  Adaptive video streaming is a key enabler for optimising the delivery of
offline encoded video content. The research focus to date has been on
optimisation, based solely on rate-quality curves. This paper adds an
additional dimension, the energy expenditure, and explores construction of
bitrate ladders based on decoding energy-quality curves rather than the
conventional rate-quality curves. Pareto fronts are extracted from the
rate-quality and energy-quality spaces to select optimal points. Bitrate
ladders are constructed from these points using conventional rate-based rules
together with a novel quality-based approach. Evaluation on a subset of
YouTube-UGC videos encoded with x.265 shows that the energy-quality ladders
reduce energy requirements by 28-31% on average at the cost of slightly higher
bitrates. The results indicate that optimising based on energy-quality curves
rather than rate-quality curves and using quality levels to create the rungs
could potentially improve energy efficiency for a comparable quality of
experience.
