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• IJG swings again, and misses

  1er février 2010, par Mans — Multimedia

  Earlier this month the IJG unleashed version 8 of its ubiquitous libjpeg library on the world. Eager to try out the “major breakthrough in image coding technology” promised in the README file accompanying v7, I downloaded the release. A glance at the README file suggests something major indeed is afoot :

  Version 8.0 is the first release of a new generation JPEG standard to overcome the limitations of the original JPEG specification.

  The text also hints at the existence of a document detailing these marvellous new features, and a Google search later a copy has found its way onto my monitor. As I read, however, my state of mind shifts from an initial excited curiosity, through bewilderment and disbelief, finally arriving at pure merriment.

  Already on the first page it becomes clear no new JPEG standard in fact exists. All we have is an unsolicited proposal sent to the ITU-T by members of the IJG. Realising that even the most brilliant of inventions must start off as mere proposals, I carry on reading. The summary informs me that I am about to witness the introduction of three extensions to the T.81 JPEG format :

  1. An alternative coefficient scan sequence for DCT coefficient serialization
  2. A SmartScale extension in the Start-Of-Scan (SOS) marker segment
  3. A Frame Offset definition in or in addition to the Start-Of-Frame (SOF) marker segment

  Together these three extensions will, it is promised, “bring DCT based JPEG back to the forefront of state-of-the-art image coding technologies.”

  Alternative scan

  The first of the proposed extensions introduces an alternative DCT coefficient scan sequence to be used in place of the zigzag scan employed in most block transform based codecs.

  

  Alternative scan sequence

  The advantage of this scan would be that combined with the existing progressive mode, it simplifies decoding of an initial low-resolution image which is enhanced through subsequent passes. The author of the document calls this scheme “image-pyramid/hierarchical multi-resolution coding.” It is not immediately obvious to me how this constitutes even a small advance in image coding technology.

  At this point I am beginning to suspect that our friend from the IJG has been trapped in a half-world between interlaced GIF images transmitted down noisy phone lines and today’s inferno of SVC, MVC, and other buzzwords.

  (Not so) SmartScale

  Disguised behind this camel-cased moniker we encounter a method which, we are told, will provide better image quality at high compression ratios. The author has combined two well-known (to us) properties in a (to him) clever way.

  The first property concerns the perceived impact of different types of distortion in an image. When encoding with JPEG, as the quantiser is increased, the decoded image becomes ever more blocky. At a certain point, a better subjective visual quality can be achieved by down-sampling the image before encoding it, thus allowing a lower quantiser to be used. If the decoded image is scaled back up to the original size, the unpleasant, blocky appearance is replaced with a smooth blur.

  The second property belongs to the DCT where, as we all know, the top-left (DC) coefficient is the average of the entire block, its neighbours represent the lowest frequency components etc. A top-left-aligned subset of the coefficient block thus represents a low-resolution version of the full block in the spatial domain.

  In his flash of genius, our hero came up with the idea of using the DCT for down-scaling the image. Unfortunately, he appears to possess precious little knowledge of sampling theory and human visual perception. Any block-based resampling will inevitably produce sharp artefacts along the block edges. The human visual system is particularly sensitive to sharp edges, so this is one of the most unwanted types of distortion in an encoded image.

  Despite the obvious flaws in this approach, I decided to give it a try. After all, the software is already written, allowing downscaling by factors of 8/8..16.

  Using a 1280×720 test image, I encoded it with each of the nine scaling options, from unity to half size, each time adjusting the quality parameter for a final encoded file size of no more than 200000 bytes. The following table presents the encoded file size, the libjpeg quality parameter used, and the SSIM metric for each of the images.

  Scale	Size	Quality	SSIM
  8/8	198462	59	0.940
  8/9	196337	70	0.936
  8/10	196133	79	0.934
  8/11	197179	84	0.927
  8/12	193872	89	0.915
  8/13	197153	92	0.914
  8/14	188334	94	0.899
  8/15	198911	96	0.886
  8/16	197190	97	0.869

  Although the smaller images allowed a higher quality setting to be used, the SSIM value drops significantly. Numbers may of course be misleading, but the images below speak for themselves. These are cut-outs from the full image, the original on the left, unscaled JPEG-compressed in the middle, and JPEG with 8/16 scaling to the right.

  

  

  Looking at these images, I do not need to hesitate before picking the JPEG variant I prefer.

  Frame offset

  The third and final extension proposed is quite simple and also quite pointless : a top-left cropping to be applied to the decoded image. The alleged utility of this feature would be to enable lossless cropping of a JPEG image. In a typical image workflow, however, JPEG is only used for the final published version, so the need for this feature appears quite far-fetched.

