The AV1 Constrained Directional Enhancement Filter (CDEF) Steinar - - PowerPoint PPT Presentation

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The AV1 Constrained Directional Enhancement Filter (CDEF) Steinar - - PowerPoint PPT Presentation

Jun 21, 2023 710 likes •894 views

The AV1 Constrained Directional Enhancement Filter (CDEF) Steinar Midtskogen Cisco Jean-Marc Valin Mozilla ICASSP 2018 The AV1 Video Codec Royalty-free licensing Created by the Alliance for Open Media Officially released March

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SLIDE 1

The AV1 Constrained Directional Enhancement Filter (CDEF)

Steinar Midtskogen

Cisco

Jean-Marc Valin

Mozilla ICASSP 2018

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SLIDE 2

The AV1 Video Codec

  • Royalty-free licensing
  • Created by the Alliance for Open Media
  • Officially released March 28th 2018
  • Based on VP9 (Google) with technology from

Thor (Cisco) and Daala (Mozilla)

Deblocking Filter Constrained Directional Enhancement Filter (CDEF) Loop Restoration Coding loop (enhancement blocks)

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SLIDE 3

CDEF Overview

  • Applied after deblocking filter, in coding loop
  • Reduces ringing (and other) artifacts
  • Low hardware and software complexity
  • Main ideas

– Non-linear filter – Direction search – Direction-adaptive taps – Applied to both luma and chroma

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SLIDE 4

Non-Linear Filter

  • Blurs ringing while preserving edges

– Behaves like an FIR at low contrast – Ignores large contrasts (edges)

  • Fully vectorizable, no division

Output value Weight Pixel difference Filter parameters Center value Non-linear function

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SLIDE 5

Constraint Function

  • Parameterized by strength and damping

– Trade-off between ringing removal and blurring – Typically use higher strength at lower bitrate

Strength: end of linear region Damping: point of zero output

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SLIDE 6

Direction Estimation

  • Runs on 8x8 decoded blocks (no signaling)

– Small enough for tracking curves – Large enough to reliably estimate direction

  • Find direction that minimizes “prediction” error

– Fully vectorizable

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SLIDE 7

Directional Filter

  • Sum of two direction-dependent sets of taps

– Primary taps along direction (higher strength) – Secondary taps off direction (lower strength)

Primary taps Secondary taps

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SLIDE 8

Example

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SLIDE 9

Example

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SLIDE 10

Example

Directions

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SLIDE 11

Example

Directions 8 x 8

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SLIDE 12

Example

Directions 8 x 8

X

Filter

Primary Secondary

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SLIDE 13

Example

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SLIDE 14

Example (Before)

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SLIDE 15

Example (After)

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SLIDE 16

Signaling

  • Limited side information
  • Two levels of signaling

– Frame-level list of 1-8 presets – Preset selection at 64x64 level (0-3 bits)

  • No 8x8 signaling, no direction signaling
  • No signaling when 64x64 filter block is skipped
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SLIDE 17

Results

  • PSNR BD-rate improvement

– 1.1% for high-latency, 3.7% for low-latency

  • Significant subjective improvement (HL)