How to calculate linear index of fuzziness of an image?

Hi,

i am trying to calculate linear index of fuzziness of an image using this formula:

i have to see a positive value. But i see a negative value, so i think i am doing something wrong. here is my try and output_image

q=(pi/2)*(1-output_image/255); %I is output image and 255 is maximum gray level. output_iamge is 288*384 double and grayscale image
y= sum(sum(min(q,1-q)))/(width*height); %M*N is width*height