# What's the Difference Between UQI and SSIM Measures for Image Similarity / Quality?

I'm studying image quality assessment (IQA) problem and I'm currently learning about quality measures UQI and SSIM, I know UQI is a predecessor of SSIM, but they look exactly the same for me...

Namely, UQI is a private case of SSIM for C1 = 0 and C2 = 0.