you must define the dim factor in a way that is
1.between 0 and 1,
2. it has smooth transition.
for example here in order to make it more realistic, I used a power2 spacing instead of even spacing.
A = imread('luna.bmp'); % read image 'luna.bmp'
figure(1);imshow(A) % show the original image
dim = linspace(0,1,90);
dim = dim.^2; % create a dimming factor ranging from 0 to 1 with power2 spacing.
dim = repmat(dim,[180 1 3]); % repeat this factor for all rows
Adim = A;
Adim(:,180:-1:91,:) = uint8(dim.*single(Adim(:,180:-1:91,:))); % dim the right side of the picture
figure(2);imshow(Adim)
