Typedetection
Sometimes the capabilities of a pad are not specificied. The disksrc, for
example, does not know what type of file it is reading. Before you can attach
an element to the pad of the disksrc, you need to determine the media type in
order to be able to choose a compatible element.
To solve this problem, a plugin can provide the GStreamer
core library with a typedefinition library with a typedefinition. The typedefinition
will contain the following information:
The MIME type we are going to define.
An optional string with a list of possible file extensions this
type usually is associated with. the list entries are separated with
a space. eg, ".mp3 .mpa .mpg".
An optional typefind function.
The typefind functions give a meaning to the MIME types that are used
in GStreamer. The typefind function is a function with the following definition:
typedef GstCaps *(*GstTypeFindFunc) (GstBuffer *buf, gpointer priv);
This typefind function will inspect a GstBuffer with data and will output
a GstCaps structure describing the type. If the typefind function does not
understand the buffer contents, it will return NULL.
GStreamer has a typefind element in its core elements
that can be used to determine the type of a given pad.
The next example will show how a typefind element can be inserted into a pipeline
to detect the media type of a file. It will output the capabilities of the pad into
an XML representation.
#include <gst/gst.h>
void type_found (GstElement *typefind, GstCaps* caps);
int
main(int argc, char *argv[])
{
GstElement *bin, *disksrc, *typefind;
gst_init(&argc,&argv);
if (argc != 2) {
g_print("usage: %s <filename>\n", argv[0]);
exit(-1);
}
/* create a new bin to hold the elements */
bin = gst_bin_new("bin");
g_assert(bin != NULL);
/* create a disk reader */
disksrc = gst_elementfactory_make("disksrc", "disk_source");
g_assert(disksrc != NULL);
gtk_object_set(GTK_OBJECT(disksrc),"location", argv[1],NULL);
/* create the typefind element */
typefind = gst_elementfactory_make("typefind", "typefind");
g_assert(typefind != NULL);
/* add objects to the main pipeline */
gst_bin_add(GST_BIN(bin), disksrc);
gst_bin_add(GST_BIN(bin), typefind);
gtk_signal_connect (GTK_OBJECT (typefind), "have_type",
type_found, NULL);
gst_pad_connect(gst_element_get_pad(disksrc,"src"),
gst_element_get_pad(typefind,"sink"));
/* start playing */
gst_element_set_state(GST_ELEMENT(bin), GST_STATE_PLAYING);
gst_bin_iterate(GST_BIN(bin));
gst_element_set_state(GST_ELEMENT(bin), GST_STATE_NULL);
exit(0);
}
We create a very simple pipeline with only a disksrc and the typefind element
in it. The sinkpad of the typefind element has been connected to the src pad
of the disksrc.
We attached a signal 'have_type' to the typefind element which will be called
when the type of the media stream as been detected.
the typefind function will loop over all the registered types and will execute
each of the typefind functions. As soon as a function returns a GstCaps pointer,
the type_found function will be called:
void
type_found (GstElement *typefind, GstCaps* caps)
{
xmlDocPtr doc;
xmlNodePtr parent;
doc = xmlNewDoc ("1.0");
doc->root = xmlNewDocNode (doc, NULL, "Capabilities", NULL);
parent = xmlNewChild (doc->root, NULL, "Caps1", NULL);
gst_caps_save_thyself (caps, parent);
xmlDocDump (stdout, doc);
}
In the type_found function we can print or inspect the type that has been
detected using the GstCaps APIs. In this example, we just print out the
XML representation of the caps structure to stdout.
A more usefull option would be to use the registry to look up an element
that can handle this particular caps structure, or we can also use the
autoplugger to connect this caps structure to, for example, a videosink.