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I'm trying to add a reticle to a live video feed, and need the reticle to be able to move based on some external data.

I can draw a box on the video with ffmpeg:

ffmpeg -i input.mpg -vf drawbox=x=100:y=100:w=20:h=20:c=red output.mpg

(Note that for now I'm just testing with video files since that makes some parts of it simpler.)

However, there doesn't appear to be a way to adjust the x and y values externally once the command has started. This would be ideal if there was a way to easily read them in e.g. from stdin or /dev/shm/reticle-pos.txt, but I can't find a way to do that.

Another option I've tried is to draw the reticle externally with imagemagick and then overlay it with ffmpeg:

while true; do
    for x in {100..500..1}; do
        convert -size 1080x720 xc:none -fill none -stroke red -strokewidth 2 \
            -draw "ellipse $x,100 10,10 0,360" -scale 1080x720 /dev/shm/newreticle.png
        cp /dev/shm/newreticle.png /dev/shm/reticle.png
        sleep 0.001
    done
done &

ffmpeg -i input.mpg -i /dev/shm/reticle.png -filter_complex overlay output.mpg

However this doesn't work, as it appears that ffmpeg only reads the image file once, rather than on each frame.

How can I draw a reticle over top of the video, in a way that lets me control the reticle's position?

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using loop option:

ffmpeg -i input.mpg -loop 1 -f image2 -i /dev/shm/reticle.png -filter_complex overlay output.mpg
  • ffmpeg reads reticle.png on every frame by using loop option of image2 demuxer.
  • Update rectile.png atomically by writing a new reticle to a temp file first and then rename to rectile.png.
  • For details: https://ffmpeg.org/ffmpeg-formats.html#image2-1
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I've figured out a way to do this. It's probably not the best way, but it works:

reticle(){
    convert -size 1080x720 xc:none \
        -fill none -stroke red -strokewidth 2 -draw "ellipse $1,$2 10,10 0,360" \
        -size 1080x720 -depth 8 rgba:-
}
export -f reticle

parallel -k reticle {} 100 ::: {100..500} \
| ffmpeg \
    -i input.mpg \
    -f rawvideo -pixel_format bgr32 -video_size 1080x720 -framerate 30 -i pipe:0 \
    -filter_complex overlay \
    -y -c libx264 output.mpg

This works by having imagemagick output raw pixel data (-size 1080x720 -depth 8 rgba:-). The pixel data for each frame gets concatenated together with the rest into a single raw video stream. ffmpeg is then able to read that raw video stream (-f rawvideo -pixel_format bgr32 -video_size 1080x720 -framerate 30 -i pipe:0) and overlay it onto the main video.

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