Python gets stuck at pipe.stdin.write(image.tostring())

Question

I am reading each frame of video and adding time stamp to it as given below.

command = ['ffmpeg',
            '-y', # (optional) overwrite output file if it exists
            '-f', 'rawvideo', #Input is raw video
            '-pix_fmt', 'bgr24', #Raw video format
            '-s', str(int(width)) + 'x' + str(int(height)), # size of one frame
            '-i', '-', # The input comes from a pipe
            '-an', # Tells FFMPEG not to expect any audio
            '-vcodec', 'mpeg4',
            '-b:v', '10M', #Sets a maximum bit rate
            Output_name]
    #Open the pipe
    pipe = sp.Popen(command, stdin=sp.PIPE, stderr=sp.PIPE)

    print('Processing....')
    print(' ')
    #Reads through each frame, calculates the timestamp, places it on the frame and exports the frame to the output video.
    #import pdb
    #pdb.set_trace()
    while current_frame < total_frames:
        success, image = video.read()
        if success:
            elapsed_time = video.get(cv2.CAP_PROP_POS_MSEC)
            current_frame = video.get(cv2.CAP_PROP_POS_FRAMES)
            timestamp = initial + dt.timedelta(microseconds = elapsed_time*1000)
            cv2.putText(image, 'Date: ' + str(timestamp)[0:10], (50,int(height-150)), cv2.FONT_HERSHEY_COMPLEX_SMALL, 2, (255, 255, 255), 3)
            cv2.putText(image, 'Time: ' + str(timestamp)[11:-4], (50,int(height-100)), cv2.FONT_HERSHEY_COMPLEX_SMALL, 2, (255, 255, 255), 3)
            pipe.stdin.write(image.tostring())
            print('frame number',current_frame)
        else:
            print('video reader fail')    
    video.release()
    pipe.stdin.close()
    pipe.stderr.close()

However, after around 18k frames, Python gets stuck at 'pipe.stdin.write(image.tostring())'. It does not produce any error, but simply hangs. How to resolve this issue?

Thanks in advance.

Sounds like the stdin buffer is filling because ffmpeg is not consuming it. — jordanm, Mar 09 '20 at 18:54
@IbrahimYousuf: Currently, Python is stuck at frame 17224.0 and the 'top' is showing as in the attached Figure. — Vandana Rajan, Mar 09 '20 at 19:04
I couldn't attach in the comments. Its attached to the question. — Vandana Rajan, Mar 09 '20 at 19:22

Rotem · Accepted Answer · 2020-03-09T23:43:45.317

I think I solved the puzzle:

stderr buffer is filled up and the process gets stuck.
I managed to reproduce the problem under Windows 10.

FFmpeg writes statuses to stderr from time to time.
You are using stderr=sp.PIPE, but not reading from stderr.
After encoding many frames, the stderr buffer filled up, and the process gets stuck.

You may either remove stderr=sp.PIPE, or making sure to read the data from stderr.

Reading data from stderr may be preformed using a Thread:

# Read from pipe.stdrr for "draining the pipe"
def drain_stderr():
    while True:
        try:
            stderr_output = pipe.stderr.readline()
        except:
            pass

I created a "self contained" code sample that generates synthetic video file, and executes the code using the synthetic video as input.

Here is the testing code sample:

import numpy as np
import cv2
import subprocess as sp
import threading
import datetime as dt

# Generate synthetic video file - resolution 640x480, 30000 frames, 1 fps
# H.264 encoded video (for testing):
#########################################################################
input_name = 'test.mp4'
width, height = 640, 480
total_frames = 30000
sp.run('ffmpeg -y -f lavfi -i testsrc=size={}x{}:rate=1 -vcodec libx264 -crf 23 -t {} {}'.format(width, height, total_frames, input_name))
#########################################################################


# Read from pipe.stdrr for "draining the pipe"
def drain_stderr():
    while keep_drain_stderr:
        try:
            stderr_output = pipe.stderr.readline()
        except:
            pass


Output_name = 'out.mp4'

command = ['ffmpeg',
            '-y', # (optional) overwrite output file if it exists
            '-f', 'rawvideo', #Input is raw video
            '-pix_fmt', 'bgr24', #Raw video format
            '-s', str(int(width)) + 'x' + str(int(height)), # size of one frame
            '-i', '-', # The input comes from a pipe
            '-an', # Tells FFMPEG not to expect any audio
            '-vcodec', 'mpeg4',
            '-b:v', '10M', #Sets a maximum bit rate
            Output_name]

# Open the pipe
pipe = sp.Popen(command, stdin=sp.PIPE, stderr=sp.PIPE)

keep_drain_stderr = True
thread = threading.Thread(target=drain_stderr)
thread.start()


# Open video file for reading
video = cv2.VideoCapture(input_name)

print('Processing....')
print(' ')

#Reads through each frame, calculates the timestamp, places it on the frame and exports the frame to the output video.
#import pdb
#pdb.set_trace()
initial = dt.timedelta(microseconds=0*1000)
current_frame = 0
while current_frame < total_frames:
    success, image = video.read()
    if success:
        elapsed_time = video.get(cv2.CAP_PROP_POS_MSEC)
        current_frame = video.get(cv2.CAP_PROP_POS_FRAMES)
        timestamp = initial + dt.timedelta(microseconds=elapsed_time*1000)
        cv2.putText(image, 'Date: ' + str(timestamp)[0:10], (50,int(height-150)), cv2.FONT_HERSHEY_COMPLEX_SMALL, 2, (255, 255, 255), 3)
        cv2.putText(image, 'Time: ' + str(timestamp)[11:-4], (50,int(height-100)), cv2.FONT_HERSHEY_COMPLEX_SMALL, 2, (255, 255, 255), 3)
        pipe.stdin.write(image.tostring())

        print('frame number', current_frame)
    else:
        print('video reader fail')

keep_drain_stderr = False
video.release()
pipe.stdin.close()
pipe.stderr.close()

#Wait 3 seconds before killing FFmpeg
try:
    pipe.wait(3)
except (sp.TimeoutExpired):
    pipe.kill()

thread.join()

Yep. That was right. I simply removed stderr from the sp.Popen and it worked. Thanks a lot. — Vandana Rajan, Mar 10 '20 at 12:01

Python gets stuck at pipe.stdin.write(image.tostring())

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