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I'm capturing an rtsp stream using ffmpeg every minute (underwater) and only want to discard any photos that are captured which is less than 10% light. This is my very simple cron script, which I'd like to augment.

#!/bin/bash
ffmpeg -loglevel panic -rtsp_transport tcp -y -i "rtsp://user:pass@ipaddress/stream" -frames:v 1 -strftime 1 "/dir/%Y-%m-%d_%H-%M-%S_underwater_cam.jpg"
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    What is your question? How to detect black images? Please edit your question, as it stands it is unclear what you're asking. – Panki Nov 23 '20 at 9:48
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I figured out a way, however it is not the most elegant solution from a processing standpoint. Adapted from this imagemagick discussion.

#!/bin/bash

set -ea
DIRECTORY=/camera/$(date +%Y-%m-%d)

[ $test -d $DIRECTORY ] && : || mkdir -p $DIRECTORY;

#Loglevel panic is to ignore the transform errors this camera is putting out.  
#Forcing the RTSP transport as TCP to reduce corrupt images. 
#Using date with strftime, however this could probably be done as a variable.
ffmpeg -loglevel panic -rtsp_transport tcp -y -i "rtsp://user:pass@ipaddress/stream" -frames:v 1 -strftime 1 "$DIRECTORY/%Y-%m-%d_%H-%M-%S_underwater_cam.jpg"

cd $DIRECTORY
#Find the most recent image, as it's assumed to be the picture above.
image=`ls -t $DIRECTORY | head -n1`

#threshold in percentage of light using imagemagick
thresh=10
mean=`convert $image -format "%[mean]" info:`
mean=`convert xc: -format "%[fx:100*$mean/quantumrange]" info:`
test=`convert xc: -format "%[fx: ($mean<$thresh)?1:0]" info:`

#example statement[ $test -eq 1 ] && echo "mean=$mean; too dark" || echo "mean=$mean; not too dark"
[ $test -eq 1 ] && rm $DIRECTORY/$image || :

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