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• Saving the openGL context as a video output

  2 juin 2023, par psiyum

  I am currently trying to save the animation made in openGL to a video file. I have tried using openCV's videowriter but to no advantage. I have successfully been able to generate a snapshot and save it as bmp using the SDL library. If I save all snapshots and then generate the video using ffmpeg, that is like collecting 4 GB worth of images. Not practical.
How can I write video frames directly during rendering ?
Here the code i use to take snapshots when I require :

  



  void snapshot(){&#xA;SDL_Surface* snap = SDL_CreateRGBSurface(SDL_SWSURFACE,WIDTH,HEIGHT,24, 0x000000FF, 0x0000FF00, 0x00FF0000, 0);&#xA;char * pixels = new char [3 *WIDTH * HEIGHT];&#xA;glReadPixels(0, 0,WIDTH, HEIGHT, GL_RGB, GL_UNSIGNED_BYTE, pixels);&#xA;&#xA;for (int i = 0 ; i <height>pixels) &#x2B; snap->pitch * i, pixels &#x2B; 3 * WIDTH * (HEIGHT-i - 1), WIDTH*3 );&#xA;&#xA;delete [] pixels;&#xA;SDL_SaveBMP(snap, "snapshot.bmp");&#xA;SDL_FreeSurface(snap);&#xA;}&#xA;</height>

  



  I need the video output. I have discovered that ffmpeg can be used to create videos from C++ code but have not been able to figure out the process. Please help !

  



  EDIT : I have tried using openCV CvVideoWriter class but the program crashes ("segmentation fault") the moment it is declared.Compilation shows no errors ofcourse. Any suggestions to that ?

  



  SOLUTION FOR PYTHON USERS (Requires Python2.7,python-imaging,python-opengl,python-opencv, codecs of format you want to write to, I am on Ubuntu 14.04 64-bit) :

  



  def snap():
    pixels=[]
    screenshot = glReadPixels(0,0,W,H,GL_RGBA,GL_UNSIGNED_BYTE)
    snapshot = Image.frombuffer("RGBA",W,H),screenshot,"raw","RGBA",0,0)
    snapshot.save(os.path.dirname(videoPath) + "/temp.jpg")
    load = cv2.cv.LoadImage(os.path.dirname(videoPath) + "/temp.jpg")
    cv2.cv.WriteFrame(videoWriter,load)


  



  Here W and H are the window dimensions (width,height). What is happening is I am using PIL to convert the raw pixels read from the glReadPixels command into a JPEG image. I am loading that JPEG into the openCV image and writing to the videowriter. I was having certain issues by directly using the PIL image into the videowriter (which would save millions of clock cycles of I/O), but right now I am not working on that. Image is a PIL module cv2 is a python-opencv module.

  


• Saving the openGL context as a video output

  16 septembre 2016, par activatedgeek

  I am currently trying to save the animation made in openGL to a video file. I have tried using openCV’s videowriter but to no advantage. I have successfully been able to generate a snapshot and save it as bmp using the SDL library. If I save all snapshots and then generate the video using ffmpeg, that is like collecting 4 GB worth of images. Not practical.
  How can I write video frames directly during rendering ?
  Here the code i use to take snapshots when I require :

  void snapshot(){
  
  SDL_Surface* snap = SDL_CreateRGBSurface(SDL_SWSURFACE,WIDTH,HEIGHT,24, 0x000000FF, 0x0000FF00, 0x00FF0000, 0);
  
  char * pixels = new char [3 *WIDTH * HEIGHT];
  
  glReadPixels(0, 0,WIDTH, HEIGHT, GL_RGB, GL_UNSIGNED_BYTE, pixels);
  
  
  
  for (int i = 0 ; i <height>pixels) + snap->pitch * i, pixels + 3 * WIDTH * (HEIGHT-i - 1), WIDTH*3 );
  
  
  
  delete [] pixels;
  
  SDL_SaveBMP(snap, "snapshot.bmp");
  
  SDL_FreeSurface(snap);
  
  }
  
  </height>

  I need the video output. I have discovered that ffmpeg can be used to create videos from C++ code but have not been able to figure out the process. Please help !

  EDIT : I have tried using openCV CvVideoWriter class but the program crashes ("segmentation fault") the moment it is declared.Compilation shows no errors ofcourse. Any suggestions to that ?

  SOLUTION FOR PYTHON USERS (Requires Python2.7,python-imaging,python-opengl,python-opencv, codecs of format you want to write to, I am on Ubuntu 14.04 64-bit) :

  def snap():
  
      pixels=[]
  
      screenshot = glReadPixels(0,0,W,H,GL_RGBA,GL_UNSIGNED_BYTE)
  
      snapshot = Image.frombuffer("RGBA",W,H),screenshot,"raw","RGBA",0,0)
  
      snapshot.save(os.path.dirname(videoPath) + "/temp.jpg")
  
      load = cv2.cv.LoadImage(os.path.dirname(videoPath) + "/temp.jpg")
  
      cv2.cv.WriteFrame(videoWriter,load)

  Here W and H are the window dimensions (width,height). What is happening is I am using PIL to convert the raw pixels read from the glReadPixels command into a JPEG image. I am loading that JPEG into the openCV image and writing to the videowriter. I was having certain issues by directly using the PIL image into the videowriter (which would save millions of clock cycles of I/O), but right now I am not working on that. Image is a PIL module cv2 is a python-opencv module.

• How to reduce bitrate with out change video quality in FFMPEG

  28 mai 2021, par Muthu GM

  I'm using FFMPEG C library. I'm use modified muxing.c example to encode video. Video quality is reduce frame by frame when I control bitrate (like 1080 x 720 - bitrate 680k ). But same image I using FFMPEG command line tool to encode same bitrate 680k image quality to not change.

  


  What is the reason for same image and bitrate encoded video quality reduce whan I encode using C API and reason why quality did not change command line tool.

  


  I use :

  


  Command line arg :

  


  

  • ffmpeg -framerate 5 image%d.jpg  -c:v libx264 -b:v 64k -pix_fmt yuv420p out.mp4

  


  


  Muxing.c(modified) Codec setting :

  


  

  • fps = 5;

  


  • CODEC_ID = H264 (libx264);

  


  • Pixel_fmt = yuv420;

  


  • Image decoder = MJPEG;

  


  • bitrate = 64000;

  


  


  The video size are same but quality is reduce frame by frame in muxing.c
but same bitrate video quality is perfect.

  


  please provide how to I reduce bitrate with out change quality using FFMPEG C API.