This chapter will discuss how you can manipulate your pipeline in several ways from your application on. Parts of this chapter are downright hackish, so be assured that you'll need some programming knowledge before you start reading this. Topics that will be discussed here include how you can insert data into a pipeline from your application, how to read data from a pipeline, how to manipulate the pipeline's speed, length, starting point and how to listen to a pipeline's data processing. Probes are best envisioned as pad listeners. They are attached to a pad in a pipeline, and you can add callback functions to this probe. Those callback functions will be called whenever data is being sent over this pad. The callback can then decide whether the data should be discarded or it can replace the piece of data with another piece of data. In this callback, it can also trigger actions in the application itself. For pipeline manipulation, probes are rather limited, but for pipeline tracking, they can be very useful. Many people have expressed the wish to use their own sources to inject data into a pipeline. Some people have also expressed the wish to grab the output in a pipeline and take care of the actual output inside their application. While either of these methods are stongly discouraged, &GStreamer; offers hacks to do this. However, there is no support for those methods. If it doesn't work, you're on your own. Also, synchronization, thread-safety and other things that you've been able to take for granted so far are no longer guanranteed if you use any of those methods. It's always better to simply write a plugin and have the pipeline schedule and manage it. See the Plugin Writer's Guide for more information on this topic. Also see the next section, which will explain how to embed plugins statically in your application. After all those disclaimers, let's start. There's three possible elements that you can use for the above-mentioned purposes. Those are called fakesrc (an imaginary source), fakesink (an imaginary sink) and identity (an imaginary filter). The same method applies to each of those elements. Here, we will discuss how to use those elements to insert (using fakesrc) or grab (using fakesink or identity) data from a pipeline, and how to set negotiation. The three before-mentioned elements (fakesrc, fakesink and identity) each have a handoff signal that will be called in the _get ()- (fakesrc) or _chain ()-function (identity, fakesink). In the signal handler, you can set (fakesrc) or get (identity, fakesink) data to/from the provided buffer. Note that in the case of fakesrc, you have to set the size of the provided buffer using the sizemax property. For both fakesrc and fakesink, you also have to set the signal-handoffs property for this method to work. Note that your handoff function should not block, since this will block pipeline iteration. Also, do not try to use all sort of weird hacks in such functions to accomplish something that looks like synchronization or so; it's not the right way and will lead to issues elsewhere. If you're doing any of this, you're basically misunderstanding the &GStreamer; design. Sometimes, when using fakesrc as a source in your pipeline, you'll want to set a specific format, for example a video size and format or an audio bitsize and number of channels. You can do this by forcing a specific GstCaps on the pipeline, which is possible by using filtered caps. You can set a filtered caps on a link by using gst_pad_link_filtered (), where the third argument is the format to force on the link. This example application will generate black/white (it switches every second) video to an X-window output by using fakesrc as a source and using filtered caps to force a format. Since the depth of the image depends on your X-server settings, we use a colorspace conversion element to make sure that the output to your X server will have the correct bitdepth. You can also set timestamps on the provided buffers to override the fixed framerate. #include <string.h> /* for memset () */ #include <gst/gst.h> static void cb_handoff (GstElement *fakesrc, GstBuffer *buffer, GstPad *pad, gpointer user_data) { static gboolean white = FALSE; /* this makes the image black/white */ memset (GST_BUFFER_DATA (buffer), white ? 0xff : 0x0, GST_BUFFER_SIZE (buffer)); white = !white; } gint main (gint argc, gchar *argv[]) { GstElement *pipeline, *fakesrc, *conv, *videosink; GstCaps *filter; /* init GStreamer */ gst_init (&argc, &argv); /* setup pipeline */ pipeline = gst_pipeline_new ("pipeline"); fakesrc = gst_element_factory_make ("fakesrc", "source"); conv = gst_element_factory_make ("ffmpegcolorspace", "conv"); videosink = gst_element_factory_make ("ximagesink", "videosink"); /* setup */ filter = gst_caps_new_simple ("video/x-raw-rgb", "width", G_TYPE_INT, 384, "height", G_TYPE_INT, 288, "framerate", G_TYPE_DOUBLE, (gdouble) 1.0, "bpp", G_TYPE_INT, 16, "depth", G_TYPE_INT, 16, "endianness", G_TYPE_INT, G_BYTE_ORDER, NULL); gst_element_link_filtered (fakesrc, conv, filter); gst_element_link (conv, videosink); gst_bin_add_many (GST_BIN (pipeline), fakesrc, conv, videosink, NULL); /* setup fake source */ g_object_set (G_OBJECT (fakesrc), "signal-handoffs", TRUE, "sizemax", 384 * 288 * 2, "sizetype", 2, NULL); g_signal_connect (fakesrc, "handoff", G_CALLBACK (cb_handoff), NULL); /* play */ gst_element_set_state (pipeline, GST_STATE_PLAYING); while (gst_bin_iterate (GST_BIN (pipeline))) ; /* clean up */ gst_element_set_state (pipeline, GST_STATE_NULL); gst_object_unref (GST_OBJECT (pipeline)); return 0; } The Plugin Writer's Guide describes in great detail how to write elements for the &GStreamer; framework. In this section, we will solely discuss how to embed such elements statically in your application. This can be useful for application-specific elements that have no use elsewhere in &GStreamer;. Dynamically loaded plugins contain a structure that's defined using GST_PLUGIN_DEFINE (). This structure is loaded when the plugin is loaded by the &GStreamer; core. The structure contains an initialization function (usually called plugin_init) that will be called right after that. It's purpose is to register the elements provided by the plugin with the &GStreamer; framework. If you want to embed elements directly in your application, the only thing you need to do is to manually run this structure using _gst_plugin_register_static (). The initialization will then be called, and the elements will from then on be available like any other element, without them having to be dynamically loadable libraries. In the example below, you would be able to call gst_element_factory_make ("my-element-name", "some-name") to create an instance of the element. /* * Here, you would write the actual plugin code. */ [..] static gboolean register_elements (GstPlugin *plugin) { return gst_element_register (plugin, "my-element-name", GST_RANK_NONE, MY_PLUGIN_TYPE); } static GstPluginDesc plugin_desc = { GST_VERSION_MAJOR, GST_VERSION_MINOR, "my-private-plugins", "Private elements of my application", register_elements, NULL, "0.0.1", "LGPL", "my-application", "http://www.my-application.net/", GST_PADDING_INIT }; /* * Call this function right after calling gst_init (). */ void my_elements_init (void) { _gst_plugin_register_static (&plugin_desc); }