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• Finding Optimal Code Coverage

  7 mars 2012, par Multimedia Mike — Programming

  A few months ago, I published a procedure for analyzing code coverage of the test suites exercised in FFmpeg and Libav. I used it to add some more tests and I have it on good authority that it has helped other developers fill in some gaps as well (beginning with students helping out with the projects as part of the Google Code-In program). Now I’m wondering about ways to do better.

  Current Process
  When adding a test that depends on a sample (like a demuxer or decoder test), it’s ideal to add a sample that’s A) small, and B) exercises as much of the codebase as possible. When I was studying code coverage statistics for the WC4-Xan video decoder, I noticed that the sample didn’t exercise one of the 2 possible frame types. So I scouted samples until I found one that covered both types, trimmed the sample down, and updated the coverage suite.

  I started wondering about a method for finding the optimal test sample for a given piece of code, one that exercises every code path in a module. Okay, so that’s foolhardy in the vast majority of cases (although I was able to add one test spec that pushed a module’s code coverage from 0% all the way to 100% — but the module in question only had 2 exercisable lines). Still, given a large enough corpus of samples, how can I find the smallest set of samples that exercise the complete codebase ?

  This almost sounds like an NP-complete problem. But why should that stop me from trying to find a solution ?

  Science Project
  Here’s the pitch :

  • Instrument FFmpeg with code coverage support
  • Download lots of media to exercise a particular module
  • Run FFmpeg against each sample and log code coverage statistics
  • Distill the resulting data in some meaningful way in order to obtain more optimal code coverage

  That first step sounds harsh– downloading lots and lots of media. Fortunately, there is at least one multimedia format in the projects that tends to be extremely small : ANSI. These are files that are designed to display elaborate scrolling graphics using text mode. Further, the FATE sample currently deployed for this test (TRE_IOM5.ANS) only exercises a little less than 50% of the code in libavcodec/ansi.c. I believe this makes the ANSI video decoder a good candidate for this experiment.

  Procedure
  First, find a site that hosts a lot ANSI files. Hi, sixteencolors.net. This site has lots (on the order of 4000) artpacks, which are ZIP archives that contain multiple ANSI files (and sometimes some other files). I scraped a list of all the artpack names.

  In an effort to be responsible, I randomized the list of artpacks and downloaded periodically and with limited bandwidth ('wget --limit-rate=20k').

  Run ‘gcov’ on ansi.c in order to gather the full set of line numbers to be covered.

  For each artpack, unpack the contents, run the instrumented FFmpeg on each file inside, run ‘gcov’ on ansi.c, and log statistics including the file’s size, the file’s location (artpack.zip:filename), and a comma-separated list of line numbers touched.

  Definition of ‘Optimal’
  The foregoing procedure worked and yielded useful, raw data. Now I have to figure out how to analyze it.

  I think it’s most desirable to have the smallest files (in terms of bytes) that exercise the most lines of code. To that end, I sorted the results by filesize, ascending. A Python script initializes a set of all exercisable line numbers in ansi.c, then iterates through each each file’s stats line, adding the file to the list of candidate samples if its set of exercised lines can remove any line numbers from the overall set of lines. Ideally, that set of lines should devolve to an empty set.

  I think a second possible approach is to find the single sample that exercises the most code and then proceed with the previously described method.

  Initial Results
  So far, I have analyzed 13324 samples from 357 different artpacks provided by sixteencolors.net.

  Using the first method, I can find a set of samples that covers nearly 80% of ansi.c :

