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• CGO : How do I write to a file in Golang using a pointer to the C data ?

  24 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 an array.

  I’m trying to write the frame data, pointed to by a uint8 pointer from C, to a .ppm file in golang.

  Once I have this done, for proof of concept that FFmpeg is doing what I expect it to, I want to set the frames to a texture in OpenGl to make a video player with cool transitions ; any pointers to do that nice and efficiently would be so very helpful ! I’m guessing I need to write some shader code to draw the ppm as a texture...

  I’m starting to understanding how to cast the pointers between C and Go types, but how can I access the data and write it in Go with the same result as C ? In C I just have to set the pointer offset for the data and state how much of it to write :

  for (y = 0; y < height; y++) {
  
      fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);
  
  }

  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];
  
      int linesize[AV_NUM_DATA_POINTERS];
  
  }

  Golang goav wrapper

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

  My Golang code

  package main
  
  
  
  import "github.com/giorgisio/goav/avutil"
  
  
  
  func saveFrame(videoFrame *avutil.Frame, width int, height int, iFrame int) {
  
      var szFilename string
  
      var y int
  
      var file *os.File
  
      var err error
  
  
  
      szFilename = ""
  
  
  
      // Open file
  
      szFilename = fmt.Sprintf("frame%d.ppm", iFrame)
  
  
  
      if file, err = os.Open(szFilename); err != nil {
  
          log.Println("Error Reading")
  
      }
  
  
  
      // Write header
  
      fh := []byte(fmt.Sprintf("P6\n%d %d\n255\n", width, height))
  
      file.Write(fh)
  
      var b byte = 0
  
      // Write pixel data
  
      for y = 0; y < height; y++ {
  
          d := avutil.Data(videoFrame) // d should be a pointer to the first byte of data
  
          l := avutil.Linesize(videoFrame)
  
  
  
          // I'm basically lost trying to figure out how to write this to a file
  
          data := make([]byte, width*3)
  
  
  
          addr := int(*d) + y*l // figure out the address
  
  
  
          for i := 0; i < l; i++ {
  
              // This is where I'm having the problem, I get an "invalid
  
              // memory address or nil pointer dereference" error
  
              byteArrayPtr := (*byte)(unsafe.Pointer(uintptr(addr) + uintptr(i)*unsafe.Sizeof(b)))
  
              data = append(data, *byteArrayPtr)
  
              fmt.Println(*byteArrayPtr)
  
          }
  
          file.Write(data)
  
      }
  
  
  
      file.Close()
  
  }

  So, how can I write to a file using a pointer to the data, like you can do in C ?

• Possible to stream video over 115kbps ?

  15 mai 2014, par Arcadio Alivio Sincero

  I need some advice from people experienced with streaming video.

  I have a task to put together a system that allows video coming from RS-170 (composite) video cameras and have them displayed on an iPad. The catch is that no wireless (no Wi-Fi, no bluetooth) is allowed. Only a wired interface.

  The physical I/O options on an iPad are apparently extremely limited, but I did manage to come across a company named Redpark that makes an RS232-to-Lightning cable. So my proposed solution is to have the video feeds go into a box with software that digitizes and encodes the video, and then sends it over RS232 to the iPad using that cable. The catch here is that the maximum bandwidth on that cable is 115kbps.

  My preliminary testing of this setup on a prototype system have been less than stellar so far. I set up two PCs, each with serial ports, and hooked them together with a null modem. I then set the baud rates of the ports to 115kpbs and then attempted to stream a web cam video feed over the serial connection in real-time using ffmpeg. The results weren’t very encouraging, but I at least did manage to get some sort of image to show up.

  I guess I need to play around with the ffmpeg encoding options some more. But I need to ask : am I wasting my time with this idea, or should what I am asking here be possible ?

