This chapter contains a fairly random selection of things that can be useful to keep in mind when writing &GStreamer;-based applications. It's up to you how much you're going to use the information provided here. We will shortly discuss how to debug pipeline problems using &GStreamer; applications. Also, we will touch upon how to acquire knowledge about plugins and elements and how to test simple pipelines before building applications around them. Always connect to the error signal of your topmost pipeline to be notified of errors in your pipeline. Always check return values of &GStreamer; functions. Especially, check return values of gst_element_link () and gst_element_set_state (). Always use your pipeline object to keep track of the current state of your pipeline. Don't keep private variables in your application. Also, don't update your user interface if a user presses the play button. Instead, connect to the state-changed signal of your topmost pipeline and update the user interface whenever this signal is triggered. Applications can make use of the extensive &GStreamer; debugging system to debug pipeline problems. Elements will write output to this system to log what they're doing. It's not used for error reporting, but it is very useful for tracking what an element is doing exactly, which can come in handy when debugging application issues (such as failing seeks, out-of-sync media, etc.). Most &GStreamer;-based applications accept the commandline option --gst-debug=LIST and related family members. The list consists of a comma-separated list of category/level pairs, which can set the debugging level for a specific debugging category. For example, --gst-debug=oggdemux:5 would turn on debugging for the Ogg demuxer element. You can use wildcards as well. A debugging level of 0 will turn off all debugging, and a level of 5 will turn on all debugging. Intermediate values only turn on some debugging (based on message severity; 2, for example, will only display errors and warnings). Here's a list of all available options: --gst-debug-help will print available debug categories and exit. --gst-debug-level=LEVEL will set the default debug level (which can range from 0 (no output) to 5 (everything)). --gst-debug=LIST takes a comma-separated list of category_name:level pairs to set specific levels for the individual categories. Example: GST_AUTOPLUG:5,avidemux:3. --gst-debug-no-color will disable color debugging. --gst-debug-disable disables debugging alltogether. --gst-plugin-spew enables printout of errors while loading &GStreamer; plugins. &GStreamer; contains a bunch of conversion plugins that most applications will find useful. Specifically, those are videoscalers (videoscale), colorspace convertors (ffmpegcolorspace), audio format convertors and channel resamplers (audioconvert) and audio samplerate convertors (audioscale). Those convertors don't do anything when not required, they will act in passthrough mode. They will activate when the hardware doesn't support a specific request, though. All applications are recommended to use those elements. &GStreamer; comes with a default set of command-line utilities that can help in application development. We will discuss only gst-launch and gst-inspect here. gst-launch is a simple script-like commandline application that can be used to test pipelines. For example, the command gst-launch sinesrc ! alsasink will run a pipeline which generates a sine-wave audio stream and plays it to your ALSA audio card. gst-launch also allows the use of threads (using curly brackets, so { and }) and bins (using brackets, so ( and )). You can use dots to imply padnames on elements, or even omit the padname to automatically select a pad. Using all this, the pipeline gst-launch filesrc location=file.ogg ! oggdemux name=d { d. ! theoradec ! ffmpegcolorspace ! xvimagesink } { d. ! vorbisdec ! alsasink } will play an Ogg file containing a Theora video-stream and a Vorbis audio-stream. You can also use autopluggers such as decodebin on the commandline. See the manual page of gst-launch for more information. gst-inspect can be used to inspect all properties, signals, dynamic parameters and the object hierarchy of an element. This acn be very useful to see which GObject properties or which signals (and using what arguments) an element supports. Run gst-inspect fakesrc to get an idea of what it does. See the manual page of gst-inspect for more information.