用gstreamer抓取视频的缩略图

通过构建不同的gstreamer管道，可以有多种方法来抓取视频文件中的缩略图，以下作一简单介绍。

1、从gdkpixbufsink中获取图像 该方法通过gdkpixbufsink的"last-pixbuf"来获取图像的pixbuf。

        descr = g_strdup_printf ("uridecodebin uri=%s ! ffmpegcolorspace ! videoscale ! gdkpixbufsink name=sink", fileurl);

        pipeline = gst_parse_launch (descr, &error);

        if (error != NULL) {

            printf ("could not construct pipeline: %s", error->message);

            g_error_free (error);

            return FALSE;

        }


        sink = gst_bin_get_by_name (GST_BIN (pipeline), "sink");


        ret = gst_element_set_state (pipeline, GST_STATE_PAUSED);

        switch (ret) {

        case GST_STATE_CHANGE_FAILURE:

            printf ("failed to play the file/n");

            return FALSE;

        case GST_STATE_CHANGE_NO_PREROLL:

            printf ("live sources not supported yet/n");

            return FALSE;

        default:

            break;

        }


        ret = gst_element_get_state (pipeline, NULL, NULL, 5 * GST_SECOND);

        if (ret == GST_STATE_CHANGE_FAILURE) {

            printf ("failed to play the file/n");

            return FALSE;

        }


        format = GST_FORMAT_TIME;

        gst_element_query_duration (pipeline, &format, &duration);


        if (duration != -1)

            position = duration * 1 / 100;

        else

            position = 1 * GST_SECOND;


        gst_element_seek_simple (pipeline, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH, position);

        g_object_get (G_OBJECT (sink), "last-pixbuf", &pixbuf, NULL);

        if(pixbuf) {

            width = gdk_pixbuf_get_width(pixbuf);

            height = gdk_pixbuf_get_height(pixbuf);

            gdk_pixbuf_save (pixbuf, snapshot_name, "jpeg", &error, NULL);

        }


        gst_element_set_state (pipeline, GST_STATE_NULL);

        gst_object_unref (pipeline);

2、通过appsink获取图像 与第一种方法不同的是采用了appsink插件，利用其"pull-preroll"事件来获取图像Buffer，随后把GST_BUFFER_DATA转换为需要的pixbuf。
下面贴出与方法1不同的部分代码：

       descr = g_strdup_printf ("uridecodebin uri=%s ! ffmpegcolorspace ! videoscale ! appsink name=sink", fileurl);

       ........

       g_signal_emit_by_name (sink, "pull-preroll", &buffer, NULL);


       if (buffer) {

           GstCaps *caps;

           GstStructure *s;


           caps = GST_BUFFER_CAPS (buffer);

           if (!caps) {

               printf ("could not get snapshot format/n");

               return FALSE;

        }

        s = gst_caps_get_structure (caps, 0);


        res = gst_structure_get_int (s, "width", &width);

        res |= gst_structure_get_int (s, "height", &height);


        if (!res) {

                printf ("could not get snapshot dimension/n");

                return FALSE;

        }


        pixbuf = gdk_pixbuf_new_from_data (GST_BUFFER_DATA (buffer),

                        GDK_COLORSPACE_RGB, FALSE, 8, width, height,

                        GST_BUFFER_SIZE (buffer)/height, NULL, NULL);

        ............

3、利用playbin来获取图像 利用playbin的"frame"属性来获取图像buffer。然后将buffer通过colorspace以及videoscale等转换为所需要的图像pixbuf。
totem使用的是该方法，所以此处不再赘述，详细见totem源码的gstscreenshot.c。