• Running a py script in the Cloud

  12 janvier 2018, par Anay Bose

  I’m new to Google’s cloud & Virtual Machine(VM) instances, and I need some clarifications on a couple of points. I have a python script ; it imports a long range of functions. I need to run those functions in parallel. I’m using multiprocessing and Process, not threads. These functions are basically image and media processors, and they use many other tools like FFMPEG, imagemagick and Avisynth in addition to a wide range of python modules, including moviepy. Now, I would like to run some 50 functions in parallel assigning a CPU for each process. Images, media and avi files are stored in seperate folders. I’m on Windows7 Core-i7 machine. So, need cloud computing power.

  Now, my question can I run such a python script/app in the cloud that requires a very complicated file system and non-python tools i.e. ffmpeg, avisynth and avi files ?

  Can Google VMs emulate my local machine and empower me with more cores and memory to run such a program ? if not, then what are my options ? Is their any tutorials that I can follow ? I need your suggestions. I have given below an example script and some codes to help facilitate your understanding about my situation.

  from __future__ import unicode_literals
  
  import youtube_dl
  
  import os
  
  import time
  
  import sys
  
  reload(sys)  
  
  sys.setdefaultencoding('utf-8')
  
  from multiprocessing import Process
  
  from utils import *
  
  
  
  from clip31 import VIDEO31
  
  from clip32 import VIDEO32
  
  from clip189 import VIDEO189
  
  from clip16 import VIDEO16
  
  from clip39 import VIDEO39
  
  
  
  
  
  if __name__== '__main__':
  
  
  
      # 1. CALLING A FUNCTION
  
      folder = "bodyforce3\\16"
  
      serial = "16"
  
      images = get_filepaths("../16")
  
      videos = get_filepaths("12__media")
  
      pngs = get_filepaths("../pngs")
  
  
  
      Process(target=VIDEO192, args=(folder, serial, color1, color2, color3, images, videos)).start()
  
  
  
  
  
      # 2. CALLING A FUNCTION
  
      folder = "bodyforce3\\20"
  
      serial = "20"
  
      images = get_filepaths("../20")
  
      videos = get_filepaths("18__media")
  
  
  
      Process(target=VIDEO32, args=(folder, serial, color1, color2, color3, images, videos)).start()
  
  
  
  
  
      # 3. CALLING A FUNCTION
  
      folder = "bodyforce3\\14"
  
      serial = "14"
  
      images = get_filepaths("../14")
  
      videos = get_filepaths("16__media")
  
  
  
      Process(target=VIDEO91, args=(folder, serial, color1, color2, color3, images, videos)).start()

  I copy avi files in functions like this :

  src = "clip50_files"
  
  src_files = os.listdir(src)
  
  for file_name in src_files:
  
      full_file_name = os.path.join(src, file_name)
  
      if (os.path.isfile(full_file_name)):
  
          shutil.copy(full_file_name, folder)

  I call ffmpeg commands like this, and they are included within py functions.

  ###########################
  
  #### FFMPEG OPERATIONS ####
  
  ###########################
  
  
  
  print "Starting FFMPEG operations ..."
  
  
  
  if os.path.isfile(os.path.join(folder, "bounce-(3).avi")):
  
      os.remove(os.path.join(folder, "bounce-(3).avi"))
  
  
  
  
  
  infile = folder + "/bounce-(3).avs"
  
  outfile = folder + "/bounce-(3).avi"
  
  codec = "rawvideo"
  
  pix_fmt = "bgra"
  
  
  
  try:
  
      subprocess.call(["ffmpeg",
  
                       "-i" ,infile, 
  
                       "-c:v" ,codec, 
  
                       "-pix_fmt", pix_fmt, 
  
                       outfile],
  
                      stdout=open(os.devnull, 'w'), 
  
                      stderr=subprocess.STDOUT)
  
  except subprocess.CalledProcessError as e:   
  
      #except subprocess.CalledProcessError as e:
  
      sys.exit(e.output)
  
  except OSError as e:
  
      sys.exit(e.strerror)
  
  
  
  
  
  print "FFMPEG operations ended"