GStreamer c++ - seeking before starting any playback - c++

Question

I added an start-offset option to my C++ application using the GStreamer library. It basically tells the application to start mid-video instead of playing from the beginning.
The pipeline is created like this: filesrc location="file_path" ! qtdemux ! h264parse ! mppvideodec ! waylandsink. This is what gives me best results on my hardware, and I'm running under Weston (wayland)
I do the seeking with a simple gst_element_seek on the pipeline. First I tried waiting until the pipeline changes state to PLAYING or PAUSED in the bus, and then seeking once. This worked, but on some larger videos it produced an effect that after launching the app, the first video frame is displayed for a glimpse of a second and then the video actually jumps to the desired position. I don't want that, I'd like to jump right to where I want without displaying anything else earlier.
So I changed my seeking method to happen before I even set the pipeline to PLAYING state like this:
// seek if needed
if(startTime > 0) {
// First set pipeline to PAUSED and wait for async state change
gst_element_set_state(m_pipeline, GST_STATE_PAUSED);
gst_element_get_state(m_pipeline, nullptr, nullptr, GST_CLOCK_TIME_NONE);
if (!gst_element_seek (m_pipeline, 1.0, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH, GST_SEEK_TYPE_SET, startTime, GST_SEEK_TYPE_NONE, -1)) {
g_print ("Seek failed!\n");
}
gst_element_get_state(m_pipeline, nullptr, nullptr, GST_CLOCK_TIME_NONE); // wait for seek to finish and then set to PLAYING
}
m_ret = gst_element_set_state(m_pipeline, GST_STATE_PLAYING);
if (m_ret == GST_STATE_CHANGE_FAILURE) {
g_printerr("Unable to set the pipeline to the playing state.\n");
gst_object_unref(m_pipeline);
exit(0);
}
And while this also works, it unfortunately produces the exact same effect as before...the first frame is still being displayed even though the seek happens before the pipeline is started.
How can I ensure that nothing is displayed until the seek is actually done?
Update
I couldn't get any method of seeking to work like I wanted, but I achieved the effect by setting the show-preroll-frame on the GstVideoSink before constructing the pipeline, like that:
m_sink = gst_element_factory_make("waylandsink", "sink");
g_object_set(m_sink, "show-preroll-frame", FALSE, NULL); // prevent preroll frame when seeking
And it works! I'll use it for now if there's no better way. The seek is still done in PAUSED state, but video is displayed directly from the seeking point.

Answer 1

As the documentation of the gst_element_seek_simple method says:
Some elements allow for seeking in the READY state...
This is true if the pipeline is completely prerolled before the seek.
Then you can get the result you expect, showing only the frames you expect.
gst_element_set_state(m_pipeline, GST_STATE_READY);
//make sure all the pipeline is prerolled (dynamic pads linked and other stuff)
if (!gst_element_seek (m_pipeline, 1.0, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH, GST_SEEK_TYPE_SET, startTime, GST_SEEK_TYPE_NONE, -1)) {
g_print ("Seek failed!\n");
}
gst_element_get_state(m_pipeline, nullptr, nullptr, GST_CLOCK_TIME_NONE);