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• Implementing custom h264 quantization for Ffmpeg ?

  27 février 2017, par user2989813

  I have a Raspberry Pi, and I’m livestreaming using FFmpeg. Unfortunately my wifi signal varies over the course of my stream. I’m currently using raspivid to send h264 encoded video to the stream. I have set a constant resolution and FPS, but have not set bitrate nor quantization, so they are variable.

  However, the issue is that the quantization doesn’t vary enough for my needs. If my wifi signal drops, my ffmpeg streaming speed will dip below 1.0x to 0.95xish for minutes, but my bitrate drops so slowly that ffmpeg can never make it back to 1.0x. As a result my stream will run into problems and start buffering.

  I would like the following to happen :
  If Ffmpeg (my stream command)’s reported speed goes below 1.0x (slower than realtime streaming), then increase quantization compression (lower bitrate) exponentially until Ffmpeg speed stabilizes at 1.0x. Prioritize stabilizing at 1.0x as quickly as possible.

  My understanding is that the quantization logic Ffmpeg is using should be in the h264 encoder, but I can’t find any mention of quantization at all in this github : https://github.com/cisco/openh264
  My knowledge of h264 is almost zilch, so I’m trying to figure out

  A) How does h264 currently vary the quantization during my stream, if at all ?

  B) Where is that code ?

  C) How hard is it for me to implement what I’m describing ?

  Thanks in advance !!

• ffmpeg zoompan + crop

  24 février 2021, par andykais

  I need to pan across a video that is also being cropped by ffmpeg. How can I accomplish this in a single ffmpeg command ?

  


  So far I see two possible options :

  


  Option #1 : use crop variables pan across the video. This is choppy, but possibly fixed by upscaling the input.

  


  ffmpeg -i input.mp4 -vf 'crop=w=in_w/2:h=in_h:x=t*10' crop_panned_output.mp4


  


  Option #2 : create an alpha mask in the shape of the crop, and then apply it on top of the input video with a zoompan filter. ffmpeg is doing extra work in this case, because we have to pad the input, then zoom in on it, then pan across it, then apply the alpha mask. This is actually using the "zoompan" feature to "pan" though.

  


  magick -size 1920x1080 xc:none -fill black -draw "rectangle 480,0 1440,1080" rectangular-alpha-mask.png
ffmpeg -i input.mp4  -i rectangular-alpha-mask.png  -filter_complex "
  [0:v]pad=2112:1188,zoompan=z=1.1:px+0.5:d=1:fps=60:s=1920x1080[input_pan];
  [1:v]alphaextract[alf];
  [input_pan][alf]alphamerge[masked];
  color=s=1920x1080:color=red[base];
  [base][masked]overlay=shortest=1
"  mask_panned_output.mp4


  


  Are either of these the right way to do it or are they incredibly inefficient ? Is there another option ? Cropping & panning together feels like a fairly common workflow, but these solutions feel a bit hacky.

  


  Here is a visual description of what panning and cropping a video looks like :

  


  First start with a video. Here is a simple one.


  


  I want to crop the video to a certain width/height, and move from the left to the right across the video. The background is colored red for clarity


  


  The video moves from left to right across the original video, and keeps the same crop ratio.


  


• Extract alpha from video ffmpeg android

  24 juin 2018, par Yarik Denisyk

  I want to overlay transparent video on the background image. I have a video where the top half is RGB object and bottom half is an alpha mask.

  

  Now, for making this I do next steps :

  1) I am extracting all frames from video and save to the folder

  2) Each frame splitting to top and bottom half bitmap

  3) Top bitmap composite with bottom mask for extract alpha and get a frame with transparent background

  3) I am drawing each frame on the background and save to a folder

  4) Create a video using FFmpeg

  The problem is step 2, 3 and 4, they very slow. Maybe has another way to overlay transparent video on the background image ?