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• Fighting with the VP8 Spec

  4 juin 2010, par Multimedia Mike — VP8

  As stated in a previous blog post on the matter, FFmpeg’s policy is to reimplement codecs rather than adopt other codebases wholesale. And so it is with Google’s recently open sourced VP8 codec, the video portion of their Webm initiative. I happen to know that the new FFmpeg implementation is in the capable hands of several of my co-developers so I’m not even worrying about that angle.

  Instead, I thought of another of my characteristically useless exercises : Create an independent VP8 decoder implementation entirely in pure Python. Silly ? Perhaps. But it has one very practical application : By attempting to write a new decoder based on the official bitstream documentation, this could serve as a mechanism for validating said spec, something near and dear to my heart.

  What is the current state of the spec ? Let me reiterate that I’m glad it exists. As I stated during the initial open sourcing event, everything that Google produced for the initial event went well beyond my wildest expectations. Having said that, the documentation does fall short in a number of places. Fortunately, I am on the Webm mailing lists and am sending in corrections and ideas for general improvement. For the most part, I have been able to understand the general ideas behind the decoding flow based on the spec and am even able to implement certain pieces correctly. Then I usually instrument the libvpx source code with output statements in order to validate that I’m doing everything right.

  Token Blocker
  Unfortunately, I’m quite blocked right now on the chapter regarding token/DCT coefficient decoding (chapter 13 in the current document iteration). In his seminal critique of the codec, Dark Shikari complained that large segments of the spec are just C code fragments copy and pasted from the official production decoder. As annoying as that is, the biggest insult comes at the end of section 13.3 :

  While we have in fact completely described the coefficient decoding procedure, the reader will probably find it helpful to consult the reference implementation, which can be found in the file detokenize.c.

  The reader most certainly will not find it helpful to consult the file detokenize.c. The file in question implements the coefficient residual decoding with an unholy sequence of C macros that contain goto statements. Honestly, I thought I did understand the coefficient decoding procedure based on the spec’s description. But my numbers don’t match up with the official decoder. Instrumenting or tracing macro’d code is obviously painful and studying the same code is making me think I don’t understand the procedure after all. To be fair, entropy decoding often occupies a lot of CPU time for many video decoders and I have little doubt that the macro/goto approach is much faster than clearer, more readable methods. It’s just highly inappropriate to refer to it for pedagogical purposes.

  Aside : For comparison, check out the reference implementation for the VC-1 codec. It was written so clearly and naively that the implementors used an O(n) Huffman decoder. That’s commitment to clarity.

  I wonder if my FFmpeg cohorts are having better luck with the DCT residue decoding in their new libavcodec implementation ? Maybe if I can get this Python decoder working, it can serve as a more appropriate reference decoder.

  Update : Almost immediately after I posted this entry, I figured out a big problem that was holding me back, and then several more small ones, and finally decoded by first correct DCT coefficient from the stream (I’ve never been so happy to see the number -448). I might be back on track now. Even better was realizing that my original understanding of the spec was correct.

  Unrelated
  I found this image on the Doom9 forums. I ROFL’d :
  

  
  
  

  It’s probably unfair and inaccurate but you have to admit it’s funny. Luckily, quality nitpickings aren’t my department. I’m just interested in getting codecs working, tested, and documented so that more people can use them reliably.

• How to verify user permissions – Introducing the Piwik Platform

  9 novembre 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 make your plugin multilingual). This time you’ll learn how to verify user permissions. For this tutorial you will need to have basic knowledge of PHP and the Piwik platform.

  When should a plugin verify permissions ?

  Usually you want to do this before executing any action – such as deleting or fetching data – and before rendering any sensitive information that should not be accessible by everyone. For instance in an API method or Controller action. You sometimes also need to verify permissions before registering menu items or widgets.

  How does Piwik’s user management work ?

