badtv.sh

#!/bin/bash
set -o errexit -o pipefail -o nounset

# Get args.
if [[ $# -lt 1 ]]; then
    1>&2 echo "Usage: $0 MKV"
    exit 1
fi

# Input and output.
MKV_IN="$1"
MKV_OUT="$(basename "$MKV_IN" .mkv).bad.mkv"

# Two temporary files.
ORIGINAL_VIDEO="$(mktemp --tmpdir original.XXXXXXXX)"
BAD_VIDEO="$(mktemp --tmpdir downgraded.XXXXXXXX.mp4)"

function cleanup() {
    rm -f "$ORIGINAL_VIDEO" "$BAD_VIDEO"
}
trap cleanup exit

# Find video track.
VIDEO_TRACK_ID="$(mkvinfo "$MKV_IN" | \
    awk '/Track number/ {vt = $5-1} /Track type: video/ {print(vt)}')"

# Extract video track.
mkvextract tracks "$MKV_IN" "$VIDEO_TRACK_ID:$ORIGINAL_VIDEO"

# Re-encode video track.
ffmpeg -y -i "$ORIGINAL_VIDEO" -c:v libx264 -threads 1 "$BAD_VIDEO"

# Merge into new container; replacing the video track.
mkvmerge -o "$MKV_OUT" "$BAD_VIDEO" -d "!$VIDEO_TRACK_ID" "$MKV_IN"

Makefile

install:
	ln -sf $(PWD)/badtv.sh $(HOME)/.local/bin/