# What does "invariant to bias and gain changes" mean?

Reading this paper, about the ECC criterion (which is included in opencv3), In section 3, the following is stated, with regard to the following equation:

(4) $$\left\|\frac{i_r}{\left\|i_r\right\|} - \frac{i_w(p)}{\left\|i_w(p)\right\|}\right\|^2$$

It is apparent from (4) that our criterion is invariant to bias and gain changes. This also suggests that our measure is going to be invariant to any photometric distortions in brightness and/or in contrast.

What does "bias", "gain" invariant mean?

Thanks

You need to be careful when reproducing formulas! Your $$i_r$$, $$i_w$$ are actually $$\bar{i_r}$$, $$\bar{i_w}$$ in the paper and two sentences above $$(4)$$, it says these are