====== Stack two videos side-by-side with ffmpeg ======

I created a Unix shell script to do a **side-by-side comparison** between videos recorded with my two action cameras: a new **SJCAM 8 Pro** and the old **Xiaomi Yi**.

Suppose that we have two videos taken simultaneously with the two cameras:

{{.:ffmpeg:video1.jpg?320|}}
{{.:ffmpeg:video2.jpg?320|}}

The result will be a video like the following: the **central part** of each video is stacked horizontally on the final frame, two **labels are overlayed** on the bottom corners, the **audio** from the first video is mixed into the **left channel** of the resulting video and the audio from the second video goes to the **right channel**.

{{.:ffmpeg:video_hstack.jpg?640|}}

The script uses **ffmpeg** to do all the incantation. It is required that both videos share **the same resolution**.

The script can take into account that the videos may be **out of sync**: it is possible to pass **two timestamps** referring at the same instant captured into both videos. The script will **shift the videos presentation time** each-other, so that both will be synced. 

The script has some constraints. It requires the **Bash shell**, because it uses the **%%((expression))%%** to do arithmetic evaluation. The script requires also the **Python** interpreter, just to do a floating point difference and properly formats the result.

<code>
#!/bin/bash

# Take the central part of two videos and stack them horizontally.
# Audio from the first video is merged into the left channel of the
# resulting video, audio from the second video goes to the right.
# Two lables are overlayed on the bottom left and right corners.

# https://stackoverflow.com/questions/35349935/ffmpeg-crop-with-side-by-side-merge
# https://unix.stackexchange.com/questions/233832/merge-two-video-clips-into-one-placing-them-next-to-each-other
# https://trac.ffmpeg.org/wiki/AudioChannelManipulation

# Left and right videos.
#VIDEO_1='20211023135104_0322.MP4'
#VIDEO_2='YDXJ0851.mp4'
VIDEO_1="$1"
VIDEO_2="$2"

# Two timestamps which should be synced.
#TIME_1='15.482'
#TIME_2='16.282'
TIME_1="$3"
TIME_2="$4"

# Labels to be superimposed on the two halves.
#LABEL_1='SJ8Pro'
#LABEL_2='Xiaomi Yi'
LABEL_1="$5"
LABEL_2="$6"

#OUTPUT='output.mkv'
OUTPUT="$7"

if [ -z "$VIDEO_1" -o -z "$VIDEO_2" -o \
     -z "$TIME_1"  -o -z "$TIME_2"  -o \
     -z "$LABEL_1" -o -z "$LABEL_2" -o \
     -z "$OUTPUT" ]; then
    echo
    echo "Usage: $(basename $0) [video1] [video2] [time1] [time2] [label1] [label2] [output]"
    echo
    echo "  Example: $(basename $0) 20211023135104_0322.MP4 YDXJ0851.mp4 15.482 16.282 SJ8Pro 'Xiaomi Yi' output.mkv"
    echo
    exit 1
fi

# Do the math, etc...
if (( $(echo "$TIME_1 > $TIME_2" | bc -l) )) ; then
    TO_1="$(python -c "print('%0.3f' % ($TIME_1 - $TIME_2,))")"
    TO_2='0.000'
else
    TO_1='0.000'
    TO_2="$(python -c "print('%0.3f' % ($TIME_2 - $TIME_1,))")"
fi

# Font and style for labels.
FONT='fontfile=/usr/share/fonts/truetype/msttcorefonts/Arial_Bold.ttf:'
STYLE='fontcolor=yellow:fontsize=48:box=1:boxcolor=black@0.5:boxborderw=10'
# Labels placement.
L1X='20'
L2X='(w-text_w-20)'
L1Y='(h-text_h-20)'
L2Y='(h-text_h-20)'

ffmpeg -ss "$TO_1" -i "$VIDEO_1" -ss "$TO_2" -i "$VIDEO_2" \
    -filter_complex \
     "[0:v]crop=iw/2:ih:ow/2:0[left]; \
      [1:v]crop=iw/2:ih:ow/2:0[right]; \
      [left][right]hstack=shortest=1[vs]; \
      [0:a][1:a]amerge=inputs=2,pan=stereo|c0<c0+c1|c1<c2+c3[a]; \
      [vs]drawtext=${FONT}text='${LABEL_1}':${STYLE}:x=${L1X}:y=${L1Y},drawtext=${FONT}text='${LABEL_2}':${STYLE}:x=${L2X}:y=${L2Y} [v]" \
    -map '[v]' -map '[a]' \
    "$OUTPUT"
</code>