width = 160;
height = 120;
mask = zeros(width, height);
 
center_x = width / 2;
center_y = height / 2;
radius = min(width, height) / 2;
radius2 = radius ^ 2;
for i = 1 : width
  for j = 1 : height
    dx = i - center_x;
    dy = j - center_y;
    dx2 = dx ^ 2;
    dy2 = dy ^ 2;
    mask(i, j) = dx2 + dy2 <= radius2;
  end;
end;
 
picture = randn(width, height); % test image :)
masked_image = picture .* mask;
imagesc(masked_image);
 
print out.png

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