Try this -
f = imread('img.jpg');
greenChannel = f(:,:,2);
h = fspecial('average', 21); %%// Kernel to be used for averaging
%h = ones(21,21)/(21*21); %%// Same as above, but custom-made
A = imfilter(greenChannel,h); %%// Average values for each pixel
newgreenChannel = greenChannel.*(128./A);
figure,imshow(newgreenChannel)