Filter concat:out:v0 has a unconnected output | Concatenating with php-ffmpeg

Question

I'm incorporating PHP-FFMpeg to my symfony3 project which is working perfectly. However recently I've been needing to concatenate two videos into one, and it would appear it's not supported. So I created a ConcatVideoFilter class that I'm using based off of this

    public function apply(Video $video, VideoInterface $format)
    {
        $params = [];
        $count = count($this->files) + 1;
        foreach ($this->files as $i => $file) {
            $params[] = '-i';
            $params[] = $file;
        }
        $params[] = '-filter_complex';
        $params[] = 'concat=n='.$count.':v=1:a=1 [v]';
        $params[] = '-map';
        $params[] = '[v]';
        return $params;
    }

I'm getting an encoding failed error in my symfony profiler

I then copy pasted the actual ffmpeg command line to see what error it would give:

Which is giving me this error

Filter concat:out:v0 has a unconnected output

What does this error mean, and what can I do to get rid of it?

Answer 1

You need

    $params[] = '-filter_complex';
    $params[] = 'concat=n='.$count.':v=1:a=1 [v][a]';
    $params[] = '-map';
    $params[] = '[v]';
    $params[] = '-map';
    $params[] = '[a]';

Your concat filter exports an audio output but you didn't assign it an output pad and then map it.