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• H.264 and VP8 for still image coding : WebP ?

  1er octobre 2010, par Dark Shikari — H.264, VP8, google, psychovisual optimizations

  JPEG is a very old lossy image format. By today’s standards, it’s awful compression-wise : practically every video format since the days of MPEG-2 has been able to tie or beat JPEG at its own game. The reasons people haven’t switched to something more modern practically always boil down to a simple one — it’s just not worth the hassle. Even if JPEG can be beaten by a factor of 2, convincing the entire world to change image formats after 20 years is nigh impossible. Furthermore, JPEG is fast, simple, and practically guaranteed to be free of any intellectual property worries. It’s been tried before : JPEG-2000 first, then Microsoft’s JPEG XR, both tried to unseat JPEG. Neither got much of anywhere.

  Now Google is trying to dump yet another image format on us, “WebP”. But really, it’s just a VP8 intra frame. There are some obvious practical problems with this new image format in comparison to JPEG ; it doesn’t even support all of JPEG’s features, let alone many of the much-wanted features JPEG was missing (alpha channel support, lossless support). It only supports 4:2:0 chroma subsampling, while JPEG can handle 4:2:2 and 4:4:4. Google doesn’t seem interested in adding any of these features either.

  But let’s get to the meat and see how these encoders stack up on compressing still images. As I explained in my original analysis, VP8 has the advantage of H.264′s intra prediction, which is one of the primary reasons why H.264 has such an advantage in intra compression. It only has i4x4 and i16x16 modes, not i8x8, so it’s not quite as fancy as H.264′s, but it comes close.

  The test files are all around 155KB ; download them for the exact filesizes. For all three, I did a binary search of quality levels to get the file sizes close. For x264, I encoded with --tune stillimage --preset placebo. For libvpx, I encoded with --best. For JPEG, I encoded with ffmpeg, then applied jpgcrush, a lossless jpeg compressor. I suspect there are better JPEG encoders out there than ffmpeg ; if you have one, feel free to test it and post the results. The source image is the 200th frame of Parkjoy, from derf’s page (fun fact : this video was shot here ! More info on the video here.).

  Files : (x264 [154KB], vp8 [155KB], jpg [156KB])

  Results (decoded to PNG) : (x264, vp8, jpg)

  This seems rather embarrassing for libvpx. Personally I think VP8 looks by far the worst of the bunch, despite JPEG’s blocking. What’s going on here ? VP8 certainly has better entropy coding than JPEG does (by far !). It has better intra prediction (JPEG has just DC prediction). How could VP8 look worse ? Let’s investigate.

  VP8 uses a 4×4 transform, which tends to blur and lose more detail than JPEG’s 8×8 transform. But that alone certainly isn’t enough to create such a dramatic difference. Let’s investigate a hypothesis — that the problem is that libvpx is optimizing for PSNR and ignoring psychovisual considerations when encoding the image… I’ll encode with --tune psnr --preset placebo in x264, turning off all psy optimizations. 

  Files : (x264, optimized for PSNR [154KB]) [Note for the technical people : because adaptive quantization is off, to get the filesize on target I had to use a CQM here.]

  Results (decoded to PNG) : (x264, optimized for PSNR)

  What a blur ! Only somewhat better than VP8, and still worse than JPEG. And that’s using the same encoder and the same level of analysis — the only thing done differently is dropping the psy optimizations. Thus we come back to the conclusion I’ve made over and over on this blog — the encoder matters more than the video format, and good psy optimizations are more important than anything else for compression. libvpx, a much more powerful encoder than ffmpeg’s jpeg encoder, loses because it tries too hard to optimize for PSNR.

  These results raise an obvious question — is Google nuts ? I could understand the push for “WebP” if it was better than JPEG. And sure, technically as a file format it is, and an encoder could be made for it that’s better than JPEG. But note the word “could”. Why announce it now when libvpx is still such an awful encoder ? You’d have to be nuts to try to replace JPEG with this blurry mess as-is. Now, I don’t expect libvpx to be able to compete with x264, the best encoder in the world — but surely it should be able to beat an image format released in 1992 ?

  Earth to Google : make the encoder good first, then promote it as better than the alternatives. The reverse doesn’t work quite as well.