  <br />
0 bytes: bad-0494.zip:5<br />
1 bytes: grip1293.zip:-ANSI---.---<br />
1 bytes: pur-0794.zip:.<br />
2 bytes: awe9706.zip:-ANSI───.───<br />
61 bytes: echo0197.zip:-(ART)-<br />
62 bytes: hx03.zip:HX005.DAT<br />
76 bytes: imp-0494.zip:IMPVIEW.CFG<br />
82 bytes: ice0010b.zip:_cont'd_.___<br />
101 bytes: bdp-0696.zip:BDP2.WAD<br />
112 bytes: plain12.zip:--------.---<br />
181 bytes: ins1295v.zip:-°VGA°-.  н<br />
219 bytes: purg-22.zip:NEM-SHIT.ASC<br />
289 bytes: srg1196.zip:HOWTOREQ.JNK<br />
315 bytes: karma-04.zip:FASHION.COM<br />
318 bytes: buzina9.zip:ox-rmzzy.ans<br />
411 bytes: solo1195.zip:FU-BLAH1.RIP<br />
621 bytes: ciapak14.zip:NA-APOC1.ASC<br />
951 bytes: lght9404.zip:AM-TDHO1.LIT<br />
1214 bytes: atb-1297.zip:TX-ROKL.ASC<br />
2332 bytes: imp-0494.zip:STATUS.ANS<br />
3218 bytes: acepak03.zip:TR-STAT5.ANS<br />
6068 bytes: lgc-0193.zip:LGC-0193.MEM<br />
16778 bytes: purg-20.zip:EZ-HIR~1.JPG<br />
20582 bytes: utd0495.zip:LT-CROW3.ANS<br />
26237 bytes: quad0597.zip:MR-QPWP.GIF<br />
29208 bytes: mx-pack17.zip:mx-mobile-source-logo.jpg<br />
----<br />
109440 bytes total<br />

  A few notes about that list : Some of those filenames are comprised primarily of control characters. 133t, and all that. The first file is 0 bytes. I wondered if I should discard 0-length files but decided to keep those in, especially if they exercise lines that wouldn’t normally be activated. Also, there are a few JPEG and GIF files in the set. I should point out that I forced the tty demuxer using -f tty and there isn’t much in the way of signatures for this format. So, again, whatever exercises more lines is better.

  Using this same corpus, I tried approach 2– which single sample exercises the most lines of the decoder ? Answer : blde9502.zip:REQUEST.EXE. Huh. I checked it out and ‘file’ ID’s it as a MS-DOS executable. So, that approach wasn’t fruitful, at least not for this corpus since I’m forcing everything through this narrow code path.

  Think About The Future
  Where can I take this next ? The cloud ! I have people inside the search engine industry who have furnished me with extensive lists of specific types of multimedia files from around the internet. I also see that Amazon Web Services Elastic Compute Cloud (AWS EC2) instances don’t charge for incoming bandwidth.

  I think you can see where I’m going with this.

  See Also :

  • Running this experiment on 4000 artpacks / 146,000 files

• Make MP4 video using JPEG 2000 frames and ROI

  21 mars 2012, par Leandro

  I'm having a hard time trying to figure out how to solve the following problem.

  I have a set of jp2 frames. I want to save them as a .mp4 video for archiving (which is great because it supports motion compensation) and then be able to extract the frames as jp2 files again.

  I have this working with a set of tools/libraries like ffmpeg. The issue is that I also want to include a ROI in the original jp2 images.

  When I do that, the video shows nothing where the ROI originally was, and when I use ffmpeg to go back to the original files, the frames are still broken.

  I tried going from JPEG2000 w\ ROI to another format before making the mp4 file, but every tool I use seems to have the same results : the ROI gets translated as a square w/o nothing there.

  Any help would be greatly appreciated.

  Thanks !

• php stream any mp4

  3 avril 2012, par GRaecuS

  I'm developing a web app that converts videos and allows to play them through Flowplayer.

  On the current status, I use ffmpeg to convert the videos to mp4 and qtfaststart to fix their metadata for streaming. Everything is working smoothly as I can download any converted mp4 and view it correctly.

  For serving the videos to Flowplayer, I use a php file which contains the following (summarized) code :

  header("Content-Type: {$mediatype}");
  
  
  
  if ( empty($_SERVER['HTTP_RANGE']) )
  
  {
  
      if ( $filetype == 'flv' && $seekPos != 0 )
  
      {
  
          header("Content-Length: " . ($filesize + 13));
  
          print('FLV');
  
          print(pack('C', 1));
  
          print(pack('C', 1));
  
          print(pack('N', 9));
  
          print(pack('N', 9));
  
      }
  
      else
  
      {           
  
          header("Content-Length: {$filesize}");
  
      }
  
  
  
      $fh = fopen($filepath, "rb") or die("Could not open file: {$filepath}");
  
  
  
      # seek to requested file position
  
      fseek($fh, $seekPos);
  
  
  
      # output file
  
      while(!feof($fh))
  
      {
  
          # output file without bandwidth limiting
  
          echo fread($fh, $filesize);
  
      }
  
      fclose($fh);
  
  }
  
  else //violes rfc2616, which requires ignoring  the header if it's invalid
  
  {   
  
      $fp = @fopen($file, 'rb');
  
  
  
      $size   = filesize($file); // File size
  
      $length = $size;           // Content length
  
      $start  = 0;               // Start byte
  
      $end    = $size - 1;       // End byte
  
      // Now that we've gotten so far without errors we send the accept range header
  
      /* At the moment we only support single ranges.
  
