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• 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 !

  


• Add Padding to video

  26 juin 2023, par Akari

  I have a video which size is 576 x 1024, my screen phone is 576 x 2044. I want to make the height of video is 2044 (full screen) and add padding black background both top and bottom. What can I do ? I currently use FFmpeg library.

  


  I want to process video in android kotlin, and I use some commands but it it doesn't work.

  


  Example :

  


  fun makeVideoFullScreen(
    context: Context,
    inputVideoPath: String,
    screenWidth: Int,
    screenHeight: Int,
    listener : (outputPath: String, returnCode: Int) -> Unit
){
    val fileName = Utility.getCurrentTime()
    val externalDir = context.getExternalFilesDir(null)
    val file = File(externalDir, "temp_video${fileName}.mp4")
    val outputPath = file.absolutePath

    val cmd = arrayOf(
        "-i", inputVideoPath,
        "-vf", "scale=-1:$screenHeight, pad=$screenWidth:$screenHeight:(ow-iw)/2:(oh-ih)/2:black",
        "-c:v", "libx264",
        "-preset", "ultrafast",
        "-c:a", "copy",
        outputPath
    )

    FFmpeg.executeAsync(cmd
    ) { _, returnCode ->
        listener(outputPath, returnCode)
    }
}


  


• How to use ffmpeg to overlay waveforms on xstack mosaics and specify specific audio for playback

  1er mai 2022, par kellib

  I would like to make a mosaic of multiple titled streams, 1) specifying which of the audio streams to play and 2) overlay waveforms at the bottom of each of the video tiles for the audio that they belong to.

  


  I'm successfully able to create the titled mosaic of streams with the code below.

  


  However :

  


  

  1. I'm having a hard time figuring out how to specify just one of the specific audio sources. I found amix, but I don't really want to mix them, I just want to specify audio [a0], or [a1], or [a2], etc.

  


  


  and

  


  

  2. I'm having a hard time figuring out how to overlay the wave forms at the bottom of the video for each of the tiles. I struggled trying to figure out putting showwaves into the mix. Is it possible ?

  


  


  I want each tile to look like this, but since these are rtmp streams, they need to play-out the matching waveforms dynamically with each stream. https://dragonquest64.blogspot.com/2020/01/ffmpeg-audio-waveform.html

  


  If someone could point me in the right direction, that would be great. I'm getting close, but I'm pretty new to all of this, and have already spent way more time than I should have, so would love a little help.

  


  ffmpeg \
-i rtmp://my.cdn.com/srcEncoders/STREAM-1 \
-i rtmp://my.cdn.com/srcEncoders/STREAM-2 \
-i rtmp://my.cdn.com/srcEncoders/STREAM-3 \
-i rtmp://my.cdn.com/srcEncoders/STREAM-4 \
  -filter_complex " \
      [0:v] setpts=PTS-STARTPTS, scale=qvga \
    , drawtext=text=STREAM-1:fontsize=20:x=10:y=10:fontcolor=white:box=1:boxcolor=black@0.5:boxborderw=5 [a0]; \
      [1:v] setpts=PTS-STARTPTS, scale=qvga \
    , drawtext=text=STREAM-2:fontsize=20:x=10:y=10:fontcolor=white:box=1:boxcolor=black@0.5:boxborderw=5 [a1]; \
      [2:v] setpts=PTS-STARTPTS, scale=qvga \
    , drawtext=text=STREAM-3:fontsize=20:x=10:y=10:fontcolor=white:box=1:boxcolor=black@0.5:boxborderw=5 [a2]; \
      [3:v] setpts=PTS-STARTPTS, scale=qvga \ 
    , drawtext=text=STREAM-4:fontsize=20:x=10:y=10:fontcolor=white:box=1:boxcolor=black@0.5:boxborderw=5 [a3]; \
      [a0][a1][a2][a3]xstack=inputs=4:layout=0_0|0_h0|w0_0|w0_h0[out]; \
    amix=inputs=1
      " \
  -map "[out]" \
 -c:v libx264 -b:v 1000k -g 30 -keyint_min 120 -profile:v baseline -preset veryfast -f mpegts "udp://127.0.0.1:1234?pkt_size=1316"