<chapter id="chapter-other-ntoone" xreflabel="Writing a N-to-1 Element or Muxer"> <title>Writing a N-to-1 Element or Muxer</title> <para> N-to-1 elements have been previously mentioned and discussed in both <xref linkend="chapter-advanced-request"/> and in <xref linkend="chapter-scheduling"/>. The main noteworthy thing about N-to-1 elements is that each pad is push-based in its own thread, and the N-to-1 element synchronizes those streams by expected-timestamp-based logic. This means it lets all streams wait except for the one that provides the earliest next-expected timestamp. When that stream has passwed one buffer, the next earliest-expected-timestamp is calculated, and we start back where we were, until all streams have reached EOS. There is a helper base class, called <classname>GstCollectPads</classname>, that will help you to do this. </para> <para> Note, however, that this helper class will only help you with grabbing a buffer from each input and giving you the one with earliest timestamp. If you need anything more difficult, such as "don't-grab-a-new-buffer until a given timestamp" or something like that, you'll need to do this yourself. </para> <!-- Note: I'd like to have something like this in the final text, but since the code below doesn't work and this is all 0.8'y, I commented it for now. <sect1 id="section-muxer-negotiation" xreflabel="Negotiation"> <title>Negotiation</title> <para> Most container formats will have a fair amount of issues with changing content on an elementary stream. Therefore, you should not allow caps to be changed once you've started using data from them. The easiest way to achieve this is by using explicit caps, which have been explained before. However, we're going to use them in a slightly different way then what you're used to, having the core do all the work for us. </para> <para> The idea is that, as long as the stream/file headers have not been written yet and no data has been processed yet, a stream is allowed to renegotiate. After that point, the caps should be fixed, because we can only use a stream once. Caps may then only change within an allowed range (think MPEG, where changes in FPS are allowed), or sometimes not at all (such as AVI audio). In order to do that, we will, after data retrieval and header writing, set an explicit caps on each sink pad, that is the minimal caps describing the properties of the format that may not change. As an example, for MPEG audio inside an MPEG system stream, this would mean a wide caps of audio/mpeg with mpegversion=1 and layer=[1,2]. For the same audio type in MPEG, though, the samplerate, bitrate, layer and number of channels would become static, too. Since the (request) pads will be removed when the stream ends, the static caps will cease to exist too, then. While the explicit caps exist, the <function>_link ()</function>- function will not be called, since the core will do all necessary checks for us. Note that the property of using explicit caps should be added along with the actual explicit caps, not any earlier. </para> <para> Below here follows the simple example of an AVI muxer's audio caps negotiation. The <function>_link ()</function>-function is fairly normal, but the <function>-Loop ()</function>-function does some of the tricks mentioned above. There is no <function>_getcaps ()</function>- function since the pad template contains all that information already (not shown). </para> <programlisting> static GstPadLinkReturn gst_avi_mux_audio_link (GstPad *pad, const GstCaps *caps) { GstAviMux *mux = GST_AVI_MUX (gst_pad_get_parent (pad)); GstStructure *str = gst_caps_get_structure (caps, 0); const gchar *mime = gst_structure_get_name (str); if (!strcmp (str, "audio/mpeg")) { /* get version, make sure it's 1, get layer, make sure it's 1-3, * then create the 2-byte audio tag (0x0055) and fill an audio * stream structure (strh/strf). */ [..] return GST_PAD_LINK_OK; } else if !strcmp (str, "audio/x-raw-int")) { /* See above, but now with the raw audio tag (0x0001). */ [..] return GST_PAD_LINK_OK; } else [..] [..] } static void gst_avi_mux_loop (GstElement *element) { GstAviMux *mux = GST_AVI_MUX (element); [..] /* As we get here, we should have written the header if we hadn't done * that before yet, and we're supposed to have an internal buffer from * each pad, also from the audio one. So here, we check again whether * this is the first run and if so, we set static caps. */ if (mux->first_cycle) { const GList *padlist = gst_element_get_pad_list (element); GList *item; for (item = padlist; item != NULL; item = item->next) { GstPad *pad = item->data; GstCaps *caps; if (!GST_PAD_IS_SINK (pad)) continue; /* set static caps here */ if (!strncmp (gst_pad_get_name (pad), "audio_", 6)) { /* the strf is the struct you filled in the _link () function. */ switch (strf->format) { case 0x0055: /* mp3 */ caps = gst_caps_new_simple ("audio/mpeg", "mpegversion", G_TYPE_INT, 1, "layer", G_TYPE_INT, 3, "bitrate", G_TYPE_INT, strf->av_bps, "rate", G_TYPE_INT, strf->rate, "channels", G_TYPE_INT, strf->channels, NULL); break; case 0x0001: /* pcm */ caps = gst_caps_new_simple ("audio/x-raw-int", [..]); break; [..] } } else if (!strncmp (gst_pad_get_name (pad), "video_", 6)) { [..] } else { g_warning ("oi!"); continue; } /* set static caps */ gst_pad_use_explicit_caps (pad); gst_pad_set_explicit_caps (pad, caps); } } [..] /* Next runs will never be the first again. */ mux->first_cycle = FALSE; } </programlisting> <para> Note that there are other ways to achieve that, which might be useful for more complex cases. This will do for the simple cases, though. This method is provided to simplify negotiation and renegotiation in muxers, it is not a complete solution, nor is it a pretty one. </para> </sect1> --> </chapter>