[Gimp-user] About the function `Brighten in 'Lens distortion filter'

About the function `Brighten in 'Lens distortion filter'
Hi, I am Syu, I have a question and hope if some kind peoples can help me to find the answer,
it will be very helpful for me.
I am not good at English, hope you will try to understand my problem. I will try my best to write...
I am a lecturer and I want to introduce the Gimp to my students here in Japan, using to make image processiong. 
I think it is a wonderful tool and thanks all for make it possible to be free ! 
Well, I have a question about the filter namede'Lens distortion',  especially the funtion of 'Brighten'.
It is explained as ''The 'Main' or 'Edge' options must be non zero for this option to produce noticeable result."
Why? If I don't want to change the shape of the image, what should I do ?
If it is possible, would you mind to explain some mathematical algorithms about this funtion or  filter ?
I don't want to use wrong informations while I explain the funtiond  'Lens distortion'.
I promise I won't use your informations for any business purpose. 
Hope to get your reply as soon as you can.  Thank you so much!
M. Syu 

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