官网：https://gstreamer.freedesktop.org/
Demo基础教程：https://gstreamer.freedesktop.org/documentation/tutorials/basic/concepts.html
Demo下载地址：git://anongit.freedesktop.org/gstreamer/gst-docs
Goal
这一节没有代码，主要教一些列工具的使用。包括：如何不使用C编译运行GStream的管道。如何查找需要的element及能力集。如何发现媒体文件的内部结构。
GStreamer comes with提供 a set of tools which range from handy便利 to absolutely essential必需品. There is no code in this tutorial, just sit back and relax高枕无忧, and we will teach you:
- How to build and run GStreamer pipelines from the command line, without using C at all!
- How to find out what GStreamer elements you have available and their capabilities.
- How to discover the internal structure of media files.
Introduction
这些工具位于GStream的bin文件中，到指定目录(比如D:\gstreamer\1.0\x86_64\bin)使用cmd执行即可。
These tools are available in the bin directory of the GStreamer binaries文件. You need to move to this directory to execute them, because it is not added to the system’s PATH environment variable (to avoid polluting污染 it too much).
Just open a terminal (or console window) and go to the bin directory of your GStreamer installation (Read again the Installing GStreamer section to find our where this is), and you are ready to start typing打字 the commands given in this tutorial.
llinux最好使用随发行版本对应的工具。
On Linux, you should use the GStreamer version installed with your distribution, the tools should be installed with a package named
gstreamer1on Fedora style distributions, orgstreamer1.0-toolson Debian/Ubuntu style distributions.扫描二维码关注公众号，回复： 3734331 查看本文章
In order to allow for multiple versions of GStreamer to coexists共存 in the same system, these tools are versioned, this is, a GStreamer version number is appended to their name. This version is based on GStreamer 1.0, so the tools are called gst-launch-1.0, gst-inspect-1.0 and gst-discoverer-1.0
gst-launch-1.0发动执行
此工具接收关于pipeline的文本描述，然后模拟运行。此工具仅能进行简单模拟，主要是应用层相关的pipeline的操作，主要是给开发人员使用。
This tool accepts a textual description of a pipeline, instantiates it, and sets it to the PLAYING state. It allows you to quickly check if a given pipeline works, before going through the actual implementation实现 using GStreamer API calls.
Bear in mind that it can only create simple pipelines. In particular尤其, it can only simulate模仿 the interaction相关 of the pipeline with the application up to a certain level一定层次. In any case, it is extremely handy非常方便 to test pipelines quickly, and is used by GStreamer developers around the world on a daily basis.
gst-launch-1.0仅仅是开发者的一个调试工具，不要妄图在上边开发程序。
Please note that gst-launch-1.0 is primarily主要 a debugging tool for developers. You should not build applications on top of it. Instead, use the gst_parse_launch() function of the GStreamer API as an easy way to construct pipelines from pipeline descriptions.
gst-launch-1.0语法不害怕，谁都能学会。
Although the rules to construct pipeline descriptions are very simple, the concatenation串联 of multiple elements can quickly make such descriptions resemble类似 black magic. Fear not, for everyone learns the gst-launch-1.0 syntax语法, eventually.
gst-launch-1.0有很多语法，接下来会介绍一些，更详细的将参考文档。
The command line for gst-launch-1.0 consists of a list of options followed by a PIPELINE-DESCRIPTION. Some simplified instructions are given next, see the complete documentation at the reference page for gst-launch-1.0.
Elements
gst-launch-1.0命令之间使用感叹号分割，输入下边命令，试试效果
In simple form, a PIPELINE-DESCRIPTION is a list of element types separated by exclamation marks (!)感叹号. Go ahead and type in the following command:
gst-launch-1.0 videotestsrc ! videoconvert ! autovideosink
You should see a windows with an animated活动的 video pattern. Use CTRL+C on the terminal to stop the program.
这就是效果图，gst-launch-1.0会把他们连接起来，具体之前意见介绍过了。当然中间不加videocovert也是可以的。
This instantiates a new element of type videotestsrc (an element which generates a sample video pattern), an videoconvert (an element which does raw video format conversion, making sure other elements can understand each other), and an autovideosink (a window to which video is rendered). Then, GStreamer tries to link the output of each element to the input of the element appearing on its right in the description. If more than one input or output Pad is available, the Pad Caps are used to find two compatible Pads.
