Sur d’autres sites (16547)

• FFmpeg : How to color-key all colors except blue and white ?

  21 février 2018, par DanielNg

  I know how to colorkey a specific color from an overlay video based on the documentation from FFmpeg. Here is the code I used to colorkey the black color out of a video then overlay it with a base video keeping the audio from the overlaid video :

  "ffmpeg -i " + baseVid + " -i " + overlayVid + " -y" + ' -filter_complex "[1:v]colorkey=0x000000:0.6:0.0[ckout];[0:v][ckout]overlay=shortest=1[out]" -shortest -map 1:a -map "[out]" ' + output

  However, my problem is that the overlaid video also contains other colors such as blue, white, brown, grey, green. Even though they are only small portions compared to the black that has been removed, they make the colorkeyed overlaid video look really dirty. I want to keep only the two main colors (blue and white) and removing the rest from the overlaid video. How do I do this using FFmpeg. I tried this code but it did not work.

• FFMPEG How to trim last 2 seconds of multiple files ?

  28 août 2022, par user10784270

  I have over 200 video clips each exactly 5 seconds long, however, the last 2 seconds of each are just a black screen so I want to have this trimmed out. How would I do this ?

  


• ffmpeg - overlay filter alpha premultiplied performs brighter on transparent images

  26 avril 2022, par windows_air

  Description

  


  I'm using ffmpeg's overlay filter with alpha premultiplied parameter. I noticed that when a completely transparent image was overlay to the background video, the output video was much brighter.

  


  In the following demo, I will use RGBA8888 format images for overlay. Its use of RGBA (255,255,255,0) indicates full transparency (aka completely transparent white), and the result demonstrates a very noticeable brightening (the expected result should be unchanged, since the image is completely transparent).
ps : Using RGBA (0,0,0,0) (aka completely transparent black) also results in a brightened image.

  


  Code (overlay transparent white)

  


  ffmpeg.exe  -loglevel debug -i a1.mp4 -i a1.png -filter_complex "[0:v][1:v]overlay=x=0:y=0:alpha=premultiplied[v]" -map "[v]"  -c:v:0 libx264 -f mp4  output.mp4


  


  result_img_white

  


  Code (overlay transparent black)

  


  This command uses lavfi to simulate the transparent black output of RGBA. For demonstration purposes only, you may need ctrl+c to prevent infinite output.

  


  ffmpeg.exe -y -i a1.mp4 -f lavfi -i "color=color=black@0.0:size=1920x540,format=rgba" -filter_complex "[0:v][1:v]overlay=x=0:y=0:alpha=premultiplied[v]" -map "[v]"  -c:v:0 libx264 -f mp4  output.mp4


  


  result_black_img

  


  The results show that the upper part is more whitish (brighter).

  


  Related files can be downloaded here : https://gist.github.com/windowsair/5315437a97dadf3f74f886486657183d

  


  ---

  


  Back to the question, I had to use premultiplied to get the correct overlay result : Using the straight option will have a darkening effect in areas that are not fully transparent, which is why I insisted on premultiplied . Is this a ffmpeg problem ? How should I avoid the undesirable effect of brighter images from transparent image overlays ? Thank you !