The use of AppSink in C#.

Application Development
1

Hi.
※I am using deepl.
I am building a windows form app with C# (Visual Studio 2019) + Gstreamer (Ver 1.24.1) + GstSharp (1.18.0).
I need some help on how to use AppSink.

About “appSink = pipeline.GetByName(“sink”) as AppSink;” in the following program,
When I run it, appSink is null.
I guess the cast must be failing, but I can’t figure out why.
The plugin seems to be loaded.
Environment variables have also been set.
Path:C:\gstreamer\1.0\msvc_x86_64\bin
GST_PLUGIN_PATH:C:\gstreamer\1.0\msvc_x86_64\lib\gstreamer-1.0

If you know anything about it, could you please let me know?
I look forward to hearing from you.

public partial class Form1 : Form
{
private Pipeline pipeline;
private Gst.App.AppSink appSink;
private VideoOverlayAdapter overlayAdapter;
private bool pipelineInitialized = false;

public Form1()
{
    InitializeComponent();
    InitGStreamer();
    InitializePipeline();
}

private void InitGStreamer()
{
    Gst.Application.Init();
}

private void InitializePipeline()
{
    if (!Gst.Application.InitCheck())
    {
        MessageBox.Show("Failed to initialize GStreamer.");
        return;
    }

    pipeline = new Pipeline();
    var source = ElementFactory.Make("udpsrc", "source");
    if (source == null)
    {
        MessageBox.Show("Failed to create source.");
        return;
    }

    var queue = ElementFactory.Make("queue", "queue");
    var jitterBuffer = ElementFactory.Make("rtpjitterbuffer", "jitter-buffer");
    var depayloader = ElementFactory.Make("rtph264depay", "depayloader");
    var decoder = ElementFactory.Make("avdec_h264", "decoder");
    var converter = ElementFactory.Make("videoconvert", "converter");
    var sink = ElementFactory.Make("appsink", "sink");

    if (queue == null || jitterBuffer == null || depayloader == null || decoder == null || converter == null || sink == null)
    {
        MessageBox.Show("Failed to create one or more elements.");
        return;
    }

    pipeline.Add(source, queue, jitterBuffer, depayloader, decoder, converter, sink);
    if (!Element.Link(source, queue, jitterBuffer, depayloader, decoder, converter, sink))
    {
        MessageBox.Show("Failed to link elements.");
        return;
    }

    appSink = pipeline.GetByName("sink") as AppSink;

    var caps = Caps.FromString("application/x-rtp,media=(string)video,clock-rate=(int)90000,encoding-name=(string)H264,payload=(int)96");
    source["caps"] = caps;
    source["port"] = 50007;

    //appSink.EmitSignals = true;
    //appSink.Sync = false;
    //appSink.NewSample += AppSink_NewSample;

    pipelineInitialized = true;
}

}

C:\Users\KY2017>gst-inspect-1.0 app
Plugin Details:
Name app
Description Elements used to communicate with applications
Filename C:\gstreamer\1.0\msvc_x86_64\lib\gstreamer-1.0\gstapp.dll
Version 1.24.1
License LGPL
Source module gst-plugins-base
Documentation app
Source release date 2024-03-21
Binary package GStreamer Base Plug-ins source release
Origin URL Unknown package origin

appsink: AppSink
appsrc: AppSrc

2 features:
±- 2 elements

C:\Users\KY2017>

2

You still can use AppSink as Element type.

Set properties with

appSink["emit-signals"] = true;
appSink["sync"] = false;

And connect your signal with

appSink.AddSignalHandler("new-sample", AppSink_NewSample);

Another way is to instantiate AppSink and add it to pipeline by yourself

var appSink = new AppSink("sink");
pipeline.Add(appSink);
converter.Link(appSink);
3

Hi, toxicmouse7
Thanks for the reply. I tried the method you taught me.
I have an additional question.
We are still investigating, but if you know anything about it,
could you please let us know.

■
“sink.AddSignalHandler(“new-sample”, AppSink_NewSample);”, but it will be
“CS1503 Argument 2: cannot convert from ‘method group’ to ‘Delegate’”

private void AppSink_NewSample(object sender, NewSampleArgs args)
{
var sample = appSink.PullSample();
if (sample == null) return;

using (var buffer = sample.Buffer)
{
    MapInfo map;
    if (buffer.Map(out map, MapFlags.Read))
    {
        try
        {
            var data = map.Data;
            Invoke((MethodInvoker)(() =>
            {
                using (MemoryStream ms = new MemoryStream(data.ToArray()))
                {
                    pictureBox1.Image?.Dispose(); // Dispose old image before setting new one
                    pictureBox1.Image = Image.FromStream(ms);
                }
            }));
        }
        finally
        {
            buffer.Unmap(map);
        }
    }
}

}

■
｢var appSink = new AppSink(“sink”);｣ causes an error(↓) at runtime.

GLib-GObject:ERROR:…/gobject/gobject.c:3598:toggle_refs_notify: assertion failed: (tstack.n_toggle_refs == 1)
Bail out! GLib-GObject:ERROR:…/gobject/gobject.c:3598:toggle_refs_notify: assertion failed: (tstack.n_toggle_refs == 1)

■
Also, I would like to get frames from the received data, and I am aware that PullSample() in AppSink is required for this.
But,
var sink = ElementFactory.Make(“appsink”, “sink”);
appSink = pipeline.GetByName(“sink”) as AppSink;
“appsink” will be null. Then PullSample() is not available.

4

Which version of GStreamer you use? You should use GStreamer built with MinGW (not the MSVC). That should propably solve you problem with GLib exception

5

I’m using version 1.24.1.(MSVC).
I believe MinGW(1.24.1) had an error “gstreamer-1.0-0.dll does not exist” at build time(When “Gst.Application.Init()” is executed).
When MinGW is installed, “complete” is selected.

6

You should add path to bin and lib folder to PATH env variable
image

7

Thanks for the reply.
I have to add ｢lib｣ as well.
I had only added ｢bin｣.
I will give it a try.

8

I have not yet tried the procedure you gave me, but I have confirmed that AppSink_NewSample is called each time a new sample is retrieved in the following process.

sink = ElementFactory.Make(“appsink”, “sink”);
sink.Connect(“new-sample”, AppSink_NewSample);
sink[“emit-signals”] = true;
sink[“sync”] = false;
※It seems that sink can be left as Element type.

I will continue to try the steps you gave me.
Thanks a lot.