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• Video codec not supported error when adding audio to mp4

  18 février 2019, par WPMed

  I work on an application which you can use to make still photos move. It’s basically a png sequence to mp4 converter.
  Recently we introduced a feature where users can add sound effects to the video.
  Since we released this feature, some users experience problems with the exported video. When they save the file and try to play it, they get a "Video codec not supported" error message. All they get is a black screen, and the audio playing in the background. I use FFmpeg to add audio (in mp3 format) to the mp4 video.
  Here’s an example video, which plays fine on my Mac and on my Samsung Galaxy S6 and S8, but buggy on Samsung SM-A310F (Android 7.0). This is how it looks like on the device.
  I tried to re-encode this video with all the FFmpeg commands I could find, but none of them seemed to work. Can someone spot something that’s not compatible with Android by analyzing the video I linked ?

• Changing video encoding quality : Raspberry pi+v4l2 (ffmpeg or direct v4l2 API)

  24 octobre 2023, par Bill Shubert

  We are using a raspberry pi 4 to acquire video. Using the x264 library for software encoding, we were able to select a usable quality level (generally in the 16-20 range), but the compression speed was much too slow to keep up with the video feed. We switched to ffmpeg's v4l2 hardware encoder, and it's plenty fast enough to keep up with the incoming video, but now the output is poor quality, too low to be usable for our application. We have tried the ffmpeg -crf flag from 20 all the way down to 5, and it seems to have little or no effect on the video quality. I dug through the ffmpeg source code and can't find any references to "crf", "quality", or even compression rate in the v4l2 codec of ffmpeg.

  


  Is there a way to change the quality level of your encoded video using the PI's v4l2 hardware video encoder ? If so, does ffmpeg support it in some way ? Hopefully I just missed how to pass it in. If raspi+video4linux does support selecting output quality, but ffmpeg doesn't use it, then I could either patch ffmpeg or write my own code to drive v4l2, but it's not a simple API to use so I'd rather stay with ffmpeg if I can.

  


• Python OpenCV VideoCapture Color Differs from ffmpeg and Other Media Players

  17 avril 2024, par cliffsu

  I’m working on video processing in Python and have noticed a slight color difference when using cv2.VideoCapture to read videos compared to other media players.

  


  

  


  I then attempted to read the video frames directly using ffmpeg, and despite using the ffmpeg backend in OpenCV, there are still differences between OpenCV’s and ffmpeg’s output. The frames read by ffmpeg match those from other media players.

  


  

  


  Below are the videos I’m using for testing :

  


  test3.webm

  


  test.avi

  


  Here is my code :

  


  import cv2
import numpy as np
import subprocess

def read_frames(path, res):
    """Read numpy arrays of video frames. Path is the file path
       and res is the resolution as a tuple."""
    args = [
        "ffmpeg",
        "-i",
        path,
        "-f",
        "image2pipe",
        "-pix_fmt",
        "rgb24",
        "-vcodec",
        "rawvideo",
        "-",
    ]

    pipe = subprocess.Popen(
        args,
        stdout=subprocess.PIPE,
        stderr=subprocess.DEVNULL,
        bufsize=res[0] * res[1] * 3,
    )

    while pipe.poll() is None:
        frame = pipe.stdout.read(res[0] * res[1] * 3)
        if len(frame) > 0:
            array = np.frombuffer(frame, dtype="uint8")
            break

    pipe.stdout.close()
    pipe.wait()
    array = array.reshape((res[1], res[0], 3))
    array = cv2.cvtColor(array, cv2.COLOR_RGB2BGR)
    return array

ORIGINAL_VIDEO = 'test3.webm'

array = read_frames(ORIGINAL_VIDEO, (1280, 720))

cap = cv2.VideoCapture(ORIGINAL_VIDEO, cv2.CAP_FFMPEG)
while cap.isOpened():
    ret, frame = cap.read()
    if not ret:
        break
    print(frame.shape)
    cv2.imshow("Opencv Read", frame)
    cv2.imshow("FFmpeg Direct Read", array)
    cv2.waitKeyEx()
    cv2.waitKeyEx()
    break
cap.release()


  


  I’ve attempted to use different media players to compare cv2.VideoCapture and ffmpeg’s frame reading, to confirm that the issue lies with opencv. I’m looking to determine whether it’s a bug in OpenCV or if there are issues in my code.

  


  EDIT :

  


  Just use the following code to check the difference between opencv read and ffmpeg read.

  


  cv2.imshow('test', cv2.absdiff(array, frame)*10)
cv2.waitKey(0)


  


  Here is the result :