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• Broken output from libavcodec/swscale, depending on resolution

  3 juin 2014, par dtumaykin

  I am writing a video conference software, I have a H.264 stream decoded with libavcoded into IYUV and than rendered into a window with VMR9 in windowless mode. I use a DirectShow graph to do so.

  To avoid unnecessary conversion into RGB and back (see link), I convert IYUV video into YUY2 before passing it to VMR9, with libswscale.

  I noticed that with video resolution of 848x480, output video is broken, so I investigated further and came up that for some resolutions video is always broken. To exclude the libswscale from elaboration, I added support for IYUV+padding to IYUV conversion, and it worked, with all resolutions.

  Still, I was willing to avoid slow IYUV, so I implemented support for NV12 (with libswscale) and YV12 (manually, essentially the same as IYUV). After doing some tests on two different computers, I came up with strange results.

  resolution  YUY2    NV12    IYUV    YV12
  
  PC 1 (my laptop)                
  
  640x360     ok      broken  ok      broken
  
  848x480     broken  broken  ok      broken
  
  960x540     broken  broken  ok      broken
  
  1024x576    ok      ok      ok      ok
  
  1280x720    ok      ok      ok      broken
  
  1920x1080   ok      broken  ok      broken
  
  
  
  PC 2                
  
  640x360     ok      ok      ok      ok
  
  848x480     ok      broken  ok      broken
  
  960x540     ok      ok      ok      ok
  
  1024x576    ok      ok      ok      ok
  
  1280x720    ok      broken  ok      ok
  
  1920x1080   ok      ok      ok      ok

  To exclude VMR9 fault, I substituted it with EVR, but with same results.

  I know that padding is needed for memory alignment, and that the size of padding depends on CPU used (libavcodec doc), that may explain difference between two computers(first has Intel i7-3820QM, the second Intel Core 2 Quad Q6600). I suppose it has something to do with padding, because images are corrupted in certain way.
  
  You can see my blue t-shirt in lower part of image, and my face in the upper one.

  To follow is the code for the conversion. NV12 and YUY2 conversions are performed with libswscale, while IYUV and YV12 manually.

  int pixels = _outputFrame->width * _outputFrame->height;
  
  if (_outputFormat == "YUY2") {
  
      int stride = _outputFrame->width * 2;
  
      sws_scale(_convertCtx, _outputFrame->data, _outputFrame->linesize, 0, _outputFrame->height, &out, &stride);
  
  }
  
  else if (_outputFormat == "NV12") {
  
      int stride[] = { _outputFrame->width, _outputFrame->width };
  
      uint8_t * dst[] = { out, out + pixels };
  
      sws_scale(_convertCtx, _outputFrame->data, _outputFrame->linesize, 0, _outputFrame->height, dst, stride);
  
  }
  
  else if (_outputFormat == "IYUV") { // clean ffmpeg padding
  
      for (int i = 0; i < _outputFrame->height; i++) // copy Y
  
          memcpy(out + i * _outputFrame->width, _outputFrame->data[0] + i * _outputFrame->linesize[0] , _outputFrame->width);
  
      for (int i = 0; i < _outputFrame->height / 2; i++) // copy U
  
          memcpy(out + pixels + i * _outputFrame->width / 2, _outputFrame->data[1] + i * _outputFrame->linesize[1] , _outputFrame->width / 2);            
  
      for (int i = 0; i < _outputFrame->height / 2; i++) // copy V
  
          memcpy(out + pixels + pixels/4 + i * _outputFrame->width / 2, _outputFrame->data[2] + i * _outputFrame->linesize[2] , _outputFrame->width / 2);
  
  } 
  
  else if (_outputFormat == "YV12") { // like IYUV, but U is inverted with V plane
  
      for (int i = 0; i < _outputFrame->height; i++) // copy Y
  
          memcpy(out + i * _outputFrame->width, _outputFrame->data[0] + i * _outputFrame->linesize[0], _outputFrame->width);
  
      for (int i = 0; i < _outputFrame->height / 2; i++) // copy V
  
          memcpy(out + pixels + i * _outputFrame->width / 2, _outputFrame->data[2] + i * _outputFrame->linesize[2], _outputFrame->width / 2);
  
      for (int i = 0; i < _outputFrame->height / 2; i++) // copy U
  
          memcpy(out + pixels + pixels / 4 + i * _outputFrame->width / 2, _outputFrame->data[1] + i * _outputFrame->linesize[1], _outputFrame->width / 2);
  
