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• ffmpeg alpha channel of images lost

  4 mai 2021, par gilad s

  Using the following command I make images appear partially transparent. This seems to work as expected.

  


   convert original.jpg -alpha set -background none -channel A -evaluate multiply 0.2 +channel image_01.png
 convert original.jpg -alpha set -background none -channel A -evaluate multiply 0.2 +channel image_02.png


  


  After the images are created I use ffmpeg to turn them into a video :

  


   ffmpeg -i image_%02d.png -pix_fmt yuv420p -c:v libx264 result.mp4


  


  The resulting video doesn't include the partial transparency. The images appear just like the original. It seems the alpha channel of the images is lost.

  


• Converting a MOV to WEBP

  27 janvier 2020, par user3469011

  I have a transparent video that I would like to include on my website with a background behind it. I am trying to convert it to .WEBP via ffmpeg.

  I have previously succeeded in converting it to .WEBM, but many browsers don’t support it, so I thought trying .WEBP. Although .WEBP is also not supported on all browsers, there is a JS package to fix that : http://webpjs.appspot.com/

  I have tried (A) from .mov to .webp directly and (B) first from .mov to .webm to then go to .webp
  The conversion to .webp isn’t correct. The new frames are rendered over the old ones. This question had the same problem, but it doesn’t start from a .Mov like me so I doesn’t help me : Transparent animated WebP not clearing frames

  A)

  ffmpeg -i input.mov -vcodec libwebp -lossless 1 -qscale 0 -preset none -an -vsync 0 -loop 1 output4.webp

  B)

  ffmpeg -i input.mov -c:v libvpx -b:v 0.5M -filter:v "crop=in_w*4/6:in_h" -c:a libvorbis -auto-alt-ref 0 -vf scale=960:540 output.webm

  ffmpeg -i output.webm -vcodec libwebp -lossless 1 -q 60 -preset default -an -vsync 0 -loop 1 output.webp

  My goal is to try webp as a way to show transparent animations.
  (GIFs are to big, APNG is not supported and also to big).

  If someone has another alternative, let me know ! I rather not started coding them in actual html/css/js, because an animator already took the time to make them in after effects...

• Use ffmpeg to compile RGBA image sequence into two separate videos (RGB + Alpha)

  27 novembre 2018, par MirceaKitsune

  I plan on using Blender to render animated sequences for a game. I need the videos to have transparent backgrounds in the format expected by its engine. This format involves the transparency being defined as a separate grayscale video of equal FPS and duration. Since splitting the RGB and Alpha channels in Blender is more complicated, I’d prefer doing this directly from ffmpeg.

  The input directory contains an image sequence of png files in RGBA format (eg : 0000.png to 0100.png). What I need ffmpeg to do is compile them into two separate videos : One containing only the RGB channels (leave transparency black) and another containing only the grayscale alpha channel. In the end I’d have something like my_video.mp4 + my_video_mask.mp4.

  I’m familiar with compiling non-transparent image sequences into video using :

  ffmpeg -f image2 -i /path/to/sequence/%04d.png output.mp4

  But I don’t know how to extract the alpha channel and make it a separate video file. What is the simplest ffmpeg command to achieve this result ?