I believe there may be an error in the docs here . In the fifth paragraph, second sentence
"For example, in the case of the third line feature (2c) the response is calculated as the difference between the sum of image pixels under the rectangle covering the whole feature (including the two white stripes and the black stripe in the middle) and the sum of the image pixels under the black stripe multiplied by 3 in order to compensate for the differences in the size of areas."
I think the "multiplied by 3" should be "multiplied by 2" since the black area was counted as white once, so it must be subtracted one time to arrive at the sum of the two white areas, and then must be subtracted again to get white areas minus black areas, in total then requiring subtraction twice and not thrice.