簡介
在多媒體應用中,我們通常需要查詢媒體檔案的總時間、當前播放位置,以及跳轉到指定的時間點。GStreamer提供了相應的介面來實現此功能,在本文中,我們將通過示例瞭解如何查詢時間資訊,以及如何進行跳轉到指定位置。
GStreamer查詢機制
GStreamer提供了GstQuery的查詢機制,用於查詢Element或Pad的相應資訊。例如:查詢當前的播放速率,產生的延遲,是否支援跳轉等。可檢視GstQuery文件瞭解所支援的型別。
要查詢所需的資訊,首先需要構造一個查詢的型別,然後使用Element或Pad的查詢介面獲取資料,最終再解析相應結果。 下面的例子介紹瞭如何使用GstQuery查詢Pipeline的總時間:
GstQuery *query = gst_query_new_duration (GST_FORMAT_TIME); gboolean res = gst_element_query (pipeline, query); if (res) { gint64 duration; gst_query_parse_duration (query, NULL, &duration); g_print ("duration = %"GST_TIME_FORMAT, GST_TIME_ARGS (duration)); } else { g_print ("duration query failed..."); } gst_query_unref (query);
示例程式碼
在本示例中,我們通過查詢Pipeline是否支援跳轉(seeking),如果支援跳轉(有些媒體不支援跳轉,例如實時視訊),我們會在播放10秒後跳轉到其他位置。
在以前的示例中,我們在Pipeline開始執行後,只等待ERROR和EOS訊息,然後退出。本例中,我們會在訊息等在中設定等待超時時間,超時後,我們會去查詢當前播放的時間,用於顯示,這與播放器的進度條類似。
#include <gst/gst.h> /* Structure to contain all our information, so we can pass it around */ typedef struct _CustomData { GstElement *playbin; /* Our one and only element */ gboolean playing; /* Are we in the PLAYING state? */ gboolean terminate; /* Should we terminate execution? */ gboolean seek_enabled; /* Is seeking enabled for this media? */ gboolean seek_done; /* Have we performed the seek already? */ gint64 duration; /* How long does this media last, in nanoseconds */ } CustomData; /* Forward definition of the message processing function */ static void handle_message (CustomData *data, GstMessage *msg); int main(int argc, char *argv[]) { CustomData data; GstBus *bus; GstMessage *msg; GstStateChangeReturn ret; data.playing = FALSE; data.terminate = FALSE; data.seek_enabled = FALSE; data.seek_done = FALSE; data.duration = GST_CLOCK_TIME_NONE; /* Initialize GStreamer */ gst_init (&argc, &argv); /* Create the elements */ data.playbin = gst_element_factory_make ("playbin", "playbin"); if (!data.playbin) { g_printerr ("Not all elements could be created.\n"); return -1; } /* Set the URI to play */ g_object_set (data.playbin, "uri", "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm", NULL); /* Start playing */ ret = gst_element_set_state (data.playbin, 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.playbin); return -1; } /* Listen to the bus */ bus = gst_element_get_bus (data.playbin); do { msg = gst_bus_timed_pop_filtered (bus, 100 * GST_MSECOND, GST_MESSAGE_STATE_CHANGED | GST_MESSAGE_ERROR | GST_MESSAGE_EOS | GST_MESSAGE_DURATION_CHANGED); /* Parse message */ if (msg != NULL) { handle_message (&data, msg); } else { /* We got no message, this means the timeout expired */ if (data.playing) { gint64 current = -1; /* Query the current position of the stream */ if (!gst_element_query_position (data.playbin, GST_FORMAT_TIME, ¤t)) { g_printerr ("Could not query current position.\n"); } /* If we didn't know it yet, query the stream duration */ if (!GST_CLOCK_TIME_IS_VALID (data.duration)) { if (!gst_element_query_duration (data.playbin, GST_FORMAT_TIME, &data.duration)) { g_printerr ("Could not query current duration.\n"); } } /* Print current position and total duration */ g_print ("Position %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r", GST_TIME_ARGS (current), GST_TIME_ARGS (data.duration)); /* If seeking is enabled, we have not done it yet, and the time is right, seek */ if (data.seek_enabled && !data.seek_done && current > 10 * GST_SECOND) { g_print ("\nReached 10s, performing seek...