  }

  out is an output buffer. _outputFrame is libavcodec output AVFrame. _convertCtx is initialized as follows.

  if (_outputFormat == "YUY2")
  
      _convertCtx = sws_getContext(_width, _height, AV_PIX_FMT_YUV420P,
  
                                   _width, _height, AV_PIX_FMT_YUYV422, SWS_FAST_BILINEAR, nullptr, nullptr, nullptr);
  
  else if (_outputFormat == "NV12")
  
      _convertCtx = sws_getContext(_width, _height, AV_PIX_FMT_YUV420P,
  
                                   _width, _height, AV_PIX_FMT_NV12, SWS_FAST_BILINEAR, nullptr, nullptr, nullptr);

  Questions :

  1. Are manual conversions correct ?
  2. Are my assumptions correct ?
  3. Is previous two answers are positive, where is the problem ? And especially...
  4. Why it presents only with some resolutions and not others ?
  5. What additional info can I provide ?

• Neutral net or neutered

  4 juin 2013, par Mans — Law and liberty

  In recent weeks, a number of high-profile events, in the UK and elsewhere, have been quickly seized upon to promote a variety of schemes for monitoring or filtering Internet access. These proposals, despite their good intentions of protecting children or fighting terrorism, pose a serious threat to fundamental liberties. Although at a glance the ideas may seem like a reasonable price to pay for the prevention of some truly hideous crimes, there is more than first meets the eye. Internet regulation in any form whatsoever is the thin end of a wedge at whose other end we find severely restricted freedom of expression of the kind usually associated with oppressive dictatorships. Where the Internet was once a novelty, it now forms an integrated part of modern society ; regulating the Internet means regulating our lives.

  Terrorism

  Following the brutal murder of British soldier Lee Rigby in Woolwich, attempts were made in the UK to revive the controversial Communications Data Bill, also dubbed the snooper’s charter. The bill would give police and security services unfettered access to details (excluding content) of all digital communication in the UK without needing so much as a warrant.

  The powers afforded by the snooper’s charter would, the argument goes, enable police to prevent crimes such as the one witnessed in Woolwich. True or not, the proposal would, if implemented, also bring about infrastructure for snooping on anyone at any time for any purpose. Once available, the temptation may become strong to extend, little by little, the legal use of these abilities to cover ever more everyday activities, all in the name of crime prevention, of course.

  In the emotional aftermath of a gruesome act, anything with the promise of preventing it happening again may seem like a good idea. At times like these it is important, more than ever, to remain rational and carefully consider all the potential consequences of legislation, not only the intended ones.

  Hate speech

  Hand in hand with terrorism goes hate speech, preachings designed to inspire violence against people of some singled-out nation, race, or other group. Naturally, hate speech is often to be found on the Internet, where it can reach large audiences while the author remains relatively protected. Naturally, we would prefer for it not to exist.

  To fulfil the utopian desire of a clean Internet, some advocate mandatory filtering by Internet service providers and search engines to remove this unwanted content. Exactly how such censoring might be implemented is however rarely dwelt upon, much less the consequences inadvertent blocking of innocent material might have.

  Pornography

  Another common target of calls for filtering is pornography. While few object to the blocking of child pornography, at least in principle, the debate runs hotter when it comes to the legal variety. Pornography, it is claimed, promotes violence towards women and is immoral or generally offensive. As such it ought to be blocked in the name of the greater good.

  The conviction last week of paedophile Mark Bridger for the abduction and murder of five-year-old April Jones renewed the debate about filtering of pornography in the UK ; his laptop was found to contain child pornography. John Carr of the UK government’s Council on Child Internet Safety went so far as suggesting a default blocking of all pornography, access being granted to an Internet user only once he or she had registered with some unspecified entity. Registering people wishing only to access perfectly legal material is not something we do in a democracy.

  The reality is that Google and other major search engines already remove illegal images from search results and report them to the appropriate authorities. In the UK, the Internet Watch Foundation, a non-government organisation, maintains a blacklist of what it deems ‘potentially criminal’ content, and many Internet service providers block access based on this list.