  The grand finale

  Throughout the text, the author makes references to “the fundamental DCT property for image representation.” In his own words :

  This property was found by the author during implementation of the new DCT scaling features and is after his belief one of the most important discoveries in digital image coding after releasing the JPEG standard in 1992.

  The secret is to be revealed in an annex to the main text. This annex quotes in full a post by the author to the comp.dsp Usenet group in a thread with the subject why DCT. Reading the entire thread proves quite amusing. A few excerpts follow.

  The actual reason is much simpler, and therefore apparently very difficult to recognize by complicated-thinking people.

  Here is the explanation :

  What are people doing when they have a bunch of images and want a quick preview ? They use thumbnails ! What are thumbnails ? Thumbnails are small downscaled versions of the original image ! If you want more details of the image, you can zoom in stepwise by enlarging (upscaling) the image.

  So with proper understanding of the fundamental DCT property, the MPEG folks could make their videos more scalable, but, as in the case of JPEG, they are unable to recognize this simple but basic property, unfortunately, and pursue rather inferior approaches in actual developments.

  These are just phrases, and they don’t explain anything. But this is typical for the current state in this field : The relevant people ignore and deny the true reasons, and thus they turn in a circle and no progress is being made.

  However, there are dark forces in action today which ignore and deny any fruitful advances in this field. That is the reason that we didn’t see any progress in JPEG for more than a decade, and as long as those forces dominate, we will see more confusion and less enlightenment. The truth is always simple, and the DCT *is* simple, but this fact is suppressed by established people who don’t want to lose their dubious position.

  I believe a trip to the Total Perspective Vortex may be in order. Perhaps his tin-foil hat will save him.

• Summary Video Accessibility Talk

  23 avril 2013, par silvia

  I’ve just got off a call to the UK Digital TV Group, for which I gave a talk on HTML5 video accessibility (slides best viewed in Google Chrome).

  The slide provide a high-level summary of the accessibility features that we’ve developed in the W3C for HTML5, including :

  • Subtitles & Captions with WebVTT and the track element
  • Video Descriptions with WebVTT, the track element and speech synthesis
  • Chapters with WebVTT for semantic navigation
  • Audio Descriptions through synchronising an audio track with a video
  • Sign Language video synchronized with a main video

  I received some excellent questions.

  The obvious one was about why WebVTT and not TTML. While for anyone who has tried to implement TTML support, the advantages of WebVTT should be clear, for some the decision of the browsers to go with WebVTT still seems to be bothersome. The advantages of CSS over XSL-FO in a browser-context are obvious, but not as much outside browsers. So, the simplicity of WebVTT and the clear integration with HTML have to speak for themselves. Conversion between TTML and WebVTT was a feature that was being asked for.

  I received a question about how to support ducking (reduce the volume of the main audio track) when using video descriptions. My reply was to either use video descriptions with WebVTT and do ducking during the times that a cue is active, or when using audio descriptions (i.e. actual audio tracks) to add an additional WebVTT file of kind=metadata to mark the intervals in which to do ducking. In both cases some JavaScript will be necessary.

  I received another question about how to do clean audio, which I had almost forgotten was a requirement from our earlier media accessibility document. “Clean audio” consists of isolating the audio channel containing the spoken dialog and important non-speech information that can then be amplified or otherwise modified, while other channels containing music or ambient sounds are attenuated. I suggested using the mediagroup attribute to provide a main video element (without an audio track) and then the other channels as parallel audio tracks that can be turned on and off and attenuated individually. There is some JavaScript coding involved on top of the APIs that we have defined in HTML, but it can be implemented in browsers that support the mediagroup attribute.

  Another question was about the possibilities to extend the list of @kind attribute values. I explained that right now we have a proposal for a new text track kind=”forced” so as to provide forced subtitles for sections of video with foreign language. These would be on when no other subtitle or caption tracks are activated. I also explained that if there is a need for application-specific text tracks, the kind=”metadata” would be the correct choice.

  I received some further questions, in particular about how to apply styling to captions (e.g. color changes to text) and about how closely the browser are able to keep synchronization across multiple media elements. The earlier was easily answered with the ::cue pseudo-element, but the latter is a quality of implementation feature, so I had to defer to individual browsers.

  Overall it was a good exercise to summarize the current state of HTML5 video accessibility and I was excited to show off support in Chrome for all the features that we designed into the standard.

• FFmpeg Concatenation Command Fails in Flutter App

  18 février 2024, par Petro

  I'm developing a Flutter application where I need to concatenate several images into a single video file using FFmpeg. Despite following the recommended practices and trying multiple variations of the FFmpeg command, all my attempts result in failure with an exit code of 1.

  


  FFMPEG Version : ffmpeg_kit_flutter: ^6.0.3-LTS

  


  All of the files are present when this happens...


