Additional resources that may help:

These JavaScript examples determine the average brightness of the entire image, but you should be able to analyze the image in segments.

https://stackoverflow.com/questions/51253752/determining-the-brightness-of-an-image-using-canvas

https://gist.github.com/codearachnid/5983831

https://codepen.io/njmcode/pen/pvLyZq

https://stackoverflow.com/questions/13762864/image-brightness-detection-in-client-side-script

Ed Ferguson