  In practical media distribution systems, visual content usually undergoes
multiple stages of quality degradation along the delivery chain, but the
pristine source content is rarely available at most quality monitoring points
along the chain to serve as a reference for quality assessment. As a result,
full-reference (FR) and reduced-reference (RR) image quality assessment (IQA)
methods are generally infeasible. Although no-reference (NR) methods are
readily applicable, their performance is often not reliable. On the other hand,
intermediate references of degraded quality are often available, e.g., at the
input of video transcoders, but how to make the best use of them in proper ways
has not been deeply investigated. Here we make one of the first attempts to
establish a new paradigm named degraded-reference IQA (DR IQA). Specifically,
we lay out the architectures of DR IQA and introduce a 6-bit code to denote the
choices of configurations. We construct the first large-scale databases
dedicated to DR IQA and will make them publicly available. We make novel
observations on distortion behavior in multi-stage distortion pipelines by
comprehensively analyzing five multiple distortion combinations. Based on these
observations, we develop novel DR IQA models and make extensive comparisons
with a series of baseline models derived from top-performing FR and NR models.
The results suggest that DR IQA may offer significant performance improvement
in multiple distortion environments, thereby establishing DR IQA as a valid IQA
paradigm that is worth further exploration.
