Re-Engineering Rate Distortion Optimisation in Modern Video Codecs Using a Per Clip Approach
Citation:
Ringis, Daniel Joseph, Re-Engineering Rate Distortion Optimisation in Modern Video Codecs Using a Per Clip Approach, Trinity College Dublin, School of Engineering, Electronic & Elect. Engineering, 2023Download Item:
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Abstract:
The majority of internet traffic is video content. This drives the demand for video compression
to deliver high quality video at low target bitrates. Optimising the parameters of a video
codec for a specific video clip (per-clip optimisation) has been shown to yield significant
bitrate savings.
One of the key challenges to be solved in the design of a practical codec is the trade-off
between rate and distortion. A Lagrangian multiplier has been the adopted view in video
codecs. In this approach the encoder makes a number of decisions in order to minimise a
cost J, where J = D + ? R, commonly referred to as the rate distortion equation. As can be
seen J combines both a distortion D (for a frame or macroblock) and a rate R (the number of
coded bits for that unit) through the action of the Lagrangian multiplier ? . Different choices
for ? result in different R/D trade-offs.
This research investigates the impact of adjusting the rate distortion equation on com-
pression performance. A constant of proportionality, k, is used to modify the Lagrangian
multiplier used in H.265 (HEVC) and VP9. Direct optimisation methods are deployed to
maximise BD-Rate improvement for a particular clip. By doing this, we have shown that
per-clip optimisation of the Lagrangian multiplier leads to BD-Rate improvement of up to
25% in a single clip using Direct Optimisation. The average BD-Rate improvement was
approximately 2% across the 10k clip corpus. This direct optimisation comes at a high
computational cost, but can be used as an estimate of the best possible improvement possible
for a given video clip. Because of this computationally expensive method which requires multiple measurement
of rate distortion curves which meant in excess of fifty video encodes were used to generate
that level of savings. This research then focuses on reducing the computational cost of
repeated video encodes by using proxy systems or by reducing this computational load
through prediction. Approximately 60% of the BD-Rate improvements found by direct
optimisation using proxies or prediction were achieved using the reduced complexity systems.
This effectively reduced the computational complexity ten fold. Overall, our system achieves
BD-Rate improvement in approximately 90% of a large corpus with comparable results to
the direct optimisation methods.
Sponsor
Grant Number
Google Faculty Award
Ussher Studentship
Sigmedia Bursary
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APPROVED
Author: Ringis, Daniel Joseph
Advisor:
Pitie, FrancoisPublisher:
Trinity College Dublin. School of Engineering. Discipline of Electronic & Elect. EngineeringType of material:
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