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Bright Screen Flash Alert

Most tech streamers I watch in Software and Game Dev on Twitch use dark mode. They also tend to hit pages without dark mode when looking up documentation. The bright flash can knock you back. I thought it would be fun to trigger a yelling sound effect when that happens.


This script plays a sound file when the overall screen brightness crosses a certain threshold. This version works on a mac. Pull requests to produce versions for other platforms are welcome.

The Script



mkdir -p images

trap "trap - SIGTERM && kill -- -$$" \

ffmpeg -f avfoundation \
    -hide_banner \
    -loglevel panic \
    -i "$SCREEN:0" \
    -vf "fps=5,scale=100:-2" \
    -y "images/%04d.png" &

fswatch -l 0.1 -0 images/ | \
    while read -d "" event
    if [ -e "$event" ]
        BRIGHT_F=`convert "$event" \
            -colorspace gray \
            -format "%[fx:100*mean]" \
        BRIGHT_I=`printf %.0f "$BRIGHT_F"`
        echo -n "$BRIGHT_I "
        if [ $BRIGHT_I -gt $BRIGHTNESS ]
             if [ $THROTTLE -eq 0 ]
                 afplay "$AUDIO_FILE" &
                 echo "YELL"
         rm "$event"


You'll need ffmpeg, imagemagick, and fswatch to run the script. They can be installed with Homebrew via these commands if you don't already have them:

brew install ffmpeg
brew install imagemagick
brew install fswatch


Here's how to set things up:

  1. Save the code for the script above into a new file called:
  2. Download the sound file and put it in the same directory as the script
  3. Configure the screen to watch via the instructions in the "Picking A Screen" section below
  4. And to start it up, run the script from a command line terminal with:

    /bin/bash yell-when-it-gets-bright.bash

If all goes well, you'll hear the sound trigger each time your screen crosses a certain brightness threshold.

Press CTRL c in the terminal window when you're ready to stop the process.

Picking A Screen

The ffmpeg command in the script needs to be configured to watch a specific screen. Run this command to get the data you need to set it up:

ffmpeg -f avfoundation -list_devices true -i ""

That will spit out a bunch of stuff ending with something like this:

[AVFoundation indev @ 0x15a709999] AVFoundation video devices:
[AVFoundation indev @ 0x15a709999] [0] FaceTime HD Camera
[AVFoundation indev @ 0x15a709999] [1] Capture screen 0
[AVFoundation indev @ 0x15a709999] [2] Capture screen 1
[AVFoundation indev @ 0x15a709999] AVFoundation audio devices:
[AVFoundation indev @ 0x15a709999] [0] MacBook Pro Microphone

Pick the video device you want to use and update the SCREEN variable in the script. For example, the output I get when I run the command includes [1] Capture screen 0 so I setup my copy of the script with SCREEN=1

If you have multiple screen and you're not sure which one to use, try one and see if it's right. If not, try the other ones until you hit the one you want.

Watch Out For Spaces

The script is written in bash. One trick with that is that can't leave white space around the equal sign when setting varaibles. For example, this wont' work:


You have to do this instead:


The Sound File

The entire point of the script is to play a sound file when the screen gets bright. I made this one which you can download if you'd like to use it:


You can also use your own sound file by changing the path of the AUDIO_FILE variable in the script to point to the one you'd like. (The script uses the afplay command which should handle most things you throw at it.)

The Brightness Threshold

Changing the level of brightness that causes the alert is done with the BRIGHTNESS variable in the script. Play with it to find something that works for you. (As with the other variables, make sure not to leave spaces around the equal sign when updating it)

How It Works

And that's it. Enjoy!