shrink and speed up video from Sony action cam

ride.pl

#!/usr/bin/env perl

use strict;
use warnings;

use List::Util qw{max};
use List::MoreUtils qw{none};
use POSIX qw{ceil};

@ARGV == 2 or die "need video file";

my ($dir)  = $ARGV[0] =~ m{([^/]+)\.MP4$};
mkdir $_ for $dir, "$dir/discard";

my @args   = (qw{-r 1 -f image2}, "$dir/frame_%05d.jpg");
my $output = system "ffmpeg", "-i", $ARGV[0], @args;

my $max    = max map {/frame[_0]+(\d+)/} glob "$dir/frame_*.jpg";
my $height = int($max / 30); # random guess, seems to work
my $center = int($max / 2);
my $width  = int($max / 3);
my $e      = 2.71828;

my @frames = ();
my $next   = 1;

while ($next <= $max) {
  push @frames, sprintf "frame_%05d.jpg", $next;
  my $step = ($height * $e) ** -( (($next - $center) ** 2) / ((2 * $width) ** 2) );
  $next += ceil($step * 10);
}

opendir my($dh), $dir;
while (my $file = readdir $dh) {
  if (none {$file eq $_} @frames) {
    rename "$dir/$file", "$dir/discard/$file";
  }
}

system "ffmpeg", qw{-r 15/1 -f image2 -pattern_type glob -i}, "$dir/frame_*.jpg",
       qw{-c:v libx264 -r 30 -pix_fmt yuv420p -vf vflip,hflip,scale=640:-1}, $ARGV[1];