标签：through 命令 main set 绝对值 易用 format orm 创建
在常见的媒体播放器中，通常可以看到快进，快退，慢放等功能，这部分功能被称为“特技模式（Trick Mode）”，这些模式有个共同点：都通过修改播放的速率来达到相应的目的。 本文将介绍如何通过GStreamer去实现快进，快退，慢放以及单帧播放。
快进（Fast-Forward），快退（Fast-Rewind）和慢放（Slow-Motion）都是通过修改播放的速率来达到相应的目的。在GStreamer中，将1倍速作为正常的播放速率，将大于1倍速的2倍，4倍，8倍等倍速称为快进，慢放则是播放速率的绝对值小于1倍速，当播放速率小于0时，则进行倒放。
在GStreamer中，我们通过seek与step事件来控制Element的播放速率及区域。Step事件允许跳过指定的区域并设置后续的播放速率（此速率必须大于0）。Seek事件允许跳转到播放文件中的的任何位置，并且播放速率可以大于0或小于0.
在播放时间控制中，我们使用gst_element_seek_simple 来快速的跳转到指定的位置，此函数是对seek事件的封装。实际使用时，我们首先需要构造一个seek event，设置seek的绝对起始位置和停止位置，停止位置可以设置为0，这样会执行seek的播放速率直到结束。同时可以支持按buffer的方式进行seek，以及设置不同的标志指定seek的行为。
Step事件相较于Seek事件需要更少的参数，更易用于修改播放速率，但是不够灵活。Step事件只会作用于最终的sink，Seek事件则可以作用于Pipeline中所有的Element。Step操作的效率高于Seek。
在GStreamer中，单帧播放(Frame Stepping）与快进相同，也是通过事件实现。单帧播放通常在暂停的状态下，构造并发送step event每次播放一帧。
需要注意的是，seek event需要直接作用于sink element（eg： audio sink或video sink），如果直接将seek event作用于Pipeline，Pipeline会自动将事件转发给所有的sink，如果有多个sink，就会造成多次seek。通常是先获取Pipeline中的video-sink或audio-sink，然后发送seek event到指定的sink，完成seek的操作。 Seek时间的构造及发送示例如下：
GstEvent *event; gboolean result; ... // construct a seek event to play the media from second 2 to 5, flush // the pipeline to decrease latency. event = gst_event_new_seek (1.0, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH, GST_SEEK_TYPE_SET, 2 * GST_SECOND, GST_SEEK_TYPE_SET, 5 * GST_SECOND); ... result = gst_element_send_event (video_sink, event); if (!result) g_warning ("seek failed"); ...
下面通过一个完整的示例，来查看GStreamer是如何通过seek和step达到相应的播放速度。
#include <string.h> #include <stdio.h> #include <gst/gst.h> typedef struct _CustomData { GstElement *pipeline; GstElement *video_sink; GMainLoop *loop; gboolean playing; /* Playing or Paused */ gdouble rate; /* Current playback rate (can be negative) */ } CustomData; /* Send seek event to change rate */ static void send_seek_event (CustomData * data) { gint64 position; GstEvent *seek_event; /* Obtain the current position, needed for the seek event */ if (!gst_element_query_position (data->pipeline, GST_FORMAT_TIME, &position)) { g_printerr ("Unable to retrieve current position.\n"); return; } /* Create the seek event */ if (data->rate > 0) { seek_event = gst_event_new_seek (data->rate, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE, GST_SEEK_TYPE_SET, position, GST_SEEK_TYPE_END, 0); } else { seek_event = gst_event_new_seek (data->rate, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE, GST_SEEK_TYPE_SET, 0, GST_SEEK_TYPE_SET, position); } if (data->video_sink == NULL) { /* If we have not done so, obtain the sink through which we will send the seek events */ g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL); } /* Send the event */ gst_element_send_event (data->video_sink, seek_event); g_print ("Current rate: %g\n", data->rate); } /* Process keyboard input */ static gboolean handle_keyboard (GIOChannel * source, GIOCondition cond, CustomData * data) { gchar *str = NULL; if (g_io_channel_read_line (source, &str, NULL, NULL, NULL) != G_IO_STATUS_NORMAL) { return TRUE; } switch (g_ascii_tolower (str[0])) { case ‘p‘: data->playing = !data->playing; gst_element_set_state (data->pipeline, data->playing ? GST_STATE_PLAYING : GST_STATE_PAUSED); g_print ("Setting state to %s\n", data->playing ? "PLAYING" : "PAUSE"); break; case ‘s‘: if (g_ascii_isupper (str[0])) { data->rate *= 2.0; } else { data->rate /= 2.0; } send_seek_event (data); break; case ‘d‘: data->rate *= -1.0; send_seek_event (data); break; case ‘n‘: if (data->video_sink == NULL) { /* If we have not done so, obtain the sink through which we will send the step events */ g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL); } gst_element_send_event (data->video_sink, gst_event_new_step (GST_FORMAT_BUFFERS, 1, ABS (data->rate), TRUE, FALSE)); g_print ("Stepping one frame\n"); break; case ‘q‘: g_main_loop_quit (data->loop); break; default: break; } g_free (str); return TRUE; } int main (int argc, char *argv[]) { CustomData data; GstStateChangeReturn ret; GIOChannel *io_stdin; /* Initialize GStreamer */ gst_init (&argc, &argv); /* Initialize our data structure */ memset (&data, 0, sizeof (data)); /* Print usage map */ g_print ("USAGE: Choose one of the following options, then press enter:\n" " ‘P‘ to toggle between PAUSE and PLAY\n" " ‘S‘ to increase playback speed, ‘s‘ to decrease playback speed\n" " ‘D‘ to