I am using https://www.codecogs.com/latex/eqneditor.php
And added my latex as follows:
\begin{equation}
SSIM(x,y)=\frac{{\left ( 2 \mu_x\mu_y+ C_1 \right )+ \left (2 \sigma _x_y+C_2\right)}}
{\left(\mu_x^2+\mu_y^2+C_1)(\sigma_x^2+\sigma_y^2+C_2\right)}
\label{eq:SSMI}
\end{equation}
I expected the following formula but instead of I got the error:!Double subscript. ...y+C_1\right)+ ...
\sigma _x_y
->\sigma_{xy}
. – CroCo Dec 9 '17 at 21:42\left
and\right
which are not wanted here – David Carlisle Dec 9 '17 at 21:45