canvas will rotate twice as fast.
            cr.translate(
                f64::from(info.width()) / 2.0,
                f64::from(info.height()) / 2.0,
            );
            cr.rotate(angle);

            // This loop will render 10 times the string "GStreamer" in a circle
            for i in 0..10 {
                // Cairo, like most rendering frameworks, is using a stack for transformations
                // with this, we push our current transformation onto this stack - allowing us
                // to make temporary changes / render something / and then returning to the
                // previous transformations.
                cr.save();

                let angle = (360. * f64::from(i)) / 10.0;
                let red = (1.0 + f64::cos((angle - 60.0) * PI / 180.0)) / 2.0;
                cr.set_source_rgb(red, 0.0, 1.0 - red);
                cr.rotate(angle * PI / 180.0);

                // Update the text layout. This function is only updating pango's internal state.
                // So e.g. that after a 90 degree rotation it knows that what was previously going
                // to end up as a 200x100 rectangle would now be 100x200.
                pangocairo::functions::update_layout(&cr, &**layout);
                let (width, _height) = layout.get_size();
                // Using width and height of the text, we can properly possition it within
                // our canvas.
                cr.move_to(
                    -(f64::from(width) / f64::from(pango::SCALE)) / 2.0,
                    -(f64::from(info.height())) / 2.0,
                );
                // After telling the layout object where to draw itself, we actually tell
                // it to draw itself into our cairo context.
                pangocairo::functions::show_layout(&cr, &**layout);

                // Here we go one step up in our stack of transformations, removing any
                // changes we did to them since the last call to cr.save();
                cr.restore();
            }

            None
        })
        .unwrap();

    // Add a signal handler to the overlay's "caps-changed" signal. This could e.g.
    // be called when the sink that we render to does not support resizing the image
    // itself - but the user just changed the window-size. The element after the overlay
    // will then change its caps and we use the notification about this change to
    // resize our canvas's size.
    // Another possibility for when this might happen is, when our video is a network
    // stream that dynamically changes resolution when enough bandwith is available.
    overlay
        .connect("caps-changed", false, move |args| {
            let _overlay = args[0].get::<gst::Element>().unwrap().unwrap();
            let caps = args[1].get::<gst::Caps>().unwrap().unwrap();

            let mut drawer = drawer.lock().unwrap();
            drawer.info = Some(gst_video::VideoInfo::from_caps(&caps).unwrap());

            None
        })
        .unwrap();

    Ok(pipeline)
}

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

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

    for msg in bus.iter_timed(gst::CLOCK_TIME_NONE) {
        use gst::MessageView;

        match msg.view() {
            MessageView::Eos(..) => break,
            MessageView::Error(err) => {
                pipeline.set_state(gst::State::Null)?;
                return Err(ErrorMessage {
                    src: msg
                        .get_src()
                        .map(|s| String::from(s.get_path_string()))
                        .unwrap_or_else(|| String::from("None")),
                    error: err.get_error().to_string(),
                    debug: err.get_debug(),
                    source: err.get_error(),
                }
                .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 environent on macOS
    // (but not necessary in normal Cocoa applications where this is set up autmatically)
    examples_common::run(example_main);
}