GStreamer 1.0 (PyGST) を使用して Web カメラからフレームをキャプチャするコードをいくつか書きました。捕獲の正確な時間を知ることは私にとって重要です。このために、v4l2src プロパティ do-timestamp を設定し、appsink を使用してバッファー PTS をテキスト ファイルに書き込みます。
ただし、タイムスタンプは単調に増加していません。たとえば、フレーム 16 のタイムスタンプは 0.88199 秒で、フレーム 17 のタイムスタンプは 0.77462 秒です。つまり、前のフレームより 0.10737 秒早くなります。(私は問題を示す図を持っていますが、それを投稿するのに必要な評判がありません。)
キャプチャされた GstBuffers の PTS が常に単調に増加するとは限らないのは正しいですか? これが正常な動作ではない場合、私が何を台無しにしたか誰か知っていますか?
Logitech c920 ウェブカメラを使用しています。フレームはカメラで h.264 エンコードされます。コードはおおよそ次のようになります。
import gi
gi.require_version('Gst', '1.0')
from gi.repository import GObject, Gst, Gtk
GObject.threads_init()
Gst.init(None)
class Webcam:
def __init__(self, video_dev='/dev/video0', fps=30):
ts_log_fname = 'webcam_timestamps.log'
vid_fname = 'webcam.mkv'
self.ts_log = open(ts_log_fname, 'w')
self.ts_log.write('video filename: %s\n '
'\nframe_number, cam_running_ts\n' % vid_fname)
self.n_frames = 0
# Create GStreamer pipline
self.pipeline = Gst.Pipeline()
# Create bus to get events from GStreamer pipeline
self.bus = self.pipeline.get_bus()
self.bus.add_signal_watch()
self.bus.connect('message::error', self.on_error)
self.bus.enable_sync_message_emission()
self.bus.connect('sync-message::element', self.on_sync_message)
###########################
# Callable function
###########################
def on_new_sample(appsink):
"""
Function called from the pipeline by appsink.
Writes the timestampes of frame capture to a log file.
"""
# Get the buffer
smp = appsink.emit('pull-sample')
buf = smp.get_buffer()
# Nanoseconds to seconds
timestamp = np.float64(1e-9) * buf.pts
self.n_frames += 1
self.ts_log.write('%d,%0.9f\n' % (self.n_frames, timestamp))
return False
###########################
# Create GStreamer elements
###########################
# Video source:
self.v4l2src0 = Gst.ElementFactory.make('v4l2src', None)
self.v4l2src0.set_property('device', video_dev)
self.v4l2src0.set_property('do-timestamp', 'true')
# Video source filters:
vid0caps = Gst.Caps.from_string('video/x-h264,width=%d,height=%d,'
'framerate=%d/1' % (1280, 720, fps))
self.vid0filter = Gst.ElementFactory.make('capsfilter', None)
self.vid0filter.set_property('caps', vid0caps)
# Parse video:
self.vid0parse = Gst.ElementFactory.make('h264parse', None)
# Split:
self.tee0 = Gst.ElementFactory.make('tee', None)
self.tee0.set_property('name', 't0')
####
# Display branch
####
# Decode
self.vid0decode = Gst.ElementFactory.make('avdec_h264', None)
# Scale to display size:
self.disp0scale = Gst.ElementFactory.make('videoscale', None)
# Display filter caps:
disp0caps = Gst.Caps.from_string('video/x-raw,width=%d,height=%d' %
(800, 600))
# Sinks:
self.disp0sink = Gst.ElementFactory.make('autovideosink', None)
self.disp0sink.set_property('filter-caps', disp0caps)
####
# File branch
####
self.mux = Gst.ElementFactory.make('matroskamux', None)
self.file0sink = Gst.ElementFactory.make('filesink', None)
self.file0sink.set_property('location', vid_fname)
self.file0sink.set_property('sync', False)
####
# Timestamp branch
####
# Create appsink
self.ts0sink = Gst.ElementFactory.make('appsink', None)
# Setting properties of appsink
ts0caps = vid0caps # use same caps as for camera
self.ts0sink.set_property('caps', ts0caps)
self.ts0sink.set_property("max-buffers", 20) # Limit memory usage
# Tell sink to emit signals
self.ts0sink.set_property('emit-signals', True)
self.ts0sink.set_property('sync', False) # No sync
# Connect appsink to my function (writing timestamps)
self.ts0sink.connect('new-sample', on_new_sample)
self.queue0 = Gst.ElementFactory.make('queue', None)
self.queue1 = Gst.ElementFactory.make('queue', None)
self.disp_queue = Gst.ElementFactory.make('queue', None)
self.file_queue = Gst.ElementFactory.make('queue', None)
self.ts_queue = Gst.ElementFactory.make('queue', None)
# Add elements to the pipeline
self.pipeline.add(self.v4l2src0)
self.pipeline.add(self.vid0filter)
self.pipeline.add(self.vid0parse)
self.pipeline.add(self.tee0)
self.pipeline.add(self.vid0decode)
self.pipeline.add(self.disp0scale)
self.pipeline.add(self.disp0sink)
self.pipeline.add(self.mux)
self.pipeline.add(self.file0sink)
self.pipeline.add(self.ts0sink)
self.pipeline.add(self.queue0)
self.pipeline.add(self.queue1)
self.pipeline.add(self.disp_queue)
self.pipeline.add(self.file_queue)
self.pipeline.add(self.ts_queue)
###############
# Link elements
###############
# video source
if not self.v4l2src0.link(self.vid0filter):
print('video source to video filter link failed')
if not self.vid0filter.link(self.vid0parse):
print('video filter to video parse link failed')
if not self.vid0parse.link(self.tee0):
print('video parse to tee link failed')
# tee
if not self.tee0.link(self.disp_queue):
print('tee to display queue link failed')
if not self.tee0.link(self.file_queue):
print('tee to file queue link failed')
if not self.tee0.link(self.ts_queue):
print('tee to ts queue link failed')
# video display sink
if not self.disp_queue.link(self.vid0decode):
print('dispaly queue to video decode link failed')
if not self.vid0decode.link(self.disp0scale):
print('decode to videoscale link failed')
if not self.disp0scale.link(self.queue0):
print('disp0scale to queue0 link failed')
if not self.queue0.link_filtered(self.disp0sink, disp0caps):
print('queue0 to display-sink link failed')
# file sink
if not self.file_queue.link(self.mux):
print('file queue to mux link failed')
if not self.mux.link(self.queue1):
print('mux to queue1 link failed')
if not self.queue1.link(self.file0sink):
print('queue1 to file-sink link failed')
# timestamp sink
if not self.ts_queue.link(self.ts0sink):
print('ts queue to ts-sink link failed')
def run(self):
self.offset_t = datetime.now().timestamp() - self.t_start
self.pipeline.set_state(Gst.State.PLAYING)
def quit(self):
self.pipeline.set_state(Gst.State.NULL)
self.ts_log.close()
def on_sync_message(self, bus, msg):
if msg.get_structure().get_name() == 'prepare-window-handle':
msg.src.set_property('force-aspect-ratio', True)
def on_error(self, bus, msg):
print('on_error():', msg.parse_error())
