How does MATLAB calculate immse?

Question

How does Matlab calculate immse? I want to find the mse between two images. According to how to measure he similarity between two 2D complex fields in matlab?, immse is the same as MSE=mean((abs(Y(:))-abs(Y1(:))).^2) for reference image Y1 and comparison image Y. Likewise, I could calculate MSE as the summed square errors divided by the number of row*cols. When I run on one of the demo images, these different approaches don't give the same answer as immse.

Here are two MSE approaches in the sample code below. The image is from the Matlab immse demo and immse gives around an MSE=340. The other two codes give around an MSE=2.5.

Note: The code is example code, I did not use the same function name twice in the same script. And I understand if you want to complain about using size(image) but that is a detail. I am more worried about the basic flaw in my understanding that is giving me orders of magnitude differences. Thank you so much.

n01 = imread('pout.tif');
n02 = imnoise(n01,'salt & pepper', 0.02);

mse = mymse(n02,n01);
mlmse = immse(n02,n01);

function this = mymse(icomp, ibase)
    [X Y nchan] = size(ibase);
    diff = (icomp - ibase);
    this = sum(sum(diff.*diff))/(X*Y*nchan);
end

function this = mymse(icomp, ibase)
    this = mean ((abs(ibase(:)) - abs(icomp(:))).^2);
end

TallBrianL TallBrianL · Accepted Answer · 2017-09-14T22:59:21

You can check the underlying code to many matlab functions by simply doing

open <func>

in the Matlab command window.

In this case you can see that immse is doing the norm of the differences, scaled by number of points.

function this = mymse(icomp, ibase)
    this = sum((ibase(:) - icomp(:)).^2) / numel(ibase);
end

How does MATLAB calculate immse?

1 Answers