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• Controlling end time in video player via AVPacket information / setting pts/dts properly

  26 janvier 2017, par SyntheticGio

  I’m currently working in C/C++ using the FFMPEG core (libavcodec, etc.). I’m capturing a stream and writing it in chunks to different files. So imagine the stream is 5 minutes in length and I’m writing five files of one minute in length each. I’m able to do this successfully.

  Currently, each file after the first file has a start time equal to the time it would have been in the un-chunked stream. So the second video file starts at 1 minute, the third starts at 2 minutes, etc. This was inadvertent but as it turns out is beneficial in my particular use case.

  VLC or other video players that I’ve tried report this start time ’properly’, but the end time shows as the duration (not start time + duration). My gut feeling is that the player simply is making the assumption all videos start at 0 and it shows the length as the ’end time’ but I don’t actually know this so I’d like to know if there is anyway to set the AVPacket information so the player for the third video would start at 2 minutes and end at 3 minutes (for a 1 minute length video) - as an example ?

  As an alternative, if I wanted to do this the traditional way (reset each chunk to starting at time 0), I assume I’d normalize the AVPacket.pts and AVPacket.dts by subtracting the values of the final packet in the previous chunk ? This seems like this strategy would work for pts but I’m less sure about it working for dts. I feel like it would generally work for dts but there might be times when this fails, so I’d like to know if this is a safe method (or if there is a better method I should use in this case).

• Rescaling and slowing down a movie at the same time with ffmpeg

  21 décembre 2016, par ejl62

  I would like with ffmpeg to slow down a movie I am creating using the flag :

  -filter:v "setpts=2.0*PTS"

  However the height of my still images is not divisible by 2, so to avoid the error : height not divisible by 2 (1238x833), I am using the flag :

  -vf scale="trunc(iw/2)*2:trunc(ih/2)*2"

  (I also tried -vf scale=1238:-2).

  When I do this the film is generated but it isn’t slowed down, like if the -filter:v "setpts=2.0*PTS" wasn’t there.

  Is there something particular to do in order to have both option working at the same time ?

  Here is the complete command I am using :

  ffmpeg -an -i ./movie/cphmd1.%05d.ppm -vcodec libx264 -pix_fmt yuv420p -b:v 5000k -r 24 -crf 18 -filter:v "setpts=2.0*PTS" -vf scale="trunc(iw/2)*2:trunc(ih/2)*2" -preset slow -f mp4 cphmd1_slower.mp4

  Many thanks in advance !

• QSharedMemory in Real-Time process

  21 novembre 2016, par Seungsoo Kim

  I’m trying to use QSharedMemory Class to share video data between two processes.

  So I tried like following method, but it has problem in simultaneous access of two processes.

  Two process crashes when they access sequentially to same memory name(key) "SharedMemory".

  I locked them while they’re used, but also it doesn’t work well.

  How can i avoid this crash ??

  1. Writing to SharedMemory - data type is and this function called by callback.

     QBuffer buffer;
     
     buffer.open(QBuffer::ReadWrite);
     
     QDataStream out(&buffer);
     
     
     
     QByteArray outArray = QByteArray::fromRawData(reinterpret_cast<const>(data), strlen(reinterpret_cast<const>(data)));
     
     out &lt;&lt; width &lt;&lt; height &lt;&lt; step &lt;&lt; cameraId &lt;&lt; strlen(reinterpret_cast<const>(data));
     
     out.writeRawData(outArray.data(), outArray.size());
     
     
     
     int size = buffer.size();
     
     
     
     sharedMemory.setKey("SharedMemory");
     
     
     
     if (!sharedMemory.isAttached()) {
     
         printf("Cannot attach to shared memory to update!\n");
     
     }
     
     if (!sharedMemory.create(size))
     
     {
     
         printf("failed to allocate memory\n");
     
     }
     
     sharedMemory.lock();
     
     char *to = (char*)sharedMemory.data();
     
     const char *from = buffer.data().data();
     
     memcpy(to, from,qMin(sharedMemory.size(),size));
     
     sharedMemory.unlock();
     
     </const></const></const>

  2. Using data in SharedMemory. - this function is called by QThread, interval 100ms

     QSharedMemory sharedMemory("SharedMemory");
     
     sharedMemory.lock();
     
     if (!sharedMemory.attach()) {
     
         printf("failed to attach to memory\n");
     
         return;
     
     }
     
     
     
     QBuffer buffer;
     
     QDataStream in(&buffer);
     
     
     
     sharedMemory.create(1920 * 1080);
     
     buffer.setData((char*)sharedMemory.constData(), sharedMemory.size());
     
     buffer.open(QBuffer::ReadOnly);
     
     sharedMemory.unlock();
     
     sharedMemory.detach();
     
     
     
     int r_width = 0;    
     
     int r_height = 0;
     
     int r_cameraId = 0;
     
     int r_step = 0;
     
     int r_strlen = 0;
     
     in >> r_width >> r_height >> r_step >> r_cameraId >> r_strlen;
     
     
     
     char* receive = new char[r_strlen];
     
     in.readRawData(receive, r_strlen);
     
     //unsigned char* r_receive = new unsigned char[r_strlen];
     
     //r_receive = (unsigned char*)receive;
     
     
     
     QPixmap backBuffer = QPixmap::fromImage(QImage((unsigned char*)receive, r_width, r_height, r_step, QImage::Format::Format_RGB888));
     
     ui.label->setPixmap(backBuffer.scaled(ui.label->size(), Qt::KeepAspectRatio));
     
     ui.label->show();

  please share your idea ! thank you !