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Versions: 00 01 02 draft-ietf-netvc-testing

Network Working Group                                           T. Daede
Internet-Draft                                                J. Moffitt
Intended status: Informational                                   Mozilla
Expires: April 21, 2016                                 October 19, 2015


              Video Codec Testing and Quality Measurement
                      draft-daede-netvc-testing-02

Abstract

   This document describes guidelines and procedures for evaluating an
   internet video codec specified at the IETF.  This covers subjective
   and objective tests, test conditions, and materials used for the
   test.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Subjective Metrics  . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Still Image Pair Comparison . . . . . . . . . . . . . . .   2
   3.  Objective Metrics . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  PSNR  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  PSNR-HVS-M  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.3.  SSIM  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.4.  Fast Multi-Scale SSIM . . . . . . . . . . . . . . . . . .   4
   4.  Comparing and Interpreting Results  . . . . . . . . . . . . .   4
     4.1.  Graphing  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Bjontegaard . . . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  Ranges  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Test Sequences  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Sources . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Test Sets . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Operating Points  . . . . . . . . . . . . . . . . . . . .   7
       5.3.1.  High Latency  . . . . . . . . . . . . . . . . . . . .   7
       5.3.2.  Unconstrained Low Latency . . . . . . . . . . . . . .   8
       5.3.3.  Constrained Low Latency . . . . . . . . . . . . . . .   8
   6.  Automation  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   When developing an internet video codec, changes and additions to the
   codec need to be decided based on their performance tradeoffs.  In
   addition, measurements are needed to determine when the codec has met
   its performance goals.  This document specifies how the tests are to
   be carried about to ensure valid comparisons and good decisions.

2.  Subjective Metrics

   Subjective testing is the preferable method of testing video codecs.

   Because the IETF does not have testing resources of its own, it has
   to rely on the resources of its participants.  For this reason, even
   if the group agrees that a particular test is important, if no one
   volunteers to do it, or if volunteers do not complete it in a timely
   fashion, then that test should be discarded.  This ensures that only
   important tests be done in particular, the tests that are important
   to participants.

2.1.  Still Image Pair Comparison





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   A simple way to determine superiority of one compressed image over
   another is to visually compare two compressed images, and have the
   viewer judge which one has a higher quality.  This is mainly used for
   rapid comparisons during development.  For this test, the two
   compressed images should have similar compressed file sizes, with one
   image being no more than 5% larger than the other.  In addition, at
   least 5 different images should be compared.

3.  Objective Metrics

   Objective metrics are used in place of subjective metrics for easy
   and repeatable experiments.  Most objective metrics have been
   designed to correlate with subjective scores.

   The following descriptions give an overview of the operation of each
   of the metrics.  Because implementation details can sometimes vary,
   the exact implementation is specified in C in the Daala tools
   repository [DAALA-GIT].

   All of the metrics described in this document are to be applied to
   the luma plane only.  In addition, they are single frame metrics.
   When applied to the video, the scores of each frame are averaged to
   create the final score.

   Codecs are allowed to internally use downsampling, but must include a
   normative upsampler, so that the metrics run at the same resolution
   as the source video.  In addition, some metrics, such as PSNR and
   FASTSSIM, have poor behavior on downsampled images, so it must be
   noted in test results if downsampling is in effect.

3.1.  PSNR

   PSNR is a traditional signal quality metric, measured in decibels.
   It is directly drived from mean square error (MSE), or its square
   root (RMSE).  The formula used is:

   20 * log10 ( MAX / RMSE )

   or, equivalently:

   10 * log10 ( MAX^2 / MSE )

   which is the method used in the dump_psnr.c reference implementation.

3.2.  PSNR-HVS-M

   The PSNR-HVS metric performs a DCT transform of 8x8 blocks of the
   image, weights the coefficients, and then calculates the PSNR of



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   those coefficients.  Several different sets of weights have been
   considered.  [PSNRHVS] The weights used by the dump_pnsrhvs.c tool in
   the Daala repository have been found to be the best match to real MOS
   scores.

3.3.  SSIM

   SSIM (Structural Similarity Image Metric) is a still image quality
   metric introduced in 2004 [SSIM].  It computes a score for each
   individual pixel, using a window of neighboring pixels.  These scores
   can then be averaged to produce a global score for the entire image.
   The original paper produces scores ranging between 0 and 1.

