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• How to read alpha channel from .webm video using ffmpeg in c++

  27 octobre 2022, par Pasin Kunamart

  Background

  


  I have a .webm file (pix_fmt : yuva420p) converted from .mov video file in order to reduce file size and I would like to read the video data using c++, so I followed using this repo as a reference.

  


  This works perfectly on .mov video.

  


  Problem

  


  By using same repo, however, there is no alpha channel data (pure zeros on that channel) for .webm video but I can get the alpha data from .mov video.

  


  Apparently many people already noticed that after the video conversion, ffmpeg somehow detect video as yuv420p + alpha_mode : 1 and thus alpha channel is not used but there is no one discuss about workaround of this.

  


  I tried forcing pixel format during this part to use yuva420p but that just broke the whole program.

  


    // Set up sws scaler
    if (!sws_scaler_ctx) {
        auto source_pix_fmt = correct_for_deprecated_pixel_format(av_codec_ctx->pix_fmt);
        sws_scaler_ctx = sws_getContext(width, height, source_pix_fmt,
                                        width, height, AV_PIX_FMT_RGB0,
                                        SWS_BILINEAR, NULL, NULL, NULL);
    }


  


  I also verified my video that it contains alpha channel using other source so I am sure there is alpha channel in my webm video but I cannot fetch the data using ffmpeg.

  


  Is there a way to fetch the alpha data ? Other video format or using other libraries work as well as long as it does have some file compression but I need to access the data in c++.

  


  Note : This is the code I used for converting video to webm

  


  ffmpeg -i input.mov -c:v libvpx-vp9 -pix_fmt yuva420p output.webm


  


• lavc/videotoolboxenc : warn when alpha quality not set

  20 mai 2023, par Rick Kern

  lavc/videotoolboxenc : warn when alpha quality not set
  Added a warn log when alpha quality cannot be set, but continue encoding.
  Signed-off-by : Rick Kern <kernrj@gmail.com>
  

  • [DH] libavcodec/videotoolboxenc.c

• Rounded corners in subtitle (Advanced Substation Alpha [.ass])

  3 mars, par Leander Mihm

  Is it possible to have rounded corners with the BorderStyle 4 in .ass (Advanced Substation Alpha) ?
I only found out that the BorderStyle 4 exists, because I was looking at this stackoverflow. Is there any good and complete documentation of the Advanced Substation Alpha format ?

  


  I'm currently using the following configuration :

  


  BorderStyle=4
Outline=10