heavily blur one fixed part of the image

Hi,

Sorry if it's written somewhere. Please direct me to the info by which I 
can heavily blur one fixed part of the image, say the lower-right 
rectangular/oval corner. 

I guess you've already known what I'm trying to do -- to me, for this 
case, blur the corner out of shape is more important than preserving the 
original content. That's why the "heavily blur" is in the subject.

Please help.

Thanks.

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Tong (remove underscore(s) to reply)
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