title: How to adjust exposure of an image in php gd
body: <p>I read that exposure is the amount of light allowed to fall on each area unit of a photographic medium (photographic film or image sensor) during the process of taking a photograph, according to wiki. I am not from image processing background. So,I am still confused about it.  </p>
  
  <p>But I cannot find any tutorial on exposure adjustment using php. 
   Is there a way to develop exposure adjustment function in php gd?  Can anyone advise? Thank you. </p>