  It is quite simple as it only differentiates between a few roles : View permission, Admin permission and Super User permission. If you manage multiple websites with Piwik a user can be assigned to different roles as a user might have no permission for some websites but view or admin permission for another set of websites.

  Worth mentioning is that roles inherit from each other. This means the role admin automatically includes the role view and a super user automatically covers the view and admin role.

  Getting started

  In this post, we assume that you have already set up your development environment and created a plugin. If not, visit the Piwik Developer Zone where you’ll find the tutorial Setting up Piwik and other Guides that help you to develop a plugin.

  Verifying user permissions

  To protect your data the platform offers many convenient methods in the \Piwik\Piwik class. There you will find methods that either start with check, is or has. While methods that start with check throw an exception in case a condition is not met, the other methods return a boolean true or false.

  Use methods that throw an exception if you want to stop any further execution in case a user does not have an appropriate role. The platform will catch the exception and display an error message or ask the user to log in.

  public function deleteAllMessages()
  {
      // delete messages only if user has super user access, otherwise show an error message
      Piwik::checkUserSuperUserAccess();
   
      $this->getModel()->deleteAllMessages();
  }
  Télécharger

  Use methods that return a boolean for instance when registering menu items or widgets.

  public function configureAdminMenu(MenuAdmin $menu)
  {
      if (Piwik::hasUserSuperUserAccess()) {
          $menu->addPlatformItem('Plugins', $this->urlForDefaultAction());
      }
  }
  Télécharger

  It is important to be aware that just because the menu item won’t be displayed in the UI a user can still open the registered URL manually. Therefore you have to check for permissions in the actual controller action as well.

  View permission

  A user having a view permission should be only able to view reports but not make any changes apart from his personal settings. The methods that end with UserHasSomeViewAccess make sure a user has at least view permission for one website whereas the methods *UserHasViewAccess($idSites = array(1,2,3)) check whether a user has view access for all of the given websites.

  Piwik::checkUserHasSomeViewAccess();
   
  Piwik::checkUserHasViewAccess($idSites = array(1,2,3));
  Télécharger

  As a plugin developer you would usually use the latter example to verify the permissions for specific websites. Use the first example in case you develop something like an “All Websites Dashboard” where you only want to make sure the user has a view permission for at least one website.

  Admin permission

  A user having an admin permission cannot only view reports but also change website related settings. The methods to check for this role are similar to the ones before, just swap the term View with Admin.

  Piwik::checkUserHasSomeAdminAccess();
   
  Piwik::checkUserHasAdminAccess($idSites = array(1,2,3));
  Télécharger

  Super user permission

  A user having the super user permission is allowed to access all of the data stored in Piwik and change any settings. To check if a user has this role use one of the methods that end with UserSuperUserAccess.

  Piwik::checkUserHasSuperUserAccess();

  As a plugin developer you would check for this permission for instance in places where your plugin shows an activity log over all users or where it offers the possibility to change any system wide settings.

  Getting information about the currently logged in user

  Sometimes you might want to know which user is currently logged in. This can be useful if you want to persist user related information in the database or if you want to send an email to the currently logged in user. You can easily get this information by calling the following methods :

  $login = Piwik::getCurrentUserLogin()
  $email = Piwik::getCurrentUserEmail()
  Télécharger

  Advanced features

  Of course there is more that you can do. For instance you can verify whether a user is an anonymous user or whether a user has a specific role. You can also perform any operation in the context of a super user even if the current user does not have this role. Would you like to know more about those features ? Check out the Piwik class reference, the Security guide and the Manage Users user guide.

  If you have any feedback regarding our APIs or our guides in the Developer Zone feel free to send it to us.

• How to verify user permissions – Introducing the Piwik Platform

  9 novembre 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 make your plugin multilingual). This time you’ll learn how to verify user permissions. For this tutorial you will need to have basic knowledge of PHP and the Piwik platform.

  When should a plugin verify permissions ?

