GStreamer基础教程11

摘要

　　通常我们的播放引擎需要和GUI进行集成，在使用GStreamer时，GStreamre会负责媒体的播放及控制，GUI会负责处理用户的交互操作以及创建显示的窗口。本例中我们将结合QT介绍如何指定GStreamer将视频输出到指定窗口，以及如何利用GStreamer上报的信息去更新GUI。

与GUI集成

我们知道与GUI集成有两个方面需要注意：

• 显示窗口的管理。

　　由于显示窗口通常由GUI框架创建，所以我们需要将具体的窗口信息告诉GStreamer。由于各个平台使用不同的方式传递窗口句柄，GStreamer提供了一个抽象接口（GstVideoOverlay），用于屏蔽平台的差异，我们可以直接将GUI创建的窗口ID传递给GStreamer。

• GUI界面的更新

　　大多数GUI框架都需要在主线程中去做UI的刷新操作，但GStreamer内部可能会创建多个线程，这就需要通过GstBus及GUI自带的通信机制将所有GStreamer产生的消息传递到GUI主线程，再由GUI主线程对界面进行刷新。

　　下面我们将以QT为例来了解如何处理GStreamer与GUI框架的集成。

示例代码

qtoverlay.h

#ifndef _QTOVERLAY_
#define _QTOVERLAY_

#include <gst/gst.h>

#include <QWidget>
#include <QPushButton>
#include <QHBoxLayout>
#include <QVBoxLayout>
#include <QSlider>
#include <QTimer>

class PlayerWindow : public QWidget
{
    Q_OBJECT
public:
  PlayerWindow(GstElement *p);

  WId getVideoWId() const ;
  static gboolean postGstMessage(GstBus * bus, GstMessage * message, gpointer user_data);

private slots:
  void onPlayClicked() ;
  void onPauseClicked() ;
  void onStopClicked() ;
  void onAlbumAvaiable(const QString &album);
  void onState(GstState st);
  void refreshSlider();
  void onSeek();
  void onEos();

signals:
  void sigAlbum(const QString &album);
  void sigState(GstState st);
  void sigEos();
  
private:
  GstElement *pipeline;
  QPushButton *playBt;
  QPushButton *pauseBt;
  QPushButton *stopBt;
  QWidget *videoWindow;
  QSlider *slider;
  QHBoxLayout *buttonLayout;
  QVBoxLayout *playerLayout;
  QTimer *timer;

  GstState state;
  gint64 totalDuration;
};

#endif

qtoverlay.cpp

#include <gst/video/videooverlay.h>
#include <QApplication>
#include "qtoverlay.h"

PlayerWindow::PlayerWindow(GstElement *p)
    :pipeline(p)
    ,state(GST_STATE_NULL)
    ,totalDuration(GST_CLOCK_TIME_NONE)
{
    playBt = new QPushButton("Play");
    pauseBt = new QPushButton("Pause");
    stopBt = new QPushButton("Stop");
    videoWindow = new QWidget();
    slider = new QSlider(Qt::Horizontal);
    timer = new QTimer();

    connect(playBt, SIGNAL(clicked()), this, SLOT(onPlayClicked()));
    connect(pauseBt, SIGNAL(clicked()), this, SLOT(onPauseClicked()));
    connect(stopBt, SIGNAL(clicked()), this, SLOT(onStopClicked()));
    connect(slider, SIGNAL(sliderReleased()), this, SLOT(onSeek()));

    buttonLayout = new QHBoxLayout;
    buttonLayout->addWidget(playBt);
    buttonLayout->addWidget(pauseBt);
    buttonLayout->addWidget(stopBt);
    buttonLayout->addWidget(slider);

    playerLayout = new QVBoxLayout;
    playerLayout->addWidget(videoWindow);
    playerLayout->addLayout(buttonLayout);

    this->setLayout(playerLayout);

    connect(timer, SIGNAL(timeout()), this, SLOT(refreshSlider()));
    connect(this, SIGNAL(sigAlbum(QString)), this, SLOT(onAlbumAvaiable(QString)));
    connect(this, SIGNAL(sigState(GstState)), this, SLOT(onState(GstState)));
    connect(this, SIGNAL(sigEos()), this, SLOT(onEos()));
}

