This guide explains how to convert MOV files with transparency, also known as an alpha channel, into WEBM files with an alpha channel. The conversion is done with FFmpeg using a PowerShell script on Windows.

The script converts all .mov files from this folder:

C:\temp\ffmpeg

The finished .webm files are saved in the same folder.

Example:

C:\temp\ffmpeg\A14-PETER.mov

becomes:

C:\temp\ffmpeg\A14-PETER.webm

The WEBM file is created with these settings:

Format: WEBM
Codec: VP9
Alpha channel: yes
Frame rate: 25 fps
Scan type: progressive
Bitrate: approx. 4 Mbit/s

If the folder does not already exist, create it as follows:

1. Open Windows Explorer.
2. Go to:

C:\

1. Create a new folder there:

temp

1. Open the temp folder.
2. Create another folder inside it:

ffmpeg

The full path should then be:

C:\temp\ffmpeg

Alternatively, the folder can be created in PowerShell:

New-Item -ItemType Directory -Path "C:\temp\ffmpeg" -Force

For the script to work, ffmpeg.exe must be located in this folder:

C:\temp\ffmpeg\ffmpeg.exe

The folder should therefore contain at least this file:

C:\temp\ffmpeg\ffmpeg.exe

Optionally, ffprobe.exe can also be placed in the same folder. This is useful for checking files later.

Copy all MOV files that should be converted to:

C:\temp\ffmpeg

Example:

C:\temp\ffmpeg\A14-PETER (vMix - Output 3).mov

The script automatically processes all files with the extension:

.mov

Open PowerShell and enter:

notepad C:\temp\ffmpeg\convert.ps1

If asked whether the file should be created, confirm with Yes.

Then paste the following content into Notepad:

$ErrorActionPreference = "Stop"
 
$folder = "C:\temp\ffmpeg"
$ffmpegLocal = Join-Path $folder "ffmpeg.exe"
 
if (Test-Path $ffmpegLocal) {
    $ffmpeg = $ffmpegLocal
} else {
    $ffmpeg = "ffmpeg"
}
 
$movFiles = Get-ChildItem -Path $folder -Filter "*.mov" -File
 
if ($movFiles.Count -eq 0) {
    Write-Host "No .mov files found in $folder."
    exit 0
}
 
foreach ($file in $movFiles) {
    $input = $file.FullName
    $output = Join-Path $folder ($file.BaseName + ".webm")
 
    Write-Host ""
    Write-Host "Converting:"
    Write-Host "  Input : $input"
    Write-Host "  Output: $output"
 
    $cmd = "`"$ffmpeg`" -y -i `"$input`" -vf `"yadif=0:-1:0,fps=25`" -c:v libvpx-vp9 -pix_fmt yuva420p -auto-alt-ref 0 -b:v 4M -minrate 4M -maxrate 4M -crf 25 `"$output`""
 
    cmd /c $cmd
 
    if ($LASTEXITCODE -eq 0) {
        Write-Host "OK: $output"
    } else {
        Write-Host "ERROR: FFmpeg exit code $LASTEXITCODE for $input"
    }
}
 
Write-Host ""
Write-Host "All conversions completed."

Save the file.

The script path is then:

C:\temp\ffmpeg\convert.ps1

Go to the folder:

C:\temp\ffmpeg

The file convert.ps1 should be located there.
Right-click the file and choose Run with PowerShell.
If the conversion starts, everything is working correctly.

OR:

Open PowerShell and change to the folder:

cd C:\temp\ffmpeg

If PowerShell blocks script execution, enter this first:

Set-ExecutionPolicy -Scope Process -ExecutionPolicy Bypass

Then start the script:

.\convert.ps1

The script now searches for all .mov files in the folder and creates .webm files from them.

This line is the most important part of the script:

$cmd = "`"$ffmpeg`" -y -i `"$input`" -vf `"yadif=0:-1:0,fps=25`" -c:v libvpx-vp9 -pix_fmt yuva420p -auto-alt-ref 0 -b:v 4M -minrate 4M -maxrate 4M -crf 25 `"$output`""

Meaning of the most important parameters:

After a successful conversion, the WEBM file is located in the same folder as the MOV file.

Example:

C:\temp\ffmpeg\A14-PETER (vMix - Output 3).webm

To check whether the file contains alpha channel information, run this in PowerShell:

.\ffmpeg.exe -i "C:\temp\ffmpeg\A14-PETER (vMix - Output 3).webm"

In the output, the video stream should contain a hint like this:

alpha_mode

or a pixel format such as:

yuva420p

This indicates that the file was saved with an alpha channel.

This means that there is no MOV file in the folder:

C:\temp\ffmpeg

Solution: copy the MOV files into this folder.

This means that ffmpeg.exe is missing.

Solution: copy ffmpeg.exe into this folder:

C:\temp\ffmpeg

Possible causes:

1. The original MOV file does not contain an alpha channel.
2. The MOV file had already been exported without alpha.
3. The player does not display WEBM alpha correctly.

For testing, open the WEBM file in an application that supports WEBM with alpha.

1. Create the folder:

C:\temp\ffmpeg

1. Place ffmpeg.exe in this folder.
2. Copy MOV files into this folder.
3. Create the script:

C:\temp\ffmpeg\convert.ps1

1. Run the script:

cd C:\temp\ffmpeg
Set-ExecutionPolicy -Scope Process -ExecutionPolicy Bypass
.\convert.ps1

1. Use the WEBM files from the same folder.