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• flac : Fix channel order for mono files.

  28 juin 2014, par Erik de Castro Lopo

  flac : Fix channel order for mono files.
  * The default channel mask for mono files was 0x0001 (front left) but it
  
    makes more sense to use 0x0004 (front center) for such files.
  * Also FLAC will accept not only mono WAV files with 0x0001 mask, but also
  
    with 0x0002 (requested at https://sourceforge.net/p/flac/bugs/390/)
  
    and 0x0004 (e.g. SoX creates mono files with this mask).
  * The comment about channel support was updated.
  * The error message
  
       "Use —channel-map=none option to store channels in current order ; FLAC files
  
       must also be decoded with —channel-map=none to restore correct order."
  
    is misleading : FLAC never changes the order of channels.
  
    Decoding with this options also sets the channel mask of the resulting WAV
  
    file to 0. Without this option the mask is equal to the value of
  
    WAVEFORMATEXTENSIBLE_CHANNEL_MASK tag.
  Patch-from : lvqcl <lvqcl.mail@gmail.com>
  

  • [DH] src/flac/decode.c
  • [DH] src/flac/encode.c

• Trying to crop an image in ffmpeg using alphamerge but produces wrong alpha

  12 septembre 2021, par Alex Styl

  I am using alphamerge in order to crop an image within a circle.

  


  What I have so far is :

  


  ffmpeg -f lavfi -i "color=c=white:size=240x240" -i avatar.png -i mask.png -filter_complex \
 "[1][2]alphamerge[img]; \
 [0][img]overlay[out]" -c:v png -map "[out]" -pix_fmt rgba -t 5 -y out.mp4 2>&1


  


  with avatar.png and mask.png respectively being :

  


  

  


  This produces the following output (1 frame of the output video) :

  


  

  


  which is unexpected, given the original input is much darker than this.

  


  How can I crop the 'avatar.png' using the 'mask.png' so that the output is the avatar.png cropped in a circle and keeping the same alpha ?

  


  PS : The important bit here is for me to be able to crop the original image and maintaining the correct colors/apha of the original image. If there is an other way of doing this (other than alphamerge) I am happy to hear it.

  


• Capture multiple individual streams ALSA FFMPEG

  3 novembre 2017, par user3170450

  There are similar questions available but I couldn’t understand them properly as per my use case. As I am new to ALSA so I would like to explain my use case first.

  I am opening my application via google chrome and I need to to capture the audio being played in that chrome window. I did it successfully by capturing the speaker sound with following command :

  ffmpeg -f pulse -i alsa_output.pci-0000_00_1b.0.analog-stereo.monitor -ac 1 -ar 16000 test.wav

  But the real problem is I will be opening multiple windows at the same time and I need to capture each audio stream separately. There is something like Virtual Audio device but I don’t know how to configure and use them for my use case.

  Please guide me in right direction.

  Just for a note my system has only one physical sound card available which I am capturing for one window.