WId PlayerWindow::getVideoWId() const {
    return videoWindow->winId();
}

void PlayerWindow::onPlayClicked() {
    GstState st = GST_STATE_NULL;
    gst_element_get_state (pipeline, &st, NULL, GST_CLOCK_TIME_NONE);
    if (st < GST_STATE_PAUSED) {
        // Pipeline stopped, we need set overlay again
        GstElement *vsink = gst_element_factory_make ("ximagesink", "vsink");
        g_object_set(GST_OBJECT(pipeline), "video-sink", vsink, NULL);
        WId xwinid = getVideoWId();
        gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (vsink), xwinid);
    }
    gst_element_set_state (pipeline, GST_STATE_PLAYING);
}

void PlayerWindow::onPauseClicked() {
    gst_element_set_state (pipeline, GST_STATE_PAUSED);
}

void PlayerWindow::onStopClicked() {
    gst_element_set_state (pipeline, GST_STATE_NULL);
}

void PlayerWindow::onAlbumAvaiable(const QString &album) {
    setWindowTitle(album);
}

void PlayerWindow::onState(GstState st) {
    if (state != st) {
        state = st;
        if (state == GST_STATE_PLAYING){
            timer->start(1000);
        }
        if (state < GST_STATE_PAUSED){
            timer->stop();
        }
    }
}

void PlayerWindow::refreshSlider() {
    gint64 current = GST_CLOCK_TIME_NONE;
    if (state == GST_STATE_PLAYING) {
        if (!GST_CLOCK_TIME_IS_VALID(totalDuration)) {
            if (gst_element_query_duration (pipeline, GST_FORMAT_TIME, &totalDuration)) {
                slider->setRange(0, totalDuration/GST_SECOND);
            }
        }
        if (gst_element_query_position (pipeline, GST_FORMAT_TIME, &current)) {
            g_print("%ld / %ld
", current/GST_SECOND, totalDuration/GST_SECOND);
            slider->setValue(current/GST_SECOND);
        }
    }
}

void PlayerWindow::onSeek() {
    gint64 pos = slider->sliderPosition();
    g_print("seek: %ld
", pos);
    gst_element_seek_simple (pipeline, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH ,
                  pos * GST_SECOND);
}

void PlayerWindow::onEos() {
    gst_element_set_state (pipeline, GST_STATE_NULL);
}

gboolean PlayerWindow::postGstMessage(GstBus * bus, GstMessage * message, gpointer user_data) {
    PlayerWindow *pw = NULL;
    if (user_data) {
        pw = reinterpret_cast<PlayerWindow*>(user_data);
    }
    switch (GST_MESSAGE_TYPE(message)) {
        case GST_MESSAGE_STATE_CHANGED: {
            GstState old_state, new_state, pending_state;
            gst_message_parse_state_changed (message, &old_state, &new_state, &pending_state);
            pw->sigState(new_state);
            break;
        }
        case GST_MESSAGE_TAG: {
            GstTagList *tags = NULL;
            gst_message_parse_tag(message, &tags);
            gchar *album= NULL;
            if (gst_tag_list_get_string(tags, GST_TAG_ALBUM, &album)) {
                pw->sigAlbum(album);
                g_free(album);
            }
            gst_tag_list_unref(tags);
            break;
        }
        case GST_MESSAGE_EOS: {
            pw->sigEos();
            break;
        }
        default:
            break;
    }
    return TRUE;
}

int main(int argc, char *argv[])
{
  gst_init (&argc, &argv);
  QApplication app(argc, argv);
  app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit ()));

  // prepare the pipeline
  GstElement *pipeline = gst_parse_launch ("playbin uri=file:///home/john/video/sintel_trailer-480p.webm", NULL);

  // prepare the ui
  PlayerWindow *window = new PlayerWindow(pipeline);
  window->resize(900, 600);
  window->show();

  // seg window id to gstreamer
  GstElement *vsink = gst_element_factory_make ("ximagesink", "vsink");
  WId xwinid = window->getVideoWId();
  gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (vsink), xwinid);
  g_object_set(GST_OBJECT(pipeline), "video-sink", vsink, NULL);

