OpenCV: How to balance the bad lighting condition(brightness, un-even shadow) on the pic (ex. face of the selfie)

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Hi my friends,

Recently I have a problem when I try to do some image processing(OpenCV / Javescript Image Processing) on the selfie photo. The reason is the bad lighting condition. It's caused by the lightings/sun of the environment pointing to the face are not balanced(not even). And it makes the right-side of nose is lighter and the rest parts on the face are darker. Please take a look below

Are there any techniques or methods making the shadow on the face more balanced and the skin tones still perform smooth and natural? Then the result shows that whole face shadow/brightness are at the same level with smooth skin tone on it.

I survey some topics and it tells that some histogram technique or gamma correction doing some effects. However, I'm too new to this field and not sure they really helps or not. Could anyone give me some expert guideline or ideas?

Thank you so much. Any suggestions will be appreciated!