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• CGO : How do you use pointers in Golang to access data from an array in C

  23 avril 2018, par nevernew

  I’m writing an app for the windows platform using FFmpeg and it’s golang wrapper goav, but I’m having trouble understanding how to use the C pointers to gain access to the data array they point to.

  I’m trying to get the data stored in the AVFrame class and use Go to write it to a file, and eventually a texture in OpenGl to make a video player with cool transitions.

  I think understanding how to cast and access the C data will make coding this a lot easier.

  I’ve stripped out all the relevant parts of the C code, the wrapper and my code, shown below :

  C code - libavutil/frame.h

  #include 
  
  
  
  typedef struct AVFrame {
  
  #define AV_NUM_DATA_POINTERS 8
  
      uint8_t *data[AV_NUM_DATA_POINTERS];
  
  }

  Golang goav wrapper - I don’t really know whats going on here with the unsafe.Pointers and casting but it gives me access to the underlying C code

  package avutil
  
  
  
  /*
  
      #cgo pkg-config: libavutil
  
      #include <libavutil></libavutil>frame.h>
  
      #include 
  
  */
  
  import "C"
  
  import (
  
      "unsafe"
  
  )
  
  
  
  type Frame C.struct_AVFrame
  
  
  
  func AvFrameAlloc() *Frame {
  
      return (*Frame)(unsafe.Pointer(C.av_frame_alloc()))
  
  }
  
  
  
  func Data(f *Frame) *uint8 {
  
      return (*uint8)(unsafe.Pointer((*C.uint8_t)(unsafe.Pointer(&amp;f.data))))
  
  }

  My Golang code

  package main
  
  
  
  import "github.com/giorgisio/goav/avutil"
  
  
  
  func main() {
  
      videoFrame := avutil.AvFrameAlloc()
  
  
  
      data := avutil.Data(videoFrame)
  
  
  
      fmt.Println(data) // here i want the values from data[0] to data[7], but how?
  
  }

• Extacting klm data from mp2 stream using c++ and ffmpeg

  19 février 2019, par Douglas

  I have an mp2 stream that has klv metadata. I stored the klv in a file using ffmpeg command line :

  ffmpeg -i input.mpg -map data-re -codec copy -f data output.klv

  I now want to do this in c++. So, I have

  FFMPEG setup …..

  Then the main loop

  // Read frames
  
  while(av_read_frame(pFormatCtx, &packet) >= 0)
  
  {
  
      // Is this a packet from the video stream?
  
      if(packet.stream_index == videoStream)
  
      {
  
          // Decode video frame
  
          avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
  
  
  
          // Did we get a video frame?
  
          if(frameFinished)
  
          {
  
              // Convert the image from its native format to RGB
  
              sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,
  
                                  pFrame->linesize, 0, pCodecCtx->height,
  
                                  pFrameRGB->data, pFrameRGB->linesize);
  
  
  
              QImage myImage(pFrameRGB->data[0], pCodecCtx->width, pCodecCtx->height, QImage::Format_RGB888);
  
  
  
              QPixmap img(QPixmap::fromImage(myImage.scaled(ui->label->width(),ui->label->height(),Qt::KeepAspectRatio)));
  
  
  
              ui->label->setPixmap(img);
  
              QCoreApplication::processEvents();
  
          }
  
      }
  
      else // klv stream
  
      {
  
          // Decode klv data
  
          qDebug() << packet.buf->size;
  
          for(int i=0; isize; i++)
  
          {
  
              qDebug() << packet.buf->data[i];
  
          }
  
      }

  The resulting klv output is different - I must be doing something wrong processing the packet. The frames are good and I’m viewing it in a qt label - so my ffmpeg setup is working on images but not the klv data. Any ideas ? THANKS

• Extracting KLV data from mp2 stream using C++ and ffmpeg

  18 juin 2019, par Douglas

  I have an mp2 stream that has klv metadata. I stored the klv in a file using ffmpeg command line :

  ffmpeg -i input.mpg -map data-re -codec copy -f data output.klv

  I now want to do this in c++. So, I have

  FFMPEG setup …..

  Then the main loop

  // Read frames
  
  while(av_read_frame(pFormatCtx, &packet) >= 0)
  
  {
  
      // Is this a packet from the video stream?
  
      if(packet.stream_index == videoStream)
  
      {
  
          // Decode video frame
  
          avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
  
  
  
          // Did we get a video frame?
  
          if(frameFinished)
  
          {
  
              // Convert the image from its native format to RGB
  
              sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,
  
                                  pFrame->linesize, 0, pCodecCtx->height,
  
                                  pFrameRGB->data, pFrameRGB->linesize);
  
  
  
              QImage myImage(pFrameRGB->data[0], pCodecCtx->width, pCodecCtx->height, QImage::Format_RGB888);
  
  
  
              QPixmap img(QPixmap::fromImage(myImage.scaled(ui->label->width(),ui->label->height(),Qt::KeepAspectRatio)));
  
  
  
              ui->label->setPixmap(img);
  
              QCoreApplication::processEvents();
  
          }
  
      }
  
      else // klv stream
  
      {
  
          // Decode klv data
  
          qDebug() << packet.buf->size;
  
          for(int i=0; isize; i++)
  
          {
  
              qDebug() << packet.buf->data[i];
  
          }
  
      }

  The resulting klv output is different - I must be doing something wrong processing the packet. The frames are good and I’m viewing it in a qt label - so my ffmpeg setup is working on images but not the klv data.