Display stream with FFmpeg, python and opencv

Situation : I have a basler camera connected to a raspberry pi, and I am trying to livestream it’s feed with FFmpg to a tcp port in my windows PC in order to monitor whats happening in front of the camera.

Things that work : I manage to set up a python script on the raspberry pi which is responsible for recording the frames, feed them to a pipe and streaming them to a tcp port. From that port, I am able to display the stream using FFplay.

My problem : FFplay is great for testing out quickly and easily if the direction you are heading is correct, but I want to “read” every frame from the stream, do some processing and then displaying the stream with opencv. That, I am not able to do yet.

Minimaly reprsented, that’s the code I use on the raspberry pi side of things :

command = ['ffmpeg',
           '-y',
           '-i', '-',
           '-an',
           '-c:v', 'mpeg4',
           '-r', '50',
           '-f', 'rtsp',
           '-rtsp_transport',
           'tcp','rtsp://192.168.1.xxxx:5555/live.sdp']

p = subprocess.Popen(command, stdin=subprocess.PIPE) 

while camera.IsGrabbing():  # send images as stream until Ctrl-C
    grabResult = camera.RetrieveResult(100, pylon.TimeoutHandling_ThrowException)
    
    if grabResult.GrabSucceeded():
        image = grabResult.Array
        image = resize_compress(image)
        p.stdin.write(image)
    grabResult.Release() 

On my PC if I use the following FFplay command on a terminal, it works and it displays the stream in real time :

ffplay -rtsp_flags listen rtsp://192.168.1.xxxx:5555/live.sdp?tcp

On my PC if I use the following python script, the stream begins, but it fails in the cv2.imshow function because I am not sure how to decode it:

import subprocess
import cv2

command = ['C:/ffmpeg/bin/ffmpeg.exe',
           '-rtsp_flags', 'listen',
           '-i', 'rtsp://192.168.1.xxxx:5555/live.sdp?tcp?', 
           '-']

p1 = subprocess.Popen(command, stdin=subprocess.PIPE, stdout=subprocess.PIPE)

while True:
    frame = p1.stdout.read()
    cv2.imshow('image', frame)
    cv2.waitKey(1)

Does anyone knows what I need to change in either of those scripts in order to get i to work?

Thank you in advance for any tips.

Answer

We may read the decoded frames from p1.stdout, convert it to NumPy array, and reshape it.

• Change command to get decoded frames in rawvideo format and BGR pixel format:

   command = ['C:/ffmpeg/bin/ffmpeg.exe',
              '-rtsp_flags', 'listen',
              '-i', 'rtsp://192.168.1.xxxx:5555/live.sdp?tcp?',
              '-f', 'rawvideo',      # Get rawvideo output format.
              '-pix_fmt', 'bgr24',   # Set BGR pixel format
              'pipe:']               # Use stdout as output
  

• Read the raw video frame from p1.stdout:

   raw_frame = p1.stdout.read(width*height*3)
  

• Convert the bytes read into a NumPy array, and reshape it to video frame dimensions:

   frame = np.frombuffer(raw_frame, np.uint8)
   frame = frame.reshape((height, width, 3))
  

Now we can show the frame by calling cv2.imshow('image', frame).

The solution assumes, we know the video frame size (width and height) from advance.

The code sample below, includes a part that reads width and height using cv2.VideoCapture, but I am not sure if it’s going to work in your case (due to '-rtsp_flags', 'listen'. (If it does work, you can try capturing using OpenCV instead of FFmpeg).

The following code is a complete “working sample” that uses public RTSP Stream for testing:

import cv2
import numpy as np
import subprocess

# Use public RTSP Stream for testing
in_stream = 'rtsp://wowzaec2demo.streamlock.net/vod/mp4:BigBuckBunny_115k.mp4'

if False:
    # Read video width, height and framerate using OpenCV (use it if you don't know the size of the video frames).

    # Use public RTSP Streaming for testing:
    cap = cv2.VideoCapture(in_stream)

    framerate = cap.get(5) #frame rate

    # Get resolution of input video
    width  = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
    height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))

    # Release VideoCapture - it was used just for getting video resolution
    cap.release()
else:
    # Set the size here, if video frame size is known
    width = 240
    height = 160


command = ['C:/ffmpeg/bin/ffmpeg.exe', # Using absolute path for example (in Linux replacing 'C:/ffmpeg/bin/ffmpeg.exe' with 'ffmpeg' supposes to work).
           #'-rtsp_flags', 'listen',   # The "listening" feature is not working (probably because the stream is from the web)
           '-rtsp_transport', 'tcp',   # Force TCP (for testing)
           '-max_delay', '30000000',   # 30 seconds (sometimes needed because the stream is from the web).
           '-i', in_stream,
           '-f', 'rawvideo',           # Video format is raw video
           '-pix_fmt', 'bgr24',        # bgr24 pixel format matches OpenCV default pixels format.
           '-an', 'pipe:']

# Open sub-process that gets in_stream as input and uses stdout as an output PIPE.
ffmpeg_process = subprocess.Popen(command, stdout=subprocess.PIPE)

while True:
    # Read width*height*3 bytes from stdout (1 frame)
    raw_frame = ffmpeg_process.stdout.read(width*height*3)

    if len(raw_frame) != (width*height*3):
        print('Error reading frame!!!')  # Break the loop in case of an error (too few bytes were read).
        break

    # Convert the bytes read into a NumPy array, and reshape it to video frame dimensions
    frame = np.frombuffer(raw_frame, np.uint8).reshape((height, width, 3))

    # Show the video frame
    cv2.imshow('image', frame)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
  

ffmpeg_process.stdout.close()  # Closing stdout terminates FFmpeg sub-process.
ffmpeg_process.wait()  # Wait for FFmpeg sub-process to finish

cv2.destroyAllWindows()

Sample frame (just for fun):

---

Update:

Reading width and height using FFprobe:

When we don’t know the video resolution from advance, we may use FFprobe for getting the information.

Here is a code sample for reading width and height using FFprobe:

import subprocess
import json

# Use public RTSP Stream for testing
in_stream = 'rtsp://wowzaec2demo.streamlock.net/vod/mp4:BigBuckBunny_115k.mp4'

probe_command = ['C:/ffmpeg/bin/ffprobe.exe',
                 '-loglevel', 'error',
                 '-rtsp_transport', 'tcp',  # Force TCP (for testing)]
                 '-select_streams', 'v:0',  # Select only video stream 0.
                 '-show_entries', 'stream=width,height', # Select only width and height entries
                 '-of', 'json', # Get output in JSON format
                 in_stream]

# Read video width, height using FFprobe:
p0 = subprocess.Popen(probe_command, stdout=subprocess.PIPE)
probe_str = p0.communicate()[0] # Reading content of p0.stdout (output of FFprobe) as string
p0.wait()
probe_dct = json.loads(probe_str) # Convert string from JSON format to dictonary.

# Get width and height from the dictonary
width = probe_dct['streams'][0]['width']
height = probe_dct['streams'][0]['height']