  


  Environment :
Flutter app targeting Android
Using ffmpeg-kit-flutter for FFmpeg operations

  


  Objective :
To concatenate multiple images stored in the app's file system into a video.

  


  Code Snippet :
I'm generating a list of image paths, writing them to a file (ffmpeg_list.txt), and using that file with FFmpeg's concat demuxer. Here's a simplified version of my code :

  


  Future<void> _createVideoFromImages() async {&#xA;  final Directory appDir = await getApplicationDocumentsDirectory();&#xA;  final Uuid uuid = Uuid();&#xA;  final String videoFileName = uuid.v4();&#xA;  final String outputPath = &#x27;${appDir.path}/$videoFileName.mp4&#x27;;&#xA;  &#xA;  final Directory tempImageDir = await Directory(&#x27;${appDir.path}/tempImages&#x27;).create();&#xA;  final StringBuffer ffmpegInput = StringBuffer();&#xA;  int index = 0;&#xA;&#xA;  for (var image in _images) {&#xA;    String newFileName = &#x27;img${index&#x2B;&#x2B;}${Path.extension(image.path)}&#x27;.replaceAll(&#x27; &#x27;, &#x27;_&#x27;);&#xA;    final String newPath = &#x27;${tempImageDir.path}/$newFileName&#x27;;&#xA;    await image.copy(newPath);&#xA;    ffmpegInput.writeln("file &#x27;$newPath&#x27;");&#xA;  }&#xA;&#xA;  final String listFilePath = &#x27;${appDir.path}/ffmpeg_list.txt&#x27;;&#xA;  await File(listFilePath).writeAsString(ffmpegInput.toString());&#xA;&#xA;  if(await File(listFilePath).exists()) {&#xA;    String ffmpegCommand = "-v verbose -f concat -safe 0 -i $listFilePath -vsync vfr -pix_fmt yuv420p -c:v libx264 -r 30 $outputPath";&#xA;    // Additional commands tried here...&#xA;    await FFmpegKit.execute(ffmpegCommand).then((session) async {&#xA;      // Error handling code...&#xA;    });&#xA;  }&#xA;}&#xA;&#xA;Result Logs:&#xA;I/flutter: file exists at /data/user/0/com.example.app/app_flutter/ffmpeg_list.txt&#xA;I/flutter: FFmpeg command: -v verbose -f concat -safe 0 -i /data/user/0/com.example.app/app_flutter/ffmpeg_list.txt -vsync vfr -pix_fmt yuv420p -c:v libx264 -r 30 /data/user/0/com.example.app/app_flutter/58fdf92b-47b0-49d1-be93-d9c95870c733.mp4&#xA;I/flutter: Failed to create video&#xA;I/flutter: FFmpeg process exited with:1&#xA;I/flutter: FFmpeg command failed with logs: ffmpeg version n6.0 Copyright (c) 2000-2023 the FFmpeg developers...&#xA;</void>

  


  Attempts :
-Simplified the FFmpeg command by removing -vsync vfr, -pix_fmt yuv420p, and adjusting -r 30 parameters.
-Tried using the -c copy option to avoid re-encoding.
-Tested with a single image to ensure basic functionality works.
-Checked file permissions and ensured all images and the list file are accessible.

  


  Despite these attempts, the command fails without providing specific error messages related to the command's execution. The verbose logs do not offer insights beyond the FFmpeg version and configuration.

  


  Questions :
Are there known issues with FFmpeg's concat that might lead to such failures ?
Are there alternative approaches ?

  


  I appreciate any insights or suggestions the community might have. Thank you !

  


  Full code :

  