   For the metric to appear more linear on BD-rate curves, the score is
   converted into a nonlinear decibel scale:

   -10 * log10 (1 - SSIM)

3.4.  Fast Multi-Scale SSIM

   Multi-Scale SSIM is SSIM extended to multiple window sizes [MSSSIM].
   This is implemented in the Fast implementation by downscaling the
   image a number of times, and computing SSIM over the same number of
   pixels, then averaging the SSIM scores together [FASTSSIM].  The
   final score is converted to decibels in the same manner as SSIM.

4.  Comparing and Interpreting Results

4.1.  Graphing

   When displayed on a graph, bitrate is shown on the X axis, and the
   quality metric is on the Y axis.  For clarity, the X axis bitrate is
   always graphed in the log domain.  The Y axis metric should also be
   chosen so that the graph is approximately linear.  For metrics such
   as PSNR and PSNR-HVS, the metric result is already in the log domain
   and is left as-is.  SSIM and FASTSSIM, on the other hand, return a
   result between 0 and 1.  To create more linear graphs, this result is
   converted to a value in decibels:

   -1 * log10 ( 1 - SSIM )

4.2.  Bjontegaard

   The Bjontegaard rate difference, also known as BD-rate, allows the
   comparison of two different codecs based on a metric.  This is
   commonly done by fitting a curve to each set of data points on the
   plot of bitrate versus metric score, and then computing the
   difference in area between each of the curves.  A cubic polynomial



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   fit is common, but will be overconstrained with more than four
   samples.  For higher accuracy, at least 10 samples and a linear
   piecewise fit should be used.  In addition, if using a truncated BD-
   rate curve, there should be at least 4 samples within the point of
   interest.

4.3.  Ranges

   The curve is split into three regions, for low, medium, and high
   bitrate.  The ranges are defined as follows:

   o  Low bitrate: 0.005 - 0.02 bpp

   o  Medium bitrate: 0.02 - 0.06 bpp

   o  High bitrate: 0.06 - 0.2 bpp

   Bitrate can be calculated from bits per pixel (bpp) as follows:

   bitrate = bpp * width * height * framerate

5.  Test Sequences

5.1.  Sources

   Lossless test clips are preferred for most tests, because the
   structure of compression artifacts in already-compressed clips may
   introduce extra noise in the test results.  However, a large amount
   of content on the internet needs to be recompressed at least once, so
   some sources of this nature are useful.  The encoder should run at
   the same bit depth as the original source.  In addition, metrics need
   to support operation at high bit depth.  If one or more codecs in a
   comparison do not support high bit depth, sources need to be
   converted once before entering the encoder.

   The JCT-VC standards organization includes a set of standard test
   clips for video codec testing, and parameters to run the clips with
   [L1100].  These clips are not publicly available, but are very useful
   for comparing to published results.

   Xiph publishes a variety of test clips collected from various
   sources.

   The Blender Open Movie projects provide a large test base of lossless
   cinematic test material.  The lossless sources are available, hosted
   on Xiph.





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5.2.  Test Sets

   Sources are divided into several categories to test different
   scenarios the codec will be required to operate in.  For easier
   comaprison, all videos in each set should have the same color
   subsampling, same resolution, and same number of frames.  In
   addition, all test videos must be publicly available for testing use,
   to allow any results

   o  Still images are useful when comparing intra coding performance.
      Xiph.org has four sets of lossless, one megapixel images that have
      been converted into YUV 4:2:0 format.  There are four sets that
      can be used:

      *  subset1 (50 images)

      *  subset2 (50 images)

      *  subset3 (1000 images)

      *  subset4 (1000 images)

   o  video-hd-2, a set that consists of the following 1920x1080 clips
      from [DERFVIDEO], cropped to 50 frames (and converted to 4:2:0 if
      necessary)

      *  aspen

      *  blue_sky

      *  crowd_run

      *  ducks_take_off

      *  factory

      *  life

      *  old_town_cross

      *  park_joy

      *  pedestrian_area

      *  red_kayak

      *  riverbed




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      *  rush_hour

      *  station2

   o  A video conferencing test set, with 1280x720 content at 60 frames
      per second.  Unlike other sets, the videos in this set are 10
      seconds long.

