Normalize Image Brightness and Saturation

Question

I have a collection of colored/black-white images and most of them have different levels of brightness and saturation.

How can I normalize those images to make them all at the same "**scale**"?

In my case, some of the images might also have a huge amount of white pixels as they are the scans of some old photos.

Thanks

Answer 1

It's hard to suggest anything very concrete without seeing your images, but one idea might be to move to a colourspace that is based on Brightness and Saturation and then to do an auto-gamma correction on the two channels of interest.

It could make a horrible mess so please make backup of your images first!

So, I am suggesting something along the lines of:

convert Original.jpg -colorspace HSL -channel B -auto-gamma -colorspace sRGB BrightnessNormalised.jpg

where I move to "Hue, Saturation and Lightness" colourspace, select the Lightness channel (it is the 3rd one so it is B out of RG&B ) and do an auto-gamma of the Lightness and save back as a brightness-corrected, sRGB JPEG.

Likewise, to normalise the saturation, you could do:

convert Original.jpg -colorspace HSL -channel G -auto-gamma -colorspace sRGB SaturationNormalised.jpg

And you could do both at once, by using -channel GB like this:

convert Original.jpg -colorspace HSL -channel GB -auto-gamma -colorspace sRGB Normalised.jpg

Normalising the saturation is probably not a good idea for greyscale images but do some tests.

Normalising the brightness of images is not really a good idea if you have moody, low-key photographs or airy, light, high-key photographs. Hang onto that backup - just make a ZIP/compressed tar-ball and keep it around.