Issue in calculating Image Sub Sampling metrics.

Hi,

I have an image of size 276 X 280. I did subsampling operation using this statement. " Y = image(1:2:end, 1:2:end); ". Y is of size 138 X 140. Now, I want to calculate metrics such as PSNR, SSIM, IMMSE etc., for image Y. Since, X and Y are of different sizes, I cannot calculate them using MATLAB functions.

Is there any other way to calculate them?. Generally, what are differnt types of metrics used to measure the performance of sub sampled image?.

Kindly help me accordingly.

Thanks in advance.

Sai..