Get brightness value for a picture

I use openHAB and want to use an old mobile phone as an day/night indicator. I have installed a webcam app and it is filming the sky. I can get the latest shot via .

Can I get a "brightness value" like this: curl | some-command --get-brightness?

2 Answers

Question is answered here on Software Recommendations Stack Exchange by Steve Barnes.

Two possibilities:

Use imagemagick to check the brightness of the image will work as long as the phone camera does not have auto-exposure or it is turned off, otherwise it will only work for extreme values, (dark/light), e.g.

 convert <image> -colorspace Gray -format "%[fx:quantumrange*image.mean]" info:

exiftool can parse the EXIF information from the file and extract the "Light Value" which should give you a reasonably consistent reading, (assuming that the camera on your specific phone includes this tag).

Both tools are free, gratis & open source and available for most platforms.

Alternatively to exiftool, you can use exiv2, which I found to be much faster than exiftool and imagemagick.

Comparison (on Raspberry Pi Zero)

Tested with always the same 4056 x 3040 image

Imagemagick convert

pi@rpicamhq:/mnt/ramdisk $ time convert image.jpg -colorspace Gray -format "%[fx:quantumrange*image.mean]" info:
1039.34
real 0m1,989s
user 0m1,753s
sys 0m0,193s

Imagemagick identify format

pi@rpicamhq:/mnt/ramdisk $ time identify -format "%[mean]\n" image.jpg
1128.99
real 0m3,357s
user 0m3,133s
sys 0m0,199s
pi@rpicamhq:/mnt/ramdisk $ time identify -format "%[fx:mean]\n" image.jpg
0.0172272
real 0m3,435s
user 0m3,274s
sys 0m0,130s
pi@rpicamhq:/mnt/ramdisk $ time identify -format "%[fx:quantumrange*mean]\n" image.jpg
1128.99
real 0m3,387s
user 0m3,218s
sys 0m0,139s

Imagemagick identify exif

pi@rpicamhq:/mnt/ramdisk $ time identify -format '%[EXIF:BrightnessValue]'
real 0m0,275s
user 0m0,071s
sys 0m0,040s

exiftool

pi@rpicamhq:/mnt/ramdisk $ time exiftool -a -u -g1 image.jpg | grep 'Brightness'
Brightness Value : 0.01
real 0m2,647s
user 0m2,429s
sys 0m0,191s

exiv2

pi@rpicamhq:/mnt/ramdisk $ time exiv2 -PElt image.jpg | grep 'Brightness'
Brightness 0.01
real 0m0,097s
user 0m0,042s
sys 0m0,050s

exiv2, w/o grep

pi@rpicamhq:/mnt/ramdisk $ time exiv2 -PElt -g Brightness image.jpg
Brightness 0.01
real 0m0,070s
user 0m0,024s
sys 0m0,039s