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...
The developers of both are the same hence the similarity is indeed "By Design".
The only difference is the addition of 2 constants in SSIM (C1 and C2).
As the writers write in the SSIM paper:
Namely, UQI is a private case of SSIM for
C1 = 0 and
C2 = 0.