Properties
可以通过property=value设置属性，多个属性用空格隔开，具体支持属性可以通过接下来的工具gst-inspect-1.0获取。
Properties may be appended to elements, in the form *property=value *(multiple properties can be specified, separated by spaces). Use the gst-inspect-1.0 tool (explained next接下来解释) to find out the available properties for an element.
gst-launch-1.0 videotestsrc pattern=11 ! videoconvert ! autovideosink
You should see a static video pattern, made of circles.
Named elements
可以给element命名，命名规则是name=?,使用element的时候，只要加上名字跟一个.即可。如下边例子。给tee命名为t，后边通过t.来应用
Elements can be named using the name property, in this way complex pipelines involving branches can be created. Names allow linking to elements created previously向前的 in the description, and are indispensable to必不可少 use elements with multiple output pads, like demuxers or tees, for example.
Named elements are referred to using their name followed by a dot.
gst-launch-1.0 videotestsrc ! videoconvert ! tee name=t ! queue ! autovideosink t. ! queue ! autovideosink
You should see two video windows, showing the same sample video pattern. If you see only one, try to move it, since it is probably on top of the second window.
This example instantiates a videotestsrc, linked to a videoconvert, linked to a tee (Remember from Basic tutorial 7: Multithreading and Pad Availability that a tee copies to each of its output pads everything coming through its input pad). The tee is named simply ‘t’ (using the name property) and then linked to a queue and an autovideosink. The same tee is referred to using ‘t.’ (mind the dot) and then linked to a second queue and a second autovideosink.
To learn why the queues are necessary read Basic tutorial 7: Multithreading and Pad Availability.
Pads
我们可以手动指定pad而不是让element自动连接。pad指定必须给element命名，然后通过名称指定具体pad。有哪些pad可以通过工具gst-inspect-1.0获取。
Instead of letting GStreamer choose which Pad to use when linking two elements, you may want to specify the Pads directly. You can do this by adding a dot plus the Pad name after the name of the element (it must be a named element). Learn the names of the Pads of an element by using the gst-inspect-1.0 tool.
This is useful, for example, when you want to retrieve找到 one particular stream out of a demuxer:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux name=d d.video_00 ! matroskamux ! filesink location=sintel_video.mkv
这段代码会从网上拉流，包含音频和视频，我们只指定视频pad，打包到新的容器，然后使用属性location写到本地文件，如果这个地址可连接的话，bin目录下回生成相应的视频文件。
This fetches a media file from the internet using souphttpsrc, which is in webm format (a special kind of Matroska container, see Basic tutorial 2: GStreamer concepts). We then open the container using matroskademux. This media contains both audio and video, so matroskademux will create two output Pads, named video_00 and audio_00. We link video_00 to a matroskamux element to re-pack the video stream into a new container, and finally link it to a filesink, which will write the stream into a file named "sintel_video.mkv" (the location property specifies the name of the file).
All in all, we took a webm file, stripped it of audio, and generated a new matroska file with the video. If we wanted to keep only the audio:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux name=d d.audio_00 ! vorbisparse ! matroskamux ! filesink location=sintel_audio.mka
同上，但是音频需要额外添加一个vorbisparse，以便获取音频的能力集。matroskamux本身可以获取视频的能力集，所以上边没有vorbisparse这个element。
The vorbisparse element is required to extract some information from the stream and put it in the Pad Caps, so the next element, matroskamux, knows how to deal with the stream. In the case of video this was not necessary, because matroskademux already extracted this information and added it to the Caps.
Note that in the above two examples no media has been decoded or played. We have just moved from one container to another (demultiplexing and re-multiplexing again).
Caps filters能力集过滤器
当一个src elem含有多个pad，一个sink elem兼容所有src的pad这样src就会选择第一个pad来与sink建立连接，这样就存在二义性（因为你不知道那个pad在前那个在后）
When an element has more than one output pad, it might happen that the link to the next element is ambiguous: the next element may have more than one compatible兼容的 input pad, or its input pad may be compatible with the Pad Caps of all the output pads. In these cases GStreamer will link using the first pad that is available, which pretty much amounts to saying that GStreamer will choose one output pad at random.
Consider the following pipeline:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux ! filesink location=test
上边这个例子，你就不知道test里边到底还是音频数据还是视频数据，可以通过制定pad或者能力集过滤来达到目的，具体详见下例。
This is the same media file and demuxer as in the previous example. The input Pad Caps of filesink are ANY, meaning that it can accept any kind of media. Which one of the two output pads of matroskademux will be linked against the filesink? video_00 or audio_00? You cannot know.
