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• Extracting each individual frame from an H264 stream for real-time analysis with OpenCV

  5 mai 2017, par exclmtnpt

  Problem Outline

  I have an h264 real-time video stream (I’ll call this "the stream") being captured in Process1. My goal is to extract each frame from the stream as it comes through and use Process2 to analyze it with OpenCV. (Process1 is nodejs, Process2 is Python)

  Things I’ve tried, and their failure modes :

  • Send the stream directly from one Process1 to Process2 over a named fifo pipe :

  I succeeded in directing the stream from Process1 into the pipe. However, in Process2 (which is Python) I could not (a) extract individual frames from the stream, and (b) convert any extracted data from h264 into an OpenCV format (e.g. JPEG, numpy array).

  I had hoped to use OpenCV’s VideoCapture() method, but it does not allow you to pass a FIFO pipe as an input. I was able to use VideoCapture by saving the h264 stream to a .h264 file, and then passing that as the file path. This doesn’t help me, because I need to do my analysis in real time (i.e. I can’t save the stream to a file before reading it in to OpenCV).

  • Pipe the stream from Process1 to FFMPEG, use FFMPEG to change the stream format from h264 to MJPEG, then pipe the output to Process2 :

  I attempted this using the command :

  cat pipeFromProcess1.fifo | ffmpeg -i pipe:0 -f h264 -f mjpeg pipe:1 | cat > pipeToProcess2.fifo

  The biggest issue with this approach is that FFMPEG takes inputs from Process1 until Process1 is killed, and only then does Process2 begin to receive the data.

  Additionally, on the Process2 side, I still don’t understand how to extract individual frames from the data coming over the pipe. I open the pipe for reading (as "f") and then execute data = f.readline(). The size of data varies drastically (some reads have length on the order of 100, others length on the order of 1,000). When I use f.read() instead of f.readline(), the length is much larger, on the order of 100,000.

  If I were to know that I was getting the correct size chunk of data, I would still not know how to transform it into an OpenCV-compatible array because I don’t understand the format it’s coming over in. It’s a string, but when I print it out it looks like this :

  ��_M 0A0����tQ,\%��e���f/�H�#Y�p�f#�Kus�} F����ʳa�G������+$x�%V�� }[����Wo �1’̶A���c����*�&=Z^�o’��Ͽ� SX-ԁ涶V&H|��$
   ��<�E�� ��>�����u���7�����cR� �f�=�9 ��fs�q�ڄߧ�9v�]�Ӷ���& gr]�n�IRܜ�檯����
  � ����+ �I��w�}� ��9�o��� �w��M�m���IJ ��� �m�=�Soՙ}S �>j �,�ƙ�’���tad =i ��WY�FeC֓z �2�g� ;EXX��S��Ҁ*, ���w� _|�&�y��H��=��)� ���Ɗ3@ �h���Ѻ�Ɋ��ZzR`��)�y�� c�ڋ.��v� !u���� �S�I#�$9R�Ԯ0py z ��8 #��A�q�� �͕� ijc �bp=��۹ c SqH

  Converting from base64 doesn’t seem to help. I also tried :

  array = np.fromstring(data, dtype=np.uint8)

  which does convert to an array, but not one of a size that makes sense based on the 640x368x3 dimensions of the frames I’m trying to decode.

  • Using decoders such as Broadway.js to convert the h264 stream :

  These seem to be focused on streaming to a website, and I did not have success trying to re-purpose them for my goal.

  Clarification about what I’m NOT trying to do :

  I’ve found many related questions about streaming h264 video to a website. This is a solved problem, but none of the solutions help me extract individual frames and put them in an OpenCV-compatible format.

  Also, I need to use the extracted frames in real time on a continual basis. So saving each frame as a .jpg is not helpful.

  System Specs

  Raspberry Pi 3 running Raspian Jessie

  Additional Detail

  I’ve tried to generalize the problem I’m having in my question. If it’s useful to know, Process1 is using the node-bebop package to pull down the h264 stream (using drone.getVideoStream()) from a Parrot Bebop 2.0. I tried using the other video stream available through node-bebop (getMjpegStream()). This worked, but was not nearly real-time ; I was getting very intermittent data streams. I’ve entered that specific problem as an Issue in the node-bebop repository.

  Thanks for reading ; I really appreciate any help anyone can give !

• libavcodec resample pcm from 16bit to 24bit

  24 juillet 2014, par markus_p

  I have some basic questions regarding libavcodec.

  My task is to create pcm_s24le audio files stored in the wav container.
  Unfortunately I don’t understand precisely what needs to happen to do this. I can easily decode all my audio tracks with avcodec_decode_audio4. When I store them into a rawfile and convert them then they play fine as long as the input bit depth is already 24bit.

  For instance :

  input file 24bit file -> avcodec_decode_audio4 -> avcodec_encode_audio2(with CODEC_ID_PCM_S24LE) -> convert output file with ffmpeg -f pcm_s24le ... -> play with vlc -> works !

  input file 16bit -> avcodec_decode_audio4 -> avcodec_encode_audio2w(with CODEC_ID_PCM_S24LE) -> convert output file wiht ffmpeg -f pcm_s24le ... -> Play with vlc -> white noise !!

  I don’t understand what I have to do. Do I need to use the swresample library ? But it seems as swresample only converts the format and the sample_rate but not the "codec" (thus to pcm_s24le)

  I would appreciate if someone could tell me the steps involved. And maybe also enlighten me a bit what for instance the output format of avcodec_decode_audio4 is. I assume pcm_s16le. Is that correct ? Or does it depend on the input format ?

  Regards
  Markus

• Making changes on a video before a download [on hold]

  1er février 2016, par Chris Russo

  We have a website with lots of videos (>10,000) and lots of users (>50,000). Each video might be around 1,2 Gbs.

  The goal is to generate a unique video for each one of the members or users of the website making a small variation either on the audio track of the video, or the keyframes on the video itself, using low frequency beeps, or a few random pixels on specific keyframes.

  I understand that this operation can be done using SoX for the audio track, or FFmpeg in the case of the video, manipulating the keyframes.

  Here’s my question, as we have to choose between 2 options at this point :

  In terms of resources : would it be convenient or even possible to generate in real time a clone of the video and make this variations at the moment that the user’s request the download (please considerate the big volume of videos and users).

  Or it would be convenient to face this problem using HDD, pre-generating and storing the already manipulated videos ready to be served ?

  I understand that storing all this data will require a few many Tera’s but I’m not sure about how much RAM and Microprocessing could be demanded to perform this operations in real time, over videos that might be as big as 1Gbs.