  // connect to interesting signals
  GstBus *bus = gst_element_get_bus(pipeline);
  gst_bus_add_watch(bus, &PlayerWindow::postGstMessage, window);
  gst_object_unref(bus);

  // run the pipeline
  GstStateChangeReturn sret = gst_element_set_state (pipeline, GST_STATE_PLAYING);
  if (sret == GST_STATE_CHANGE_FAILURE) {
    gst_element_set_state (pipeline, GST_STATE_NULL);
    gst_object_unref (pipeline);
    // Exit application
    QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
  }

  int ret = app.exec();

  window->hide();
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);

  return ret;
}

qtoverlay.pro

QT += core gui widgets
TARGET = qtoverlay

INCLUDEPATH += /usr/include/glib-2.0
INCLUDEPATH += /usr/lib/x86_64-linux-gnu/glib-2.0/include
INCLUDEPATH += /usr/include/gstreamer-1.0
INCLUDEPATH += /usr/lib/x86_64-linux-gnu/gstreamer-1.0/include
LIBS += -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0 -lgstvideo-1.0

SOURCES += qtoverlay.cpp
HEADERS += qtoverlay.h

分别保存以上内容到各个文件，执行下列命令即可得到可执行程序。如果找不到头文件及库文件，需要根据实际路径修改qtoverlay.pro文件中的内容。

qmake -o Makefile qtoverlay.pro
make

源码分析

  // prepare the pipeline
  GstElement *pipeline = gst_parse_launch ("playbin uri=file:///home/jleng/video/sintel_trailer-480p.webm", NULL);

  // prepare the ui
  PlayerWindow *window = new PlayerWindow(pipeline);
  window->resize(900, 600);
  window->show();

　　在main函数中对GStreamer进行初始化及创建了QT的应用对象后，构造了Pipline，构造GUI窗口对象。在PlayerWindow的构造函数中初始化按钮及窗口，同时创建定时刷新进度条的Timer。

  // seg window id to gstreamer
  GstElement *vsink = gst_element_factory_make ("ximagesink", "vsink");
  WId xwinid = window->getVideoWId();
  gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (vsink), xwinid);
  g_object_set(GST_OBJECT(pipeline), "video-sink", vsink, NULL);
  ...
  gst_bus_add_watch(bus, &PlayerWindow::postGstMessage, window);
  ...
  GstStateChangeReturn sret = gst_element_set_state (pipeline, GST_STATE_PLAYING);
  ...
  int ret = app.exec();
  ...

　　接着我们单独创建了ximagesink用于视频渲染，同时我们将Qt创建的视频窗口ID设置给GStreamer，让GStreamer得到渲染的窗口ID，接着使用g_object_set()将自定义的Sink通过“video-sink”属性设置到playbin中。
　　同时，我们设置了GStreamer的消息处理函数，所有的消息都会在postGstMessage函数中被转发。为了后续调用GUI对象中的接口，我们需要将GUI窗口指针作为user-data，在postGstMessage中再转换为GUI对象。
　　接着设置Pipeline的状态为PLAYING开始播放。
　　最后调用GUI框架的事件循环，exec()会一直执行，直到关闭窗口。
　　由于GStreamer的GstBus会默认使用GLib的主循环及事件处理机制，所以必须要保证GLi默认的MainLoop在某个线程中运行。在本例中，Qt在Linux下会自动使用GLib的主循环，所以我们无需额外进行处理。

gboolean PlayerWindow::postGstMessage(GstBus * bus, GstMessage * message, gpointer user_data) {
    PlayerWindow *pw = NULL;
    if (user_data) {
        pw = reinterpret_cast<PlayerWindow*>(user_data);
    }
    switch (GST_MESSAGE_TYPE(message)) {
        case GST_MESSAGE_STATE_CHANGED: {
            GstState old_state, new_state, pending_state;
            gst_message_parse_state_changed (message, &old_state, &new_state, &pending_state);
            pw->sigState(new_state);
            break;
        }
        case GST_MESSAGE_TAG: {
            GstTagList *tags = NULL;
            gst_message_parse_tag(message, &tags);
            gchar *album= NULL;
            if (gst_tag_list_get_string(tags, GST_TAG_ALBUM, &album)) {
                pw->sigAlbum(album);
                g_free(album);
            }
            gst_tag_list_unref(tags);
            break;
        }
        case GST_MESSAGE_EOS: {
            pw->sigEos();
            break;
        }
        default:
            break;
    }
    return TRUE;
}