  Future<void> _createVideoFromImages() async {&#xA;    final Directory appDir = await getApplicationDocumentsDirectory();&#xA;    final Uuid uuid = Uuid();&#xA;    final String videoFileName = uuid.v4();&#xA;    final String outputPath = &#x27;${appDir.path}/$videoFileName.mp4&#x27;;&#xA;    final String singleImagePath = _images[0]!.path;&#xA;&#xA;// Create a directory to store renamed images to avoid any naming conflict&#xA;    final Directory tempImageDir = await Directory(&#x27;${appDir.path}/tempImages&#x27;).create();&#xA;&#xA;    final StringBuffer ffmpegInput = StringBuffer();&#xA;    int index = 0; // To ensure unique filenames&#xA;&#xA;    for (var image in _images) {&#xA;      // Generate a new filename by replacing spaces with underscores and adding an index&#xA;      String newFileName = &#x27;img${index&#x2B;&#x2B;}${p.extension(image!.path)}&#x27;.replaceAll(&#x27; &#x27;, &#x27;_&#x27;);&#xA;      final String newPath = &#x27;${tempImageDir.path}/$newFileName&#x27;;&#xA;&#xA;      // Copy and rename the original file to the new path&#xA;      await image!.copy(newPath);&#xA;&#xA;      // Add the new, safely named file path to the ffmpegInput&#xA;      ffmpegInput.writeln("file &#x27;$newPath&#x27;");&#xA;    }&#xA;&#xA;// Write the paths to a temporary text file for FFmpeg&#x27;s concat demuxer&#xA;    final String listFilePath = &#x27;${appDir.path}/ffmpeg_list.txt&#x27;;&#xA;    await File(listFilePath).writeAsString(ffmpegInput.toString());&#xA;&#xA;    //check if file exists&#xA;    if(await File(listFilePath).exists()) {&#xA;      print("file exists at $listFilePath");&#xA;&#xA;// Use the generated list file in the concat command&#xA;      String ffmpegCommand = "-v verbose -f concat -safe 0 -i $listFilePath -vsync vfr -pix_fmt yuv420p -c:v libx264 -r 30 $outputPath";&#xA;      String ffmpegCommand2 = "-v verbose -f concat -safe 0 -i $listFilePath -c:v libx264 $outputPath";&#xA;      String ffmpegCommand3 = "-v verbose -i $singleImagePath -frames:v 1 $outputPath";&#xA;      //print command&#xA;      print("FFmpeg command: $ffmpegCommand");&#xA;&#xA;&#xA;      await FFmpegKit.execute(ffmpegCommand).then((session) async {&#xA;        // Check the session for success or failure&#xA;        final returnCode = await session.getReturnCode();&#xA;        if (returnCode!.isValueSuccess()) {&#xA;          print("Video created successfully: $outputPath");&#xA;          //okay all set, now set the video to be this:&#xA;          _actual_video_file_ready_to_upload = File(outputPath);&#xA;          print ("video path is: ${outputPath}");&#xA;        } else {&#xA;          print("Failed to create video");&#xA;          print("FFmpeg process exited with:" &#x2B; returnCode.toString());&#xA;          // Command failed; capture and log error details&#xA;          await session.getLogsAsString().then((logs) {&#xA;            print("FFmpeg command failed with logs: $logs");&#xA;          });&#xA;          // Handle failure, e.g., by showing an error message&#xA;          showSnackBarHelperERRORWrapLongString(context, "Failed to create video");&#xA;        }&#xA;      });&#xA;&#xA;      //try command 2&#xA;      if(_actual_video_file_ready_to_upload == null) {&#xA;        await FFmpegKit.execute(ffmpegCommand2).then((session) async {&#xA;          // Check the session for success or failure&#xA;          final returnCode = await session.getReturnCode();&#xA;          if (returnCode!.isValueSuccess()) {&#xA;            print("Video created successfully: $outputPath");&#xA;            //okay all set, now set the video to be this:&#xA;            _actual_video_file_ready_to_upload = File(outputPath);&#xA;            print ("video path is: ${outputPath}");&#xA;          } else {&#xA;            print("Failed to create video");&#xA;            print("FFmpeg process exited with:" &#x2B; returnCode.toString());&#xA;            // Command failed; capture and log error details&#xA;            await session.getLogsAsString().then((logs) {&#xA;              print("FFmpeg command failed with logs: $logs");&#xA;            });&#xA;            // Handle failure, e.g., by showing an error message&#xA;            showSnackBarHelperERRORWrapLongString(context, "Failed to create video");&#xA;          }&#xA;        });&#xA;      }&#xA;      //try command 3&#xA;      if(_actual_video_file_ready_to_upload == null) {&#xA;        await FFmpegKit.execute(ffmpegCommand3).then((session) async {&#xA;          // Check the session for success or failure&#xA;          final returnCode = await session.getReturnCode();&#xA;          if (returnCode!.isValueSuccess()) {&#xA;            print("Video created successfully: $outputPath");&#xA;            //okay all set, now set the video to be this:&#xA;            _actual_video_file_ready_to_upload = File(outputPath);&#xA;            print ("video path is: ${outputPath}");&#xA;          } else {&#xA;            print("Failed to create video");&#xA;            print("FFmpeg process exited with:" &#x2B; returnCode.toString());&#xA;            // Command failed; capture and log error details&#xA;            await session.getLogsAsString().then((logs) {&#xA;              print("FFmpeg command failed with logs: $logs");&#xA;            });&#xA;            // Handle failure, e.g., by showing an error message&#xA;            showSnackBarHelperERRORWrapLongString(context, "Failed to create video");&#xA;          }&#xA;        });&#xA;      }&#xA;    }else{&#xA;      print("file does not exist at $listFilePath");&#xA;    }&#xA;  }&#xA;</void>