AbdurRasheed Sadiq asked . 2023-02-11

Why do I find different SSIM values depending on whether the images

Why do I find different SSIM values depending on whether the images' intensity information is stored in the range [0 1] or [0 255]?

% Import an image using im2gray
im = im2gray(imread('peppers.png'));

% Convert this a double (without rescaling the values from the range 0-255)
% and then modify the values slightly to create a distorted image.

imdouble = double(im);
imdouble_new = imdouble * 0.5;
im_new = mat2gray(imdouble_new, [0 255]);

% NB im_new is a grayscale image with values 0-1 rather than 0-255. If we
% want to compare this image to the original using the SSIM metric we can
% do so in 2 ways.

% 1) Convert im into a 0-1 image matrix and use the ssim function.

ssim_values_1 = ssim(mat2gray(imdouble, [0 255]), im_new);
ssim_values_1(1)

% 2) Just compare the double matrices using the ssim function.

ssim_values_2 = ssim(imdouble, imdouble_new);
ssim_values_2(1)

Ans = 0.7508

% 2) Just compare the double matrices using the ssim function.
ssim_values_2 = ssim(imdouble, imdouble_new);
ssim_values_2(1)

ans = 0.6400

image processing , image analysis , ssim , grayscale , Image Segmentation and Analysis , Image Qual