  Usually you want to do this before executing any action – such as deleting or fetching data – and before rendering any sensitive information that should not be accessible by everyone. For instance in an API method or Controller action. You sometimes also need to verify permissions before registering menu items or widgets.

  How does Piwik’s user management work ?

  It is quite simple as it only differentiates between a few roles : View permission, Admin permission and Super User permission. If you manage multiple websites with Piwik a user can be assigned to different roles as a user might have no permission for some websites but view or admin permission for another set of websites.

  Worth mentioning is that roles inherit from each other. This means the role admin automatically includes the role view and a super user automatically covers the view and admin role.

  Getting started

  In this post, we assume that you have already set up your development environment and created a plugin. If not, visit the Piwik Developer Zone where you’ll find the tutorial Setting up Piwik and other Guides that help you to develop a plugin.

  Verifying user permissions

  To protect your data the platform offers many convenient methods in the \Piwik\Piwik class. There you will find methods that either start with check, is or has. While methods that start with check throw an exception in case a condition is not met, the other methods return a boolean true or false.

  Use methods that throw an exception if you want to stop any further execution in case a user does not have an appropriate role. The platform will catch the exception and display an error message or ask the user to log in.

  public function deleteAllMessages()
  {
      // delete messages only if user has super user access, otherwise show an error message
      Piwik::checkUserSuperUserAccess();
   
      $this->getModel()->deleteAllMessages();
  }
  Télécharger

  Use methods that return a boolean for instance when registering menu items or widgets.

  public function configureAdminMenu(MenuAdmin $menu)
  {
      if (Piwik::hasUserSuperUserAccess()) {
          $menu->addPlatformItem('Plugins', $this->urlForDefaultAction());
      }
  }
  Télécharger

  It is important to be aware that just because the menu item won’t be displayed in the UI a user can still open the registered URL manually. Therefore you have to check for permissions in the actual controller action as well.

  View permission

  A user having a view permission should be only able to view reports but not make any changes apart from his personal settings. The methods that end with UserHasSomeViewAccess make sure a user has at least view permission for one website whereas the methods *UserHasViewAccess($idSites = array(1,2,3)) check whether a user has view access for all of the given websites.

  Piwik::checkUserHasSomeViewAccess();
   
  Piwik::checkUserHasViewAccess($idSites = array(1,2,3));
  Télécharger

  As a plugin developer you would usually use the latter example to verify the permissions for specific websites. Use the first example in case you develop something like an “All Websites Dashboard” where you only want to make sure the user has a view permission for at least one website.

  Admin permission

  A user having an admin permission cannot only view reports but also change website related settings. The methods to check for this role are similar to the ones before, just swap the term View with Admin.

  Piwik::checkUserHasSomeAdminAccess();
   
  Piwik::checkUserHasAdminAccess($idSites = array(1,2,3));
  Télécharger

  Super user permission

  A user having the super user permission is allowed to access all of the data stored in Piwik and change any settings. To check if a user has this role use one of the methods that end with UserSuperUserAccess.

  Piwik::checkUserHasSuperUserAccess();

  As a plugin developer you would check for this permission for instance in places where your plugin shows an activity log over all users or where it offers the possibility to change any system wide settings.

  Getting information about the currently logged in user

  Sometimes you might want to know which user is currently logged in. This can be useful if you want to persist user related information in the database or if you want to send an email to the currently logged in user. You can easily get this information by calling the following methods :

  $login = Piwik::getCurrentUserLogin()
  $email = Piwik::getCurrentUserEmail()
  Télécharger

  Advanced features

  Of course there is more that you can do. For instance you can verify whether a user is an anonymous user or whether a user has a specific role. You can also perform any operation in the context of a super user even if the current user does not have this role. Would you like to know more about those features ? Check out the Piwik class reference, the Security guide and the Manage Users user guide.

  If you have any feedback regarding our APIs or our guides in the Developer Zone feel free to send it to us.