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--- doc:appunti:linux:video:ffmpeg_stack_horizontally [2021/10/28 10:52] – created niccolo
+++ doc:appunti:linux:video:ffmpeg_stack_horizontally [2021/10/28 11:43] (current) – niccolo
@@ Line 1: / Line 1: @@
 ====== Stack two videos side-by-side with ffmpeg ======
-I created a 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**.
+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**.
-{{.:ffmpeg:video1.jpg?360|}}
+Suppose that we have two videos taken simultaneously with the two cameras:
-{{.:ffmpeg:video2.jpg?360|}}
-The result is a video like the following: the **central part** of each video is stacked horizontally on the final video, 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: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>