　　我们在转换后GUI对象后，再根据消息类型进行处理。在postGstMessage中我们没有直接更新GUI，因为GStreamer的Bus处理线程与GUI主线程可能为不同线程，直接更新GUI会出错或无效。因此利用Qt的signal-slot机制在相应的槽函数中就行GUI信息的更新。这里只处理了3种消息STATE_CHANGED（状态变化），TAG（媒体元数据及编码信息），EOS（播放结束），GStreamer所支持的消息可查看官方文档GstMessage。

void PlayerWindow::onPlayClicked() {
    GstState st = GST_STATE_NULL;
    gst_element_get_state (pipeline, &st, NULL, GST_CLOCK_TIME_NONE);
    if (st < GST_STATE_PAUSED) {
        // Pipeline stopped, we need set overlay again
        GstElement *vsink = gst_element_factory_make ("ximagesink", "vsink");
        g_object_set(GST_OBJECT(pipeline), "video-sink", vsink, NULL);
        WId xwinid = getVideoWId();
        gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (vsink), xwinid);
    }
    gst_element_set_state (pipeline, GST_STATE_PLAYING);
}

　　当点击Play按钮时，onPlayClicked函数会被调用，我们在此直接调用GStreamer的接口设置Pipeline的状态。当播放结束或点击Stop时，GStreamer会在状态切换到NULL时释放所有资源，所以我们在此需要重新设置playbin的vido-sink，并指定视频输出窗口。

　　Pause，Stop的处理类似，直接调用gst_element_set_state ()将Pipeline设置为相应状态。

void PlayerWindow::refreshSlider() {
    gint64 current = GST_CLOCK_TIME_NONE;
    if (state == GST_STATE_PLAYING) {
        if (!GST_CLOCK_TIME_IS_VALID(totalDuration)) {
            if (gst_element_query_duration (pipeline, GST_FORMAT_TIME, &totalDuration)) {
                slider->setRange(0, totalDuration/GST_SECOND);
            }
        }
        if (gst_element_query_position (pipeline, GST_FORMAT_TIME, &current)) {
            g_print("%ld / %ld
", current/GST_SECOND, totalDuration/GST_SECOND);
            slider->setValue(current/GST_SECOND);
        }
    }
}

void PlayerWindow::onSeek() {
    gint64 pos = slider->sliderPosition();
    g_print("seek: %ld
", pos);
    gst_element_seek_simple (pipeline, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH ,
                  pos * GST_SECOND);
}

　　我们在构造函数中创建了Timer用于每秒刷新进度条，在refreshSlider被调用时，我们通过gst_element_query_duration() 和gst_element_query_position ()得到文件的总时间和当前时间，并刷新进度条。由于GStreamer返回时间单位为纳秒，所以我们需要通过GST_SECOND将其转换为秒用于时间显示。
　　我们同样处理了用户的Seek操作，在拉动进度条到某个位置时，获取Seek的位置，调用gst_element_seek_simple ()跳转到指定位置。我们不用关心对GStreamer的调用是处于哪个线程，GStreamer内部会自动进行处理。

总结

通过本文，我们学习到：

• 如何使用gst_video_overlay_set_window_handle ()将GUI的窗口句柄传递给GStremaer。
• 如何使用信号槽传递消息到GUI主线程。
• 如何使用Timer定时刷新GUI。

引用

https://gstreamer.freedesktop.org/documentation/video/gstvideooverlay.html?gi-language=c
https://gstreamer.freedesktop.org/documentation/tutorials/basic/toolkit-integration.html?gi-language=c
https://doc.qt.io/qt-5/qmake-manual.html

作者：John.Leng

出处：http://www.cnblogs.com/xleng/

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