1D SSIM (WSSIM 1D): Though the SSIM was developed for 2D signals (images), it can be applied to 1D time series by considering the signal as a Nt x 1 pixel image and using a 1D convolution window. The formula is still given by Eq.(7). Computationally,this is preferable to the time-frequency case because it eliminates several steps from the calculation.