\n"); gst_element_seek_simple (data.playbin, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_KEY_UNIT, 30 * GST_SECOND); data.seek_done = TRUE; } } } } while (!data.terminate); /* Free resources */ gst_object_unref (bus); gst_element_set_state (data.playbin, GST_STATE_NULL); gst_object_unref (data.playbin); return 0; } static void handle_message (CustomData *data, GstMessage *msg) { GError *err; gchar *debug_info; switch (GST_MESSAGE_TYPE (msg)) { case GST_MESSAGE_ERROR: gst_message_parse_error (msg, &err, &debug_info); g_printerr ("Error received from element %s: %s\n", GST_OBJECT_NAME (msg->src), err->message); g_printerr ("Debugging information: %s\n", debug_info ? debug_info : "none"); g_clear_error (&err); g_free (debug_info); data->terminate = TRUE; break; case GST_MESSAGE_EOS: g_print ("End-Of-Stream reached.\n"); data->terminate = TRUE; break; case GST_MESSAGE_DURATION_CHANGED: /* The duration has changed, mark the current one as invalid */ data->duration = GST_CLOCK_TIME_NONE; break; case GST_MESSAGE_STATE_CHANGED: { GstState old_state, new_state, pending_state; gst_message_parse_state_changed (msg, &old_state, &new_state, &pending_state); if (GST_MESSAGE_SRC (msg) == GST_OBJECT (data->playbin)) { g_print ("Pipeline state changed from %s to %s:\n", gst_element_state_get_name (old_state), gst_element_state_get_name (new_state)); /* Remember whether we are in the PLAYING state or not */ data->playing = (new_state == GST_STATE_PLAYING); if (data->playing) { /* We just moved to PLAYING. Check if seeking is possible */ GstQuery *query; gint64 start, end; query = gst_query_new_seeking (GST_FORMAT_TIME); if (gst_element_query (data->playbin, query)) { gst_query_parse_seeking (query, NULL, &data->seek_enabled, &start, &end); if (data->seek_enabled) { g_print ("Seeking is ENABLED from %" GST_TIME_FORMAT " to %" GST_TIME_FORMAT "\n", GST_TIME_ARGS (start), GST_TIME_ARGS (end)); } else { g_print ("Seeking is DISABLED for this stream.\n"); } } else { g_printerr ("Seeking query failed."); } gst_query_unref (query); } } } break; default: /* We should not reach here */ g_printerr ("Unexpected message received.\n"); break; } gst_message_unref (msg); }
將原始碼儲存為basic-tutorial-5.c,執行下列命令可得到編譯結果:
gcc basic-tutorial-5.c -o basic-tutorial-5 `pkg-config --cflags --libs gstreamer-1.0`
原始碼分析
示例前部分內容與其他示例類似,構造Pipeline並使其進入PLAYING狀態。之後開始監聽Bus上的訊息。
msg = gst_bus_timed_pop_filtered (bus, 100 * GST_MSECOND, GST_MESSAGE_STATE_CHANGED | GST_MESSAGE_ERROR | GST_MESSAGE_EOS | GST_MESSAGE_DURATION_CHANGED);
與以前的示例相比,我們在gst_bus_timed_pop_filtered ()中加入了超時時間(100毫秒),這使得此函式如果在100毫秒內沒有收到任何訊息就會返回超時(msg == NULL),我們會在超時中去更新當前時間,如果返回相應訊息(msg != NULL),我們在handle_message中處理相應訊息。
GStreamer內部有統一的時間型別(GstClockTime),時間計算方式為:GstClockTime = 數值 x 時間單位。GStreamer提供了3種時間單位(巨集定義):GST_SECOND(秒),GST_MSECOND(毫秒),GST_NSECOND(納秒)。例如:
10秒: 10 * GST_SECOND
100毫秒:100 * GST_MSECOND
100納秒:100 * GST_NSECOND
重新整理播放時間
/* We got no message, this means the timeout expired */ if (data.playing) { /* Query the current position of the stream */ if (!gst_element_query_position (data.pipeline, GST_FORMAT_TIME, ¤t)) { g_printerr ("Could not query current position.\n"); } /* If we didn't know it yet, query the stream duration */ if (!GST_CLOCK_TIME_IS_VALID (data.duration)) { if (!gst_element_query_duration (data.pipeline, GST_FORMAT_TIME, &data.duration)) { g_printerr ("Could not query current duration.\n"); } }
我們首先判斷Pipeline的狀態,僅在PLAYING狀態時才更新當前時間,在非PLAYING狀態時查詢可能失敗。這部分邏輯每秒大概會執行10次,頻率足夠用於介面的重新整理。這裡我們只將查詢到的時間輸出到終端。
GstElement封裝了相應的介面分別用於查詢當前時間(gst_element_query_position)和總時間(gst_element_query_duration )。
/* Print current position and total duration */ g_print ("Position %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r", GST_TIME_ARGS (current), GST_TIME_ARGS (data.duration));
