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• Slightly wrong color in MP4 videos written by PyAV

  26 septembre 2024, par Yossi Kreinin

  I am writing MP4 video files with the following PyAV-based code (getting input frames represented as numpy arrays - the sort produced by imageio.imread - as input) :

  


  class MP4:
    def __init__(self, fname, width, height, fps):
        self.output = av.open(fname, 'w', format='mp4')
        self.stream = self.output.add_stream('h264', str(fps))
        self.stream.width = width
        self.stream.height = height
        # these 2 lines can be removed and the problem still reproduces:
        self.stream.pix_fmt = 'yuv420p'
        self.stream.options = {'crf': '17'}
    def write_frame(self, pixels):
        frame = av.VideoFrame.from_ndarray(pixels, format='rgb24')
        packet = self.stream.encode(frame)
        self.output.mux(packet)
    def close(self):
        packet = self.stream.encode(None)
        self.output.mux(packet)
        self.output.close()


  


  The colors in the output MP4 video are slightly different (apparently darker) than the colors in the input images :

  


  Screen grab of an image viewer showing an input frame :

  


  

  


  Screen grab of VLC playing the output MP4 video :

  


  

  


  How can this problem be fixed ? I variously fiddled with the frame.colorspace attribute, stream options and VideoFrame.reformat but it changed nothing ; of course I could have been fiddling incorrectly.

  


  As you can see, the input has simple flat color regions, so I doubt it's any sort of compression artifact, eg YUV420 dropping some of the chroma info or other such.

  


• FFmpeg only reading one channel

  29 août 2017, par AbstractDissonance

  Trying to read a stereo wav file and ffmpeg is only reading one channel. ffprobe returns 2 channels.

  I have used -ac 2 and then added -channel_layout stereo but all return 1 channel(or basically filling half the buffer I created).

  Basically the output size of ffmpeg is about half the wav file size.

  What I would like is for it to return every channel in

  Channel1_sample1, channel2_sample1, ..., ChannelN_sample1, channel1_sample2, etc...

  But, in reality, I’d rather just have it work with stereo ;) I’m giving it plenty large enough buffer to read to, so that isn’t the problem either.

  Here is the output

  ffprobe.exe -hide_banner -v quiet -print_format flat -show_streams -i temp.wav
  
  streams.stream.0.index=0
  
  streams.stream.0.codec_name="pcm_s16le"
  
  streams.stream.0.codec_long_name="PCM signed 16-bit little-endian"
  
  streams.stream.0.profile="unknown"
  
  streams.stream.0.codec_type="audio"
  
  streams.stream.0.codec_time_base="1/44100"
  
  streams.stream.0.codec_tag_string="[1][0][0][0]"
  
  streams.stream.0.codec_tag="0x0001"
  
  streams.stream.0.sample_fmt="s16"
  
  streams.stream.0.sample_rate="44100"
  
  streams.stream.0.channels=2
  
  streams.stream.0.channel_layout="unknown"
  
  streams.stream.0.bits_per_sample=16
  
  streams.stream.0.id="N/A"
  
  streams.stream.0.r_frame_rate="0/0"
  
  streams.stream.0.avg_frame_rate="0/0"
  
  streams.stream.0.time_base="1/44100"
  
  streams.stream.0.start_pts="N/A"
  
  streams.stream.0.start_time="N/A"
  
  streams.stream.0.duration_ts=14200200
  
  streams.stream.0.duration="322.000000"
  
  streams.stream.0.bit_rate="1411200"
  
  streams.stream.0.max_bit_rate="N/A"
  
  streams.stream.0.bits_per_raw_sample="N/A"
  
  streams.stream.0.nb_frames="N/A"
  
  streams.stream.0.nb_read_frames="N/A"
  
  streams.stream.0.nb_read_packets="N/A"
  
  streams.stream.0.disposition.default=0
  
  streams.stream.0.disposition.dub=0
  
  streams.stream.0.disposition.original=0
  
  streams.stream.0.disposition.comment=0
  
  streams.stream.0.disposition.lyrics=0
  
  streams.stream.0.disposition.karaoke=0
  
  streams.stream.0.disposition.forced=0
  
  streams.stream.0.disposition.hearing_impaired=0
  
  streams.stream.0.disposition.visual_impaired=0
  
  streams.stream.0.disposition.clean_effects=0
  
  streams.stream.0.disposition.attached_pic=0
  
  streams.stream.0.disposition.timed_thumbnails=0
  
  ffmpeg.exe -i temp.wav -loglevel quiet -f s16le -ac 2 -channel_layout stereo -

  temp.wav is just a standard stereo wav file.

• Why does ffmpeg return "No such file or directory"

  4 juillet 2020, par JackNewman

  I'm trying to split a video file into 2 second increments and then merge the video back together.

  


  source_vid_path = r"C:\SplitAndMergeVids\Before\before.mp4"
ffcat_path = r'C:\SplitAndMergeVids\Chunk\video.ffcat'
chunks_path = r'C:\SplitAndMergeVids\Chunk\chunk-%03d.mp4'
segments_time = '2'
cmd_input = rf'ffmpeg -fflags +genpts -i {source_vid_path} -map 0 -c copy -f segment -segment_format mp4 -segment_time {segments_time} -segment_list {ffcat_path} -reset_timestamps 1 -v error {chunks_path}'
output = str(subprocess.run(cmd_input, shell=True, capture_output=True))
print(output)

output_path = r'C:\SplitAndMergeVids\Output\output.mp4'
second_input = rf'ffmpeg -y -v error -i {ffcat_path} -map 0 -c copy {output_path}'
output = str(subprocess.run(second_input, shell=True, capture_output=True))
print(output)


  


  First subprocess runs perfectly although the second outputs returns

  


  "Impossible to open 'chunk-000.mp4'\r\nC:\\SplitAndMergeVids\\Chunk\\video.ffcat: No such file or directory".


  


  Full output looks like

  


  CompletedProcess(args='ffmpeg -fflags +genpts -i C:\\SplitAndMergeVids\\Before\\before.mp4 -map 0 -c copy -f segment -segment_format mp4 -segment_time 2 -segment_list C:\\SplitAndMergeVids\\Chunk\\video.ffcat -reset_timestamps 1 -v error C:\\SplitAndMergeVids\\Chunk\\chunk-%03d.mp4', returncode=0, stdout=b'', stderr=b'')
CompletedProcess(args='ffmpeg -y -v error -i C:\\SplitAndMergeVids\\Chunk\\video.ffcat -map 0 -c copy C:\\SplitAndMergeVids\\Output\\output.mp4', returncode=1, stdout=b'', stderr=b"[concat @ 0000028691a6c6c0] Impossible to open 'chunk-000.mp4'\r\nC:\\SplitAndMergeVids\\Chunk\\video.ffcat: No such file or directory\r\n")


  


  When I run cmd_input and second_input manually in cmd, everything functions perfectly. I don't understand how in the first command I am making a file at "ffcat_path", then in the second command I'm using the same "ffcat_path" and it returns "No such file or directory" when it certainly does exist.