  While well-intentioned, the IWF and its blacklist should raise some concerns. Firstly, a vigilante organisation operating in secret and with no government oversight acting as the nation’s morality police has serious implications for freedom of speech. Secondly, the blocks imposed are sometimes more far-reaching than intended. In one incident, an attempt to block the cover image of the Scorpions album Virgin Killer hosted by Wikipedia (in itself a dubious decision) rendered the entire related article inaccessible as well as interfered with editing.

  Net neutrality

  Content filtering, or more precisely the lack thereof, is central to the concept of net neutrality. Usually discussed in the context of Internet service providers, this is the principle that the user should have equal, unfiltered access to all content. As a consequence, ISPs should not be held responsible for the content they deliver. Compare this to how the postal system works.

  The current debate shows that the principle of net neutrality is important not only at the ISP level, but should also include providers of essential services on the Internet. This means search engines should not be responsible for or be required to filter results, email hosts should not be required to scan users’ messages, and so on. No mandatory censoring can be effective without infringing the essential liberties of freedom of speech and press.

  Social networks operate in a less well-defined space. They are clearly not part of the essential Internet infrastructure, and they require that users sign up and agree to their terms and conditions. Because of this, they can include restrictions that would be unacceptable for the Internet as a whole. At the same time, social networks are growing in importance as means of communication between people, and as such they have a moral obligation to act fairly and apply their rules in a transparent manner.

  Facebook was recently under fire, accused of not taking sufficient measures to curb ‘hate speech,’ particularly against women. Eventually they pledged to review their policies and methods, and reducing the proliferation of such content will surely make the web a better place. Nevertheless, one must ask how Facebook (or another social network) might react to similar pressure from, say, a religious group demanding removal of ‘blasphemous’ content. What about demands from a foreign government ? Only yesterday, the Turkish prime minister Erdogan branded Twitter ‘a plague’ in a TV interview.

  Rather than impose upon Internet companies the burden of law enforcement, we should provide them the latitude to set their own policies as well as the legal confidence to stand firm in the face of unreasonable demands. The usual market forces will promote those acting responsibly.

  Further reading

  • Tory-Labour pact could save data bill, says Lord Howard
  • Internet companies warn May over ‘snooper’s charter’
  • Snooper’s charter ‘should be replaced by strengthening of existing powers’
  • Exclusive : ‘Snooper’s charter’ would not have prevented Woolwich attack, says MI5
  • Search engines urged to block more online porn sites
  • Why technology must be the solution to child abuse material online
  • Google must take more action to police explicit content, says Vince Cable
  • Facebook bows to campaign groups over ‘hate speech’
  • Facebook sexism campaign attracts thousands online
  • Türkischer Ministerpräsident : Twitter ist eine Plage
  • Valls : « La traque sur Internet doit être une priorité pour nous »
  • La Cnil, futur juge d’Internet
  • “National security matter” : Third agency caught unilaterally blocking web sites

• Compressing videos from a smartphone

  9 novembre 2016, par fejesjoco

  I have a Nexus 6p with the stock camera. It’s set to record at 1080p, 30fps. Here’s a 5 second sample (11 MB).

  Videos from this phone come out at about 17 Mbps on average. I tried to compress it with ffmpeg with -c:v libx264 -crf 23 -preset veryslow, the result comes out at about 5.5 MB, which is about 9 Mbps.

  I think this bitrate is a bit too much. When I look at torrent file listings, I can see high quality videos at 3 GB in size on average, and if such a movie is 90 minutes long on average, that is about 4-5 Mbps which sounds okay.

  I’m wondering, why the big difference ? I can notice that my video is noisy/grainy (which is expected from a phone), and that might reduce compressibility. I tried a few ffmpeg filters, like hqdn3d and atadenoise, but the noise mostly remained (maybe I didn’t play with it enough). Then I figured, the video is also shaky (which is also expected), and that might reduce compressibility too (and even makes temporal noise filtering less effective). I tried to stabilize it with the deshake filter, but that didn’t help either.

  I know I could just limit the bandwidth to whatever I like, but there must be a reason why ffmpeg thinks it needs a high bandwidth to maintain a certain quality, and a lower bandwidth would just decrease the quality.

  Why do these videos have such a high bitrate ? What’s the best way to compress them more while keeping or even increasing their quality ?