toggle playback direction\n" " ‘N‘ to move to next frame (in the current direction, better in PAUSE)\n" " ‘Q‘ to quit\n"); /* Build the pipeline */ data.pipeline = gst_parse_launch ("playbin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm", NULL); /* Add a keyboard watch so we get notified of keystrokes */ #ifdef G_OS_WIN32 io_stdin = g_io_channel_win32_new_fd (fileno (stdin)); #else io_stdin = g_io_channel_unix_new (fileno (stdin)); #endif g_io_add_watch (io_stdin, G_IO_IN, (GIOFunc) handle_keyboard, &data); /* Start playing */ ret = gst_element_set_state (data.pipeline, GST_STATE_PLAYING); if (ret == GST_STATE_CHANGE_FAILURE) { g_printerr ("Unable to set the pipeline to the playing state.\n"); gst_object_unref (data.pipeline); return -1; } data.playing = TRUE; data.rate = 1.0; /* Create a GLib Main Loop and set it to run */ data.loop = g_main_loop_new (NULL, FALSE); g_main_loop_run (data.loop); /* Free resources */ g_main_loop_unref (data.loop); g_io_channel_unref (io_stdin); gst_element_set_state (data.pipeline, GST_STATE_NULL); if (data.video_sink != NULL) gst_object_unref (data.video_sink); gst_object_unref (data.pipeline); return 0; }
通过下面的命令编译即可得到可执行文件，在终端输入相应指令可修改播放速率。
gcc basic-tutorial-7.c -o basic-tutorial-7 `pkg-config --cflags --libs gstreamer-1.0`
本例中，Pipeline的创建与其他示例相同，通过playbin播放文件，采用GLib的I/O接口来处理键盘输入。
/* Process keyboard input */ static gboolean handle_keyboard (GIOChannel *source, GIOCondition cond, CustomData *data) { gchar *str = NULL; if (g_io_channel_read_line (source, &str, NULL, NULL, NULL) != G_IO_STATUS_NORMAL) { return TRUE; } switch (g_ascii_tolower (str[0])) { case ‘p‘: data->playing = !data->playing; gst_element_set_state (data->pipeline, data->playing ? GST_STATE_PLAYING : GST_STATE_PAUSED); g_print ("Setting state to %s\n", data->playing ? "PLAYING" : "PAUSE"); break;
在终端输入P时，使用gst_element_set_state ()设置播放状态。
case ‘s‘: if (g_ascii_isupper (str[0])) { data->rate *= 2.0; } else { data->rate /= 2.0; } send_seek_event (data); break; case ‘d‘: data->rate *= -1.0; send_seek_event (data); break;
通过S和s增加和降低播放速度，d用于改变播放方向（倒放），这里在修改rate后，调用send_seek_event实现真正的处理。
/* Send seek event to change rate */ static void send_seek_event (CustomData *data) { gint64 position; GstEvent *seek_event; /* Obtain the current position, needed for the seek event */ if (!gst_element_query_position (data->pipeline, GST_FORMAT_TIME, &position)) { g_printerr ("Unable to retrieve current position.\n"); return; }
这个函数会构造一个SeekEvent发送到Pipeline以调节速率。因为Seek Event会跳转到指定的位置，但我们在此例汇总只想改变速率，不跳转到其他位置，所以首先通过gst_element_query_position ()获取当前的播放位置。
/* Create the seek event */ if (data->rate > 0) { seek_event = gst_event_new_seek (data->rate, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE, GST_SEEK_TYPE_SET, position, GST_SEEK_TYPE_END, 0); } else { seek_event = gst_event_new_seek (data->rate, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE, GST_SEEK_TYPE_SET, 0, GST_SEEK_TYPE_SET, position); }
通过gst_event_new_seek()创建SeekEvent，设置新的rate，flag，起始位置，结束位置。需要注意的是，起始位置需要小于结束位置。
if (data->video_sink == NULL) { /* If we have not done so, obtain the sink through which we will send the seek events */ g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL); } /* Send the event */ gst_element_send_event (data->video_sink, seek_event);
正如上文提到的，为了避免Pipeline执行多次的seek，我们在此处获取video-sink，并向其发送SeekEvent。我们直到执行Seek时才获取video-sink是因为实际的sink有可能会根据不同的媒体类型，在PLAYING状态时才创建。
以上部分内容就是速率的修改，关于单帧播放的情况，实现方式更加简单：
case ‘n‘: if (data->video_sink == NULL) { /* If we have not done so, obtain the sink through which we will send the step events */ g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL); } gst_element_send_event (data->video_sink, gst_event_new_step (GST_FORMAT_BUFFERS, 1, ABS (data->rate), TRUE, FALSE)); g_print ("Stepping one frame\n"); break;
我们通过gst_event_new_step()创建了StepEvent，并指定只跳转一帧，且不改变当前速率。单帧播放通常都是先暂停，然后再进行单帧播放。
以上就是通过GStreamer实现播放速率的控制，实际中，有些Element对倒放支持不是很好，不能达到理想的效果。
通过本文我们掌握了：
https://gstreamer.freedesktop.org/documentation/tutorials/basic/playback-speed.html?gi-language=c
https://gstreamer.freedesktop.org/documentation/additional/design/seeking.html?gi-language=c
https://gstreamer.freedesktop.org/documentation/additional/design/framestep.html?gi-language=c
标签：through 命令 main set 绝对值 易用 format orm 创建
原文地址：https://www.cnblogs.com/xleng/p/11401802.html