  [155KB]

• How to create a command – Introducing the Piwik Platform

  2 octobre 2014, par Thomas Steur — Development

  This is the next post of our blog series where we introduce the capabilities of the Piwik platform (our previous post was How to publish your plugin or theme on the Piwik Marketplace). This time you’ll learn how to create a new command. For this tutorial you will need to have basic knowledge of PHP.

  What is a command ?

  A command can execute any task on the command line. Piwik provides currently about 50 commands via the Piwik Console. These commands let you start the archiver, change the number of available custom variables, enable the developer mode, clear caches, run tests and more. You could write your own command to sync users or websites with another system for instance.

  Getting started

  In this series of posts, we assume that you have already set up your development environment. If not, visit the Piwik Developer Zone where you’ll find the tutorial Setting up Piwik.

  To summarize the things you have to do to get setup :

  • Install Piwik (for instance via git).
  • Activate the developer mode : ./console development:enable --full.
  • Generate a plugin : ./console generate:plugin --name="MyCommandPlugin". There should now be a folder plugins/MyCommandPlugin.
  • And activate the created plugin under Settings => Plugins.

  Let’s start creating a command

  We start by using the Piwik Console to create a new command. As you can see there is even a command that lets you easily create a new command :

  ./console generate:command

  The command will ask you to enter the name of the plugin the created command should belong to. I will simply use the above chosen plugin name “MyCommandPlugin”. It will ask you for a command name as well. I will use “SyncUsers” in this example. There should now be a file plugins/MyCommandPlugin/Commands/Syncusers.php which contains already an example to get you started easily :

  class Syncusers extends ConsoleCommand
  {
      protected function configure()
      {
          $this->setName('mycommandplugin:syncusers');
          $this->setDescription('MyCommandPlugin');
          $this->addOption('name', null, InputOption::VALUE_REQUIRED, 'Your name:');
      }
   
      /**
       * Execute command like: ./console mycommandplugin:syncusers --name="The Piwik Team"
       */
      protected function execute(InputInterface $input, OutputInterface $output)
      {
          $name    = $input->getOption('name');
   
          $message = sprintf('Syncusers: %s', $name);
   
          $output->writeln($message);
      }
  }
  Télécharger

  Any command that is placed in the “Commands” folder of your plugin will be available on the command line automatically. Therefore, the newly created command can now be executed via ./console mycommandplugin:syncusers --name="The Piwik Team".

  The code template explained

  protected function configure()
  {
      $this->setName('mycommandplugin:checkdatabase');
      $this->setDescription('MyCommandPlugin');
      $this->addOption('name', null, InputOption::VALUE_REQUIRED, 'Your name:');
  }
  Télécharger

  As the name says the method configure lets you configure your command. You can define the name and description of your command as well as all the options and arguments you expect when executing it.

  protected function execute(InputInterface $input, OutputInterface $output)
  {
      $name    = $input->getOption('name');
      $message = sprintf('Syncusers: %s', $name);
      $output->writeln($message);
  }
  Télécharger

  The actual task is defined in the execute method. There you can access any option or argument that was defined on the command line via $input and write anything to the console via $output argument.

  In case anything went wrong during the execution you should throw an exception to make sure the user will get a useful error message. Throwing an exception when an error occurs will make sure the command does exit with a status code different than 0 which can sometimes be important.

  Advanced features

  The Piwik Console is based on the powerful Symfony Console component. For instance you can ask a user for any interactive input, you can use different output color schemes and much more. If you are interested in learning more all those features have a look at the Symfony console website.

  How to test a command

  After you have created a command you are surely wondering how to test it. Ideally, the actual command is quite short as it acts like a controller. It should only receive the input values, execute the task by calling a method of another class and output any useful information. This allows you to easily create a unit or integration test for the classes behind the command. We will cover this topic in one of our future blog posts. Just one hint : You can use another command ./console generate:test to create a test. If you want to know how to test a command have a look at the Testing Commands documentation.

  Publishing your Plugin on the Marketplace

  In case you want to share your commands with other Piwik users you can do this by pushing your plugin to a public GitHub repository and creating a tag. Easy as that. Read more about how to distribute a plugin and best practices when publishing a plugin.

  Isn’t it easy to create a command ? We never even created a file ! If you have any feedback regarding our APIs or our guides in the Developer Zone feel free to send it to us.