這裡使用GST_TIME_FORMAT 和GST_TIME_ARGS 幫助我們方便地將GstClockTime的值轉換為: ”時:分:秒“格式的字串輸出。
/* If seeking is enabled, we have not done it yet, and the time is right, seek */ if (data.seek_enabled && !data.seek_done && current > 10 * GST_SECOND) { g_print ("\nReached 10s, performing seek...\n"); gst_element_seek_simple (data.pipeline, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_KEY_UNIT, 30 * GST_SECOND); data.seek_done = TRUE; }
我們同時在超時處理中判斷是否需要進行seek操作(在播放到10s時,自動跳轉到30s),這裡我們直接在Pipeline物件上使用gst_element_seek_simple()來執行跳轉操作。
gst_element_seek_simple所需要的引數為:
- element : 需要執行seek操作的Element,這裡是Pipeline。
- format:執行seek的型別,這裡使用GST_FORMAT_TIME表示我們基於時間的方式進行跳轉。其他支援的型別可以檢視 GstFormat。
- seek_flags :通過標識指定seek的行為。 常用的標識如下,其他支援的flag詳見GstSeekFlags。
- GST_SEEK_FLAG_FLUSH:在執行seek前,清除Pipeline中所有buffer中快取的資料。這可能導致Pipeline在填充的新資料被顯示之前出現短暫的等待,但能提高應用更快的響應速度。如果不指定這個標誌,Pipeline中的所有快取資料會依次輸出,然後才會播放跳轉的位置,會導致一定的延遲。
- GST_SEEK_FLAG_KEY_UNIT:對於大多數的視訊,如果跳轉的位置不是關鍵幀,需要依次解碼該幀所依賴的幀(I幀及P幀)後,才能解碼此非關鍵幀。使用這個標識後,seek會自動從最近的I幀開始播放。這個標識降低了seek的精度,提高了seek的效率。
- GST_SEEK_FLAG_ACCURATE:一些媒體檔案沒有提供足夠的索引資訊,在這種檔案中執行seek操作會非常耗時,針對這類檔案,GStreamer通過內部計算得到需要跳轉的位置,大部分的計算結果都是正確的。如果seek的位置不能達到所需精度時,可以增加此標識。但需要注意的是,使用此標識可能會導致seek耗費更多時間來尋找精確的位置。
- seek_pos :需要跳轉的位置,前面指定了seek的型別為時間,所以這裡是30秒。
訊息處理
我們在handle_message介面中處理所有Pipeline上的訊息,ERROR和EOS與以前示例處理方式相同,此例中新增了以下內容:
case GST_MESSAGE_DURATION_CHANGED: /* The duration has changed, mark the current one as invalid */ data->duration = GST_CLOCK_TIME_NONE; break;
在檔案的總時間發生變化時,我們會收到此訊息,這裡簡單的將總長度標記為非法值,在下次更新時間時進行查詢。
case GST_MESSAGE_STATE_CHANGED: { GstState old_state, new_state, pending_state; gst_message_parse_state_changed (msg, &old_state, &new_state, &pending_state); if (GST_MESSAGE_SRC (msg) == GST_OBJECT (data->pipeline)) { g_print ("Pipeline state changed from %s to %s:\n", gst_element_state_get_name (old_state), gst_element_state_get_name (new_state)); /* Remember whether we are in the PLAYING state or not */ data->playing = (new_state == GST_STATE_PLAYING);
跳轉和時間查詢操作僅在PUASED和PLAYING狀態時才能得到正確的結果,因為所有的Element只能在這2個狀態才能接收處理seek和query的指令。這裡會儲存播放的狀態便於後續使用,並且在進入PLAYING狀態時查詢當前所播放的檔案/流是否支援跳轉操作:
if (data->playing) { /* We just moved to PLAYING. Check if seeking is possible */ GstQuery *query; gint64 start, end; query = gst_query_new_seeking (GST_FORMAT_TIME); if (gst_element_query (data->pipeline, query)) { gst_query_parse_seeking (query, NULL, &data->seek_enabled, &start, &end); if (data->seek_enabled) { g_print ("Seeking is ENABLED from %" GST_TIME_FORMAT " to %" GST_TIME_FORMAT "\n", GST_TIME_ARGS (start), GST_TIME_ARGS (end)); } else { g_print ("Seeking is DISABLED for this stream.\n"); } } else { g_printerr ("Seeking query failed."); } gst_query_unref (query); }
這裡的查詢步驟與文章開始介紹的方式相同:
- 首先,通過gst_query_new_seeking()構造一個跳轉的查詢物件,使用GST_FORMAT_TIME作為引數,表明我們需要知道當前的檔案是否支援通過時間進行跳轉。我們同樣可以使用GST_FORMAT_BYTES作為引數,用於查詢是否可以根據檔案的偏移量來就行跳轉,但這種使用方式不常見。
- 接著,將查詢物件傳入gst_element_query()查詢,並得到結果。
- 最後,通過gst_query_parse_seeking()解析是否支援跳轉及所支援的範圍。
一定需要記住在使用完後釋放查詢物件。
總結
在本教程中,我們學習了:
- 如何通過GstQuery查詢Pipeline上的資訊。
- 如何通過gst_element_query_position()和gst_element_query_duration() 查詢當前時間和總時間。
- 如何通過gst_element_seek_simple()操作跳轉到任意位置。
- 在哪些狀態可以執行查詢和跳轉操作。
後續我們將介紹如何獲取媒體檔案中的後設資料(Metadata)。
引用
https://gstreamer.freedesktop.org/documentation/tutorials/basic/time-management.html?gi-language=c
https://gstreamer.freedesktop.org/documentation/additional/design/query.html?gi-language=c
https://gstreamer.freedesktop.org/documentation/gstreamer/gstquery.html?gi-language=c