Recherche avancée

Sur d’autres sites (3580)

• 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.

• ffmpeg pipe output and script [duplicate]

  27 juillet 2018, par James Magnus

  This question already has an answer here :

  • Execute command in a variable don’t execute the latter part of a pipe
    
    1 answer
    

  I need to extract raw video and pipe it to another program. This runs perfectly in a terminal :

  ffmpeg -i SampleVideo_1280x720_5mb.mp4 -s 1920x1080 -c:v rawvideo -pix_fmt yuv420p -f rawvideo - | ffplay -f rawvideo -pix_fmt yuv420p -s 1920x1080 -

  But when I try to run this command from a script it fails. Here is a very simplified version of the script :

  #!/bin/bash
  
  
  
  c='ffmpeg -i SampleVideo_1280x720_5mb.mp4 -s 1920x1080 -c:v rawvideo -pix_fmt yuv420p -f rawvideo - | ffplay -f rawvideo -pix_fmt yuv420p -s 1920x1080 -'
  
  $c

  The output

  ~/ets/videos$ ./test.sh 
  
  ffmpeg version N-91403-gd24c9e5 Copyright (c) 2000-2018 the FFmpeg developers
  
    built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.10) 20160609
  
    configuration: --extra-cflags='-fPIC -I/home/jlbeaussart/ffmpeg_build/include' --extra-ldflags=-L/home/jlbeaussart/ffmpeg_build/lib --extra-libs='-lpthread -lm' --enable-gpl --enable-libaom --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-shared --enable-libcaca --enable-libxvid --enable-libxml2 --enable-opencl --enable-opengl --enable-cuvid --enable-ffnvcodec --enable-libdrm --enable-nvenc --enable-nvdec --enable-vaapi --enable-vdpau --enable-nonfree
  
    libavutil      56. 18.102 / 56. 18.102
  
    libavcodec     58. 20.104 / 58. 20.104
  
    libavformat    58. 17.101 / 58. 17.101
  
    libavdevice    58.  4.101 / 58.  4.101
  
    libavfilter     7. 25.100 /  7. 25.100
  
    libswscale      5.  2.100 /  5.  2.100
  
    libswresample   3.  2.100 /  3.  2.100
  
    libpostproc    55.  2.100 / 55.  2.100
  
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'SampleVideo_1280x720_5mb.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2avc1mp41
  
      creation_time   : 1970-01-01T00:00:00.000000Z
  
      encoder         : Lavf53.24.2
  
    Duration: 00:00:29.57, start: 0.000000, bitrate: 1421 kb/s
  
      Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 1032 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
  
      Metadata:
  
        creation_time   : 1970-01-01T00:00:00.000000Z
  
        handler_name    : VideoHandler
  
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 383 kb/s (default)
  
      Metadata:
  
        creation_time   : 1970-01-01T00:00:00.000000Z
  
        handler_name    : SoundHandler
  
  [NULL @ 0x18d5d40] Unable to find a suitable output format for '|'
  
  |: Invalid argument

• Very basic Moviepy script failing to run

  1er juillet 2018, par cem akbulut

  So, I have this very basic script that concatenates two videos, and adds a background sound.

  def intro():
  
  
  
      voiceoverIntro = AudioFileClip(audio[3]) #get path for intro.mp3
  
  
  
  
  
      introVideo = []
  
      temp = p1vid + p2vid + p3vid
  
  
  
      for x in temp: # add product videos to introVideo if they are longer than intro.mp3
  
          x = VideoFileClip(x)
  
  
  
          if x.duration >= voiceoverIntro.duration:
  
              introVideo.append(x)
  
  
  
  
  
      if len(introVideo) >= 1:
  
  
  
          #Get the video and turn off its volume
  
          introBg = introVideo[0].volumex(0).resize((1280,720))
  
          introBg = introBg.set_audio(voiceoverIntro).set_duration(voiceoverIntro.duration)
  
  
  
  
  
          introBg.reader.close()
  
          introBg.audio.reader.close_proc()
  