       * Multiple ranges requires some more work to ensure it works correctly
  
       * and comply with the spesifications: http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html#sec19.2
  
       *
  
       * Multirange support annouces itself with:
  
       * header('Accept-Ranges: bytes');
  
       *
  
       * Multirange content must be sent with multipart/byteranges mediatype,
  
       * (mediatype = mimetype)
  
       * as well as a boundry header to indicate the various chunks of data.
  
       */
  
      header("Accept-Ranges: 0-$length");
  
      // header('Accept-Ranges: bytes');
  
      // multipart/byteranges
  
      // http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html#sec19.2
  
      if (isset($_SERVER['HTTP_RANGE']))
  
      {
  
          $c_start = $start;
  
          $c_end   = $end;
  
          // Extract the range string
  
          list(, $range) = explode('=', $_SERVER['HTTP_RANGE'], 2);
  
          // Make sure the client hasn't sent us a multibyte range
  
          if (strpos($range, ',') !== false)
  
          {
  
              // (?) Shoud this be issued here, or should the first
  
              // range be used? Or should the header be ignored and
  
              // we output the whole content?
  
              header('HTTP/1.1 416 Requested Range Not Satisfiable');
  
              header("Content-Range: bytes $start-$end/$size");
  
              // (?) Echo some info to the client?
  
              exit;
  
          }
  
          // If the range starts with an '-' we start from the beginning
  
          // If not, we forward the file pointer
  
          // And make sure to get the end byte if spesified
  
          if ($range0 == '-')
  
          {
  
  
  
              // The n-number of the last bytes is requested
  
              $c_start = $size - substr($range, 1);
  
          }
  
          else
  
          {
  
              $range  = explode('-', $range);
  
              $c_start = $range[0];
  
              $c_end   = (isset($range[1]) && is_numeric($range[1])) ? $range[1] : $size;
  
          }
  
          /* Check the range and make sure it's treated according to the specs.
  
           * http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html
  
           */
  
          // End bytes can not be larger than $end.
  
          $c_end = ($c_end > $end) ? $end : $c_end;
  
          // Validate the requested range and return an error if it's not correct.
  
          if ($c_start > $c_end || $c_start > $size - 1 || $c_end >= $size)
  
          {
  
              header('HTTP/1.1 416 Requested Range Not Satisfiable');
  
              header("Content-Range: bytes $start-$end/$size");
  
              // (?) Echo some info to the client?
  
              exit;
  
          }
  
          $start  = $c_start;
  
          $end    = $c_end;
  
          $length = $end - $start + 1; // Calculate new content length
  
          fseek($fp, $start);
  
          header('HTTP/1.1 206 Partial Content');
  
      }
  
  
  
      // Notify the client the byte range we'll be outputting
  
      header("Content-Range: bytes $start-$end/$size");
  
      header("Content-Length: $length");
  
  
  
      // Start buffered download
  
      $buffer = 1024 * 8;
  
      while(!feof($fp) && ($p = ftell($fp)) <= $end)
  
      {
  
          if ($p + $buffer > $end)
  
          {
  
              // In case we're only outputtin a chunk, make sure we don't
  
              // read past the length
  
              $buffer = $end - $p + 1;
  
          }
  
          set_time_limit(0); // Reset time limit for big files
  
          echo fread($fp, $buffer);
  
          flush(); // Free up memory. Otherwise large files will trigger PHP's memory limit.
  
      }
  
  
  
      fclose($fp);
  
  }

  Unfortunately, it is working only for the majority of the videos. For some of them, Flowplayer keeps returning Error 200, even though they were encoded correctly.

  How can I fix this ? Is it a coding problem or those videos are faulty ?