• How to create a command – Introducing the Piwik Platform

  2 octobre 2014, par Thomas Steur — Development

  This is the next post of our blog series where we introduce the capabilities of the Piwik platform (our previous post was How to publish your plugin or theme on the Piwik Marketplace). This time you’ll learn how to create a new command. For this tutorial you will need to have basic knowledge of PHP.

  What is a command ?

  A command can execute any task on the command line. Piwik provides currently about 50 commands via the Piwik Console. These commands let you start the archiver, change the number of available custom variables, enable the developer mode, clear caches, run tests and more. You could write your own command to sync users or websites with another system for instance.

  Getting started

  In this series of posts, we assume that you have already set up your development environment. If not, visit the Piwik Developer Zone where you’ll find the tutorial Setting up Piwik.

  To summarize the things you have to do to get setup :

  • Install Piwik (for instance via git).
  • Activate the developer mode : ./console development:enable --full.
  • Generate a plugin : ./console generate:plugin --name="MyCommandPlugin". There should now be a folder plugins/MyCommandPlugin.
  • And activate the created plugin under Settings => Plugins.

  Let’s start creating a command

  We start by using the Piwik Console to create a new command. As you can see there is even a command that lets you easily create a new command :

  ./console generate:command

  The command will ask you to enter the name of the plugin the created command should belong to. I will simply use the above chosen plugin name “MyCommandPlugin”. It will ask you for a command name as well. I will use “SyncUsers” in this example. There should now be a file plugins/MyCommandPlugin/Commands/Syncusers.php which contains already an example to get you started easily :

  class Syncusers extends ConsoleCommand
  {
      protected function configure()
      {
          $this->setName('mycommandplugin:syncusers');
          $this->setDescription('MyCommandPlugin');
          $this->addOption('name', null, InputOption::VALUE_REQUIRED, 'Your name:');
      }
   
      /**
       * Execute command like: ./console mycommandplugin:syncusers --name="The Piwik Team"
       */
      protected function execute(InputInterface $input, OutputInterface $output)
      {
          $name    = $input->getOption('name');
   
          $message = sprintf('Syncusers: %s', $name);
   
          $output->writeln($message);
      }
  }
  Télécharger

  Any command that is placed in the “Commands” folder of your plugin will be available on the command line automatically. Therefore, the newly created command can now be executed via ./console mycommandplugin:syncusers --name="The Piwik Team".

  The code template explained

  protected function configure()
  {
      $this->setName('mycommandplugin:checkdatabase');
      $this->setDescription('MyCommandPlugin');
      $this->addOption('name', null, InputOption::VALUE_REQUIRED, 'Your name:');
  }
  Télécharger

  As the name says the method configure lets you configure your command. You can define the name and description of your command as well as all the options and arguments you expect when executing it.

  protected function execute(InputInterface $input, OutputInterface $output)
  {
      $name    = $input->getOption('name');
      $message = sprintf('Syncusers: %s', $name);
      $output->writeln($message);
  }
  Télécharger

  The actual task is defined in the execute method. There you can access any option or argument that was defined on the command line via $input and write anything to the console via $output argument.

  In case anything went wrong during the execution you should throw an exception to make sure the user will get a useful error message. Throwing an exception when an error occurs will make sure the command does exit with a status code different than 0 which can sometimes be important.

  Advanced features

  The Piwik Console is based on the powerful Symfony Console component. For instance you can ask a user for any interactive input, you can use different output color schemes and much more. If you are interested in learning more all those features have a look at the Symfony console website.

  How to test a command

  After you have created a command you are surely wondering how to test it. Ideally, the actual command is quite short as it acts like a controller. It should only receive the input values, execute the task by calling a method of another class and output any useful information. This allows you to easily create a unit or integration test for the classes behind the command. We will cover this topic in one of our future blog posts. Just one hint : You can use another command ./console generate:test to create a test. If you want to know how to test a command have a look at the Testing Commands documentation.

  Publishing your Plugin on the Marketplace

  In case you want to share your commands with other Piwik users you can do this by pushing your plugin to a public GitHub repository and creating a tag. Easy as that. Read more about how to distribute a plugin and best practices when publishing a plugin.

  Isn’t it easy to create a command ? We never even created a file ! If you have any feedback regarding our APIs or our guides in the Developer Zone feel free to send it to us.