  
  
          return introBg

  And this function creates the second clip

  def productThreeOpener(productName):
  
      opener3 = VideoFileClip("media/number3.mp4").volumex(0).resize((1280,720))
  
  
  
  
  
  
  
      text = TextClip(productName,fontsize=60,color='white', font="Dekar")\
  
          .set_pos(("right", "center"))\
  
          .set_duration(3)\
  
          .set_start(2)\
  
          .crossfadein(1)\
  
          .crossfadeout(1)\
  
          .margin(right=200,opacity=0)
  
  
  
      opener3 = CompositeVideoClip([opener3, text])
  
  
  
      return opener3

  Now we just concatenate the two outputs

  final_clip = concatenate_videoclips([intro, productThreeOpener], method="compose")

  final_clip.resize((1280, 720)).write_videofile("output/intro.mp4", threads=8, fps=24, progress_bar=False)

  Running the code above returns this error ;

  Traceback (most recent call last):
  
  [MoviePy] >>>> Building video output/intro.mp4
  
    File "C:/Users/Akbulut/PycharmProjects/youtubeVideoEditor/vidEditor/sandbox.py", line 145, in <module>
  
  [MoviePy] Writing audio in introTEMP_MPY_wvf_snd.mp3
  
      final_clip.write_videofile("output/intro.mp4", threads=8, fps=24, progress_bar=False)
  
    File "", line 2, in write_videofile
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\decorators.py", line 54, in requires_duration
  
      return f(clip, *a, **k)
  
    File "", line 2, in write_videofile
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\decorators.py", line 137, in use_clip_fps_by_default
  
      return f(clip, *new_a, **new_kw)
  
    File "", line 2, in write_videofile
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\decorators.py", line 22, in convert_masks_to_RGB
  
      return f(clip, *a, **k)
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\video\VideoClip.py", line 318, in write_videofile
  
      progress_bar=progress_bar)
  
    File "", line 2, in write_audiofile
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\decorators.py", line 54, in requires_duration
  
      return f(clip, *a, **k)
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\audio\AudioClip.py", line 213, in write_audiofile
  
      progress_bar=progress_bar)
  
    File "", line 2, in ffmpeg_audiowrite
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\decorators.py", line 54, in requires_duration
  
      return f(clip, *a, **k)
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\audio\io\ffmpeg_audiowriter.py", line 165, in ffmpeg_audiowrite
  
      ffmpeg_params=ffmpeg_params)
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\audio\io\ffmpeg_audiowriter.py", line 71, in __init__
  
      self.proc = sp.Popen(cmd, **popen_params)
  
    File "C:\Program Files\Python36\lib\subprocess.py", line 594, in __init__
  
      _cleanup()
  
    File "C:\Program Files\Python36\lib\subprocess.py", line 205, in _cleanup
  
      res = inst._internal_poll(_deadstate=sys.maxsize)
  
    File "C:\Program Files\Python36\lib\subprocess.py", line 1025, in _internal_poll
  
      if _WaitForSingleObject(self._handle, 0) == _WAIT_OBJECT_0:
  
  OSError: [WinError 6] İşleyici geçersiz
  
  Exception ignored in: <bound method="method" of="of" object="object" at="at" 0x0000000a885019b0="0x0000000a885019b0">>
  
  Traceback (most recent call last):
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\audio\io\ffmpeg_audiowriter.py", line 134, in __del__
  
      self.close()
  
    File "C:\Program Files\Python36\lib\site-packages\moviepy\audio\io\ffmpeg_audiowriter.py", line 122, in close
  
      if self.proc:
  
  AttributeError: 'FFMPEG_AudioWriter' object has no attribute 'proc'
  
  
  
  Process finished with exit code 1
  
  </bound></module>

  I strictly followed MoviePY installation guide, installed it and all of its dependencies about 2 days ago. I work on a 64b Windows machine. Python version 3.6.1

  I’ve been stuck with this error for over a day now, and I don’t know what to try.. Any help/advice is highly appreciated.

  Thanks.