Near Optimal Per-Clip Lagrangian Multiplier Prediction in HEVC

Ringis, Daniel; Pitié, François; Kokaram, Anil

doi:10.1109/pcs50896.2021.9477476

Cited by 8 publications

(4 citation statements)

References 20 publications

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“…We show that the use of video proxies in the optimisation yields significant speed-up gains (x265:×21, libaom-av1:×230), whilst achieving comparable average gains (x265:0.82%, libaom-av1: 2.53%). This expands on work performed in Ringis et al (2021) where the objective quality metric used was limited to PSNR-Y, and expands the work from (Vibhoothi et al, 2022a;Vibhoothi et al, 2022b) by using a larger amount of videos.…”

Section: Open Access Edited Bymentioning

confidence: 69%

The disparity between optimal and practical Lagrangian multiplier estimation in video encoders

Ringis

Vibhoothi

Pitié

et al. 2023

Front. Signal Process.

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With video streaming making up 80% of the global internet bandwidth, the need to deliver high-quality video at low bitrate, combined with the high complexity of modern codecs, has led to the idea of a per-clip optimisation approach in transcoding. In this paper, we revisit the Lagrangian multiplier parameter, which is at the core of rate-distortion optimisation. Currently, video encoders use prediction models to set this parameter but these models are agnostic to the video at hand. We explore the gains that could be achieved using a per-clip direct-search optimisation of the Lagrangian multiplier parameter. We evaluate this optimisation framework on a much larger corpus of videos than that has been attempted by previous research. Our results show that per-clip optimisation of the Lagrangian multiplier leads to BD-Rate average improvements of 1.87% for x265 across a 10 k clip corpus of modern videos, and up to 25% in a single clip. Average improvements of 0.69% are reported for libaom-av1 on a subset of 100 clips. However, we show that a per-clip, per-frame-type optimisation of λ for libaom-av1 can increase these average gains to 2.5% and up to 14.9% on a single clip. Our optimisation scheme requires about 50–250 additional encodes per-clip but we show that significant speed-up can be made using proxy videos in the optimisation. These computational gains (of up to ×200) incur a slight loss to BD-Rate improvement because the optimisation is conducted at lower resolutions. Overall, this paper highlights the value of re-examining the estimation of the Lagrangian multiplier in modern codecs as there are significant gains still available without changing the tools used in the standards.

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Section: Open Access Edited Bymentioning

confidence: 69%

The disparity between optimal and practical Lagrangian multiplier estimation in video encoders

Ringis

Vibhoothi

Pitié

et al. 2023

Front. Signal Process.

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“…motion vectors, block type (Skipped/Intra/Inter), and bit allocation at the frame and clip levels. 8 The general idea which we have applied in our recent work [9][10][11] is to adjust λ away from the codec default by using a constant k as follows,…”

Section: Introductionmentioning

confidence: 99%

Per-clip and per-bitrate adaptation of the Lagrangian multiplier in video coding

Ringis,

Pitié,

Kokaram

2022

Preprint

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In the past ten years there have been significant developments in optimization of transcoding parameters on a per-clip rather than per-genre basis. In our recent work we have presented per-clip optimization for the Lagrangian multiplier in Rate controlled compression, which yielded BD-Rate improvements of approximately 2% across a corpus of videos using HEVC. However, in a video streaming application, the focus is on optimizing the rate/distortion tradeoff at a particular bitrate and not on average across a range of performance.We observed in previous work that a particular multiplier might give BD rate improvements over a certain range of bitrates, but not the entire range. Using different parameters across the range would improve gains overall. Therefore here we present a framework for choosing the best Lagrangian multiplier on a per-operating point basis across a range of bitrates. In effect, we are trying to find the para-optimal gain across bitrate and distortion for a single clip. In the experiments presented we employ direct optimization techniques to estimate this Lagrangian parameter path approximately 2,000 video clips. The clips are primarily from the YouTube-UGC dataset. We optimize both for bitrate savings as well as distortion metrics (PSNR, SSIM).

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“…6 Over the past years a lot of researchers have focused on the optimisation of a high-level parameter (target bitrate or quantisation factor or objective quality) to generate an optimal bitrate ladder for a clip as part of a Adaptive Bitrate Streaming (ABR). [7][8][9] In our previous work, 10 we showed that the RD tradeoff can be directly addressed by applying a numerical optimisation scheme to estimate the appropriate Lagrangian λ multiplier for a given clip (see Section 2) for standard dynamic range (SDR) videos. We observed an average BD-rate improvement of 1.9% for HEVC, 1.3% for VP9, 11 and 0.5% in AV1.…”

Section: Introductionmentioning

confidence: 99%

“…Zhang and Bull 15 altered λ based on distortion statistics on a frame-basis for HEVC. In our previous work, 10,11,16 we introduced the idea of an adaptive λ on a clip basis, using a single modified λ = kλ o across all the frames in a clip. Here, λ o represents the default value deployed from the relevant empirical relationship e.g.…”

Section: Introductionmentioning

confidence: 99%

Direct optimisation of λ for HDR content adaptive transcoding in AV1

Vibhoothi

Pitié

Katsenou

et al. 2022

Applications of Digital Image Processing XLV

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Since the adoption of VP9 by Netflix in 2016, royalty-free coding standards continued to gain prominence through the activities of the AOMedia consortium. AV1, 1 the latest open source standard, is now widely supported. In the early years after standardisation, HDR video tends to be under served in open source encoders for a variety of reasons including the relatively small amount of true HDR content being broadcast and the challenges in RD optimisation with that material. AV1 codec optimisation has been ongoing since 2020 including consideration of the computational load. 2 In this paper, we explore the idea of direct optimisation of the Lagrangian λ parameter used in the rate control of the encoders to estimate the optimal Rate-Distortion trade-off achievable for a High Dynamic Range signalled video clip. We show that by adjusting the Lagrange multiplier in the RD optimisation process on a frame-hierarchy basis, we are able to increase the Bjontegaard difference 3 rate gains by more than 3.98× on average without visually affecting the quality.

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Near Optimal Per-Clip Lagrangian Multiplier Prediction in HEVC

Cited by 8 publications

References 20 publications

The disparity between optimal and practical Lagrangian multiplier estimation in video encoders

The disparity between optimal and practical Lagrangian multiplier estimation in video encoders

Per-clip and per-bitrate adaptation of the Lagrangian multiplier in video coding

Direct optimisation of λ for HDR content adaptive transcoding in AV1

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