In the last article we learned how to create a GStreamer pipeline that streams a test video via an Icecast server to the web. In this article we will use GStreamer’s programmable appsrc element, in order to feed the pipeline with raw image data from our application.

Building the pipeline

First we will recreate the pipeline from the last article in C source code. We use plain C, since the original GStreamer API is a GLib based C API.

#include <gst/gst.h>

int main(int argc, char *argv)
{
    GError *error = NULL;

    gst_init(&argc, &argv);

    GstPipeline *pipeline = gst_parse_launch("videotestsrc ! vp8enc ! webmmux ! shout2send ip=127.0.0.1 port=8000 password=hackme mount=/test.webm", &error);
    if (error != NULL) {
        g_printerr("Could not create pipeline: %s\n", error->message);
        return 1;
    }
    gst_element_set_state(pipeline, GST_STATE_PLAYING);

    GMainLoop *loop = g_main_loop_new(NULL, FALSE);
    g_main_loop_run(loop);

    g_free(loop);
    g_free(pipeline);

    return 0;
}

In order to compile this code the GStreamer development files must be installed on your system. On an openSUSE Linux system, for example, you have to install the package gstreamer-plugins-base-devel. Compile and run this code from the command line:

$ cc demo1.c -o demo1 $(pkg-config --cflags --libs gstreamer-1.0)
$ ./demo1

The key in this simple program is the gst_parse_launch call. It takes the same pipeline string that we built on the command line in the previous article as an argument and creates a pipeline object. The pipeline is then started by setting its state to playing.

appsrc

So far we have only recreated the same pipeline that we called via gst-launch-1.0 before in C code. Now we will replace the videotestsrc element with an appsrc element:

#include <gst/gst.h>

extern guchar *get_next_image(gsize *size);

const gchar *format = "GRAY8";
const guint fps = 15;
const guint width = 640;
const guint height = 480;

typedef struct {
    GstClockTime timestamp;
    guint sourceid;
    GstElement *appsrc;
} StreamContext;

static StreamContext *stream_context_new(GstElement *appsrc)
{
    StreamContext *ctx = g_new0(StreamContext, 1);
    ctx->timestamp = 0;
    ctx->sourceid = 0;
    ctx->appsrc = appsrc;
    return ctx;
}

static gboolean read_data(StreamContext *ctx)
{
    gsize size;

    guchar *pixels = get_next_image(&size);
    GstBuffer *buffer = gst_buffer_new_wrapped(pixels, size);

    GST_BUFFER_PTS(buffer) = ctx->timestamp;
    GST_BUFFER_DURATION(buffer) = gst_util_uint64_scale_int(1, GST_SECOND, fps);
    ctx->timestamp += GST_BUFFER_DURATION(buffer);

    gst_app_src_push_buffer(ctx->appsrc, buffer);

    return TRUE;
}

static void enough_data(GstElement *appsrc, guint unused, StreamContext *ctx)
{
    if (ctx->sourceid != 0) {
        g_source_remove(ctx->sourceid);
        ctx->sourceid = 0;
    }
}

static void need_data(GstElement *appsrc, guint unused, StreamContext *ctx)
{
    if (ctx->sourceid == 0) {
        ctx->sourceid = g_idle_add((GSourceFunc)read_data, ctx);
    }
}

int main(int argc, char *argv[])
{
    gst_init(&argc, &argv);

    GstElement *pipeline = gst_parse_launch("appsrc name=imagesrc ! videoconvert ! vp8enc ! webmmux ! shout2send ip=127.0.0.1 port=8000 password=hackme mount=/test.webm", NULL);
    GstElement *appsrc = gst_bin_get_by_name(GST_BIN(pipeline), "imagesrc");

    gst_util_set_object_arg(G_OBJECT(appsrc), "format", "time");
    gst_app_src_set_caps(appsrc, gst_caps_new_simple("video/x-raw",
        "format", G_TYPE_STRING, format,
        "width", G_TYPE_INT, width,
        "height", G_TYPE_INT, height,
        "framerate", GST_TYPE_FRACTION, fps, 1, NULL));

    GMainLoop *loop = g_main_loop_new(NULL, FALSE);
    StreamContext *ctx = stream_context_new(appsrc);

    g_signal_connect(appsrc, "need-data", G_CALLBACK(need_data), ctx);
    g_signal_connect(appsrc, "enough-data", G_CALLBACK(enough_data), ctx);

    gst_element_set_state(pipeline, GST_STATE_PLAYING);

    g_main_loop_run(loop);

    gst_element_set_state(pipeline, GST_STATE_NULL);

    g_free(ctx);
    g_free(loop);

    return 0;
}

We assign a name (“imagesrc”) to the appsrc element by setting its name attribute in the pipeline string in line 58. The element can then be retrieved via this name by calling the function gst_bin_get_by_name. In lines 61-66 we set properties and capabilities of the appsrc element such as the image format (in this example 8 bit greyscale), width, height and frames per second.

In lines 71 and 72 we connect callback functions to the “need-data” and “enough-data” signals. The appsrc element emits the need-data signal, when it wants us to feed more image frame buffers to the pipeline and the enough-data signal when it wants us to stop.

We use an idle source to schedule calls to the read_data function in the main loop. The interesting work happens in read_data: we acquire the raw pixel data of the image for the next frame as byte array, in this example represented by a call to a function named get_next_image. The pixel data is wrapped into a GStreamer buffer and the duration and timestamp of the buffer is set. We track the time in a self-defined context object. The buffer is then sent to the appsrc via gst_app_src_push_buffer. GStreamer will take care of freeing the buffer once it’s no longer needed.

Conclusion

With little effort we created a simple C program that streams image frames from within the program itself as video to the Web by leveraging the power of GStreamer and Icecast.