You can remove this ambiguity, though, by using named pads, as in the previous sub-section, or by using Caps Filters:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux ! video/x-vp8 ! matroskamux ! filesink location=sintel_video.mkv
通过在两个element之间加入cap条件，这样会导致只有满足条件的才能通过。
A Caps Filter behaves like a pass-through element which does nothing and only accepts media with the given Caps, effectively resolving the ambiguity. In this example, between matroskademux and matroskamux we added a video/x-vp8 Caps Filter to specify that we are interested in the output pad of matroskademux which can produce this kind of video.
使用gst-inspect-1.0可以查找element可以接受和生成的pad。使用 gst-discoverer-1.0可以发现一个指定文件有包含的cap。如果要查看指定pipeline中element的cap，可以通过 gst-launch-1.0的-v属性获取。
To find out the Caps an element accepts and produces, use the gst-inspect-1.0 tool. To find out the Caps contained in a particular file, use the gst-discoverer-1.0 tool. To find out the Caps an element is producing for a particular具体的 pipeline, run gst-launch-1.0 as usual, with the –v option to print Caps information.
Examples
Play a media file using playbin (as in Basic tutorial 1: Hello world!):
运行结果如图，完全有playbin创建pipelien及所需element
gst-launch-1.0 playbin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm
效果和上图一致，重新对音视频进行编码，然后复合播放。
A fully operation playback pipeline, with audio and video (more or less the same pipeline that playbin will create internally):
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux name=d ! queue ! vp8dec ! videoconvert ! autovideosink d. ! queue ! vorbisdec ! audioconvert ! audioresample ! autoaudiosink
一个转码管道，它打开webm容器并解码两个流（通过uridecodebin），然后用不同的编解码器重新编码音频和视频分支，并将它们放回到一个Ogg容器中保存到文件sintel.ogg中。
A transcoding pipeline, which opens the webm container and decodes both streams (via uridecodebin), then re-encodes the audio and video branches with a different codec, and puts them back together in an Ogg container (just for the sake of it).
gst-launch-1.0 uridecodebin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm name=d ! queue ! theoraenc ! oggmux name=m ! filesink location=sintel.ogg d. ! queue ! audioconvert ! audioresample ! flacenc ! m.
重新缩放管道。videoscale只要输入和输出上限的帧大小不同，该元素就会执行重新缩放操作。输出上限由Caps Filter设置为320x200。
A rescaling pipeline. The videoscale element performs a rescaling operation whenever the frame size is different in the input and the output caps. The output caps are set by the Caps Filter to 320x200.
gst-launch-1.0 uridecodebin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! queue ! videoscale ! video/x-raw-yuv,width=320,height=200 ! videoconvert ! autovideosink
简单减少到此为止，复杂操作，详见参考文档。
This short description of gst-launch-1.0 should be enough to get you started. Remember that you have the complete documentation available here.
gst-inspect-1.0检查工具
这个工具有下边功能，1，罗列所有可用element类型。2，查找指定plugin的element。3，查看指定element的所有信息。
This tool has three modes of operation:
- Without arguments, it lists all available elements types, this is, the types you can use to instantiate new elements.
- With a file name as an argument, it treats the file as a GStreamer plugin, tries to open it, and lists all the elements described inside.
- With a GStreamer element name as an argument, it lists all information regarding that element.
Let's see an example of the third mode:看下边这个例子，输入.\gst-inspect-1.0.exe vp8dec，查看结果
gst-inspect-1.0 vp8dec
Factory Details:
Rank primary (256)
Long-name On2 VP8 Decoder
Klass Codec/Decoder/Video
Description Decode VP8 video streams
Author David Schleef <[email protected]>, Sebastian Dröge <[email protected]>
Plugin Details:
Name vpx
Description VP8 plugin
Filename /usr/lib64/gstreamer-1.0/libgstvpx.so
Version 1.6.4
License LGPL
Source module gst-plugins-good
Source release date 2016-04-14
Binary package Fedora GStreamer-plugins-good package
Origin URL http://download.fedoraproject.org
GObject
+----GInitiallyUnowned
+----GstObject
+----GstElement
+----GstVideoDecoder
+----GstVP8Dec
Pad Templates:
SINK template: 'sink'
Availability: Always
Capabilities:
video/x-vp8
SRC template: 'src'
Availability: Always
Capabilities:
video/x-raw
format: I420
width: [ 1, 2147483647 ]
height: [ 1, 2147483647 ]
framerate: [ 0/1, 2147483647/1 ]
Element Flags:
no flags set
Element Implementation:
Has change_state() function: gst_video_decoder_change_state
Element has no clocking capabilities.
Element has no URI handling capabilities.
Pads:
SINK: 'sink'
Pad Template: 'sink'
SRC: 'src'
Pad Template: 'src'
Element Properties:
name : The name of the object
flags: readable, writable
String. Default: "vp8dec0"
parent : The parent of the object
flags: readable, writable
Object of type "GstObject"
post-processing : Enable post processing
flags: readable, writable
Boolean. Default: false
post-processing-flags: Flags to control post processing
flags: readable, writable
Flags "GstVP8DecPostProcessingFlags" Default: 0x00000403, "mfqe+demacroblock+deblock"
(0x00000001): deblock - Deblock
(0x00000002): demacroblock - Demacroblock
(0x00000004): addnoise - Add noise
(0x00000400): mfqe - Multi-frame quality enhancement
deblocking-level : Deblocking level
flags: readable, writable
Unsigned Integer. Range: 0 - 16 Default: 4
noise-level : Noise level
flags: readable, writable
Unsigned Integer. Range: 0 - 16 Default: 0
threads : Maximum number of decoding threads
flags: readable, writable
Unsigned Integer. Range: 1 - 16 Default: 0
内容主要包含，Pad 模板(这个element含有的所有pad，这里sink pad模板仅支持video/x-vp8，src pad仅支持video/x-raw裸流，解码后数据)，element属性(各种属性，包括类型和可接受的属性范围)。
gst-inspect-1.0的更多内容，详见gst-inspect-1.0.的帮助文档。
The most relevant sections are:
- Pad Templates: This lists all the kinds of Pads this element can have, along with their capabilities. This is where you look to find out if an element can link with another one. In this case, it has only one sink pad template, accepting only
video/x-vp8(encoded video data in VP8 format) and only one source pad template, producingvideo/x-raw(decoded video data). - Element Properties: This lists the properties of the element, along with their type and accepted values.
For more information, you can check the documentation page of gst-inspect-1.0.
gst-discoverer-1.0
这个工具主要是对函数GstDiscoverer(详见第9章)的包装，接收一个URI，打印所有的media以及相关内容。使用gst-discoverer-1.0 --help可以获得可用选项，用于控制输出。
This tool is a wrapper包装 around the GstDiscoverer object shown in Basic tutorial 9: Media information gathering. It accepts a URI from the command line and prints all information regarding the media that GStreamer can extract. It is useful to find out what container and codecs have been used to produce the media, and therefore what elements you need to put in a pipeline to play it.
Use gst-discoverer-1.0 --help to obtain the list of available options, which basically control the amount of verbosity唠叨 of the output.
Let's see an example:
gst-discoverer-1.0 https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm -v
Analyzing https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm
Done discovering https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm
Topology:
container: video/webm
audio: audio/x-vorbis, channels=(int)2, rate=(int)48000
Codec:
audio/x-vorbis, channels=(int)2, rate=(int)48000
Additional info:
None
Language: en
Channels: 2
Sample rate: 48000
Depth: 0
Bitrate: 80000
Max bitrate: 0
Tags:
taglist, language-code=(string)en, container-format=(string)Matroska, audio-codec=(string)Vorbis, application-name=(string)ffmpeg2theora-0.24, encoder=(string)"Xiph.Org\ libVorbis\ I\ 20090709", encoder-version=(uint)0, nominal-bitrate=(uint)80000, bitrate=(uint)80000;
video: video/x-vp8, width=(int)854, height=(int)480, framerate=(fraction)25/1
Codec:
video/x-vp8, width=(int)854, height=(int)480, framerate=(fraction)25/1
Additional info:
None
Width: 854
Height: 480
Depth: 0
Frame rate: 25/1
Pixel aspect ratio: 1/1
Interlaced: false
Bitrate: 0
Max bitrate: 0
Tags:
taglist, video-codec=(string)"VP8\ video", container-format=(string)Matroska;
Properties:
Duration: 0:00:52.250000000
Seekable: yes
Tags:
video codec: VP8 video
language code: en
container format: Matroska
application name: ffmpeg2theora-0.24
encoder: Xiph.Org libVorbis I 20090709
encoder version: 0
audio codec: Vorbis
nominal bitrate: 80000
bitrate: 80000
Conclusion
This tutorial has shown:
- How to build and run GStreamer pipelines from the command line using the
gst-launch-1.0tool. - How to find out what GStreamer elements you have available and their capabilities, using the
gst-inspect-1.0tool. - How to discover the internal structure of media files, using
gst-discoverer-1.0.
It has been a pleasure having you here, and see you soon!