      *  TBD

   o  Game streaming content: 1920x1080, 60 frames per second, 4:2:0
      chroma subsampling. 1080p is chosen as it is currently the most
      common gaming monitor resolution [STEAM].  All clips should be two
      seconds long.

      *  TBD

   o  Screensharing content is low framerate, high resolution content
      typical of a computer desktop.

      *  screenshots - desktop screenshots of various resolutions, with
         4:2:0 subsampling

      *  Video sets TBD

5.3.  Operating Points

   Two operating modes are defined.  High latency is intended for on
   demand streaming, one-to-many live streaming, and stored video.  Low
   latency is intended for videoconferencing and remote access.

5.3.1.  High Latency

   The encoder should be run at the best quality mode available, using
   the mode that will provide the best quality per bitrate (VBR or
   constant quality mode).  Lookahead and/or two-pass are allowed, if
   supported.  One parameter is provided to adjust bitrate, but the
   units are arbitrary.  Example configurations follow:

   o  x264: -crf=x

   o  x265: -crf=x

   o  daala: -v=x

   o  libvpx: -codec=vp9 -end-usage=q -cq-level=x -lag-in-frames=25
      -auto-alt-ref=1




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5.3.2.  Unconstrained Low Latency

   The encoder should be run at the best quality mode available, using
   the mode that will provide the best quality per bitrate (VBR or
   constant quality mode), but no frame delay, buffering, or lookahead
   is allowed.  One parameter is provided to adjust bitrate, but the
   units are arbitrary.  Example configurations follow:

   o  x264: -crf-x -tune zerolatency

   o  x265: -crf=x -tune zerolatency

   o  daala: -v=x

   o  libvpx: -codec=vp9 -end-usage=q -cq-level=x -lag-in-frames=0
      -auto-alt-ref=0

5.3.3.  Constrained Low Latency

   The encoder is given one parameter, which is absolute bitrate.  No
   frame delay, buffering, or lookahead is allowed.  The maximum
   achieved bitrate deviation from the supplied parameter is determined
   by a buffer model:

   o  The buffer starts out empty.

   o  After each frame is encoded, the buffer is filled by the number of
      bits spent for the frame.

   o  The buffer is then emptied by (bitrate * frame duration) bits.

   o  The buffer fill level is checked.  If it is over the limit, the
      test is considered a failure.

   The buffer size limit is defined by the bitrate target * 0.3 seconds.

6.  Automation

   Frequent objective comparisons are extremely beneficial while
   developing a new codec.  Several tools exist in order to automate the
   process of objective comparisons.  The Compare-Codecs tool allows BD-
   rate curves to be generated for a wide variety of codecs
   [COMPARECODECS].  The Daala source repository contains a set of
   scripts that can be used to automate the various metrics used.  In
   addition, these scripts can be run automatically utilizing
   distributed computer for fast results [AWCY].





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7.  Informative References

   [AWCY]     Xiph.Org, "Are We Compressed Yet?", 2015, <https://
              arewecompressedyet.com/>.

   [COMPARECODECS]
              Alvestrand, H., "Compare Codecs", 2015,
              <http://compare-codecs.appspot.com/>.

   [DAALA-GIT]
              Xiph.Org, "Daala Git Repository", 2015,
              <http://git.xiph.org/?p=daala.git;a=summary>.

   [DERFVIDEO]
              Terriberry, T., "Xiph.org Video Test Media", n.d., <https:
              //media.xiph.org/video/derf/>.

   [FASTSSIM]
              Chen, M. and A. Bovik, "Fast structural similarity index
              algorithm", 2010, <http://live.ece.utexas.edu/publications
              /2011/chen_rtip_2011.pdf>.

   [L1100]    Bossen, F., "Common test conditions and software reference
              configurations", JCTVC L1100, 2013,
              <http://phenix.int-evry.fr/jct/>.

   [MSSSIM]   Wang, Z., Simoncelli, E., and A. Bovik, "Multi-Scale
              Structural Similarity for Image Quality Assessment", n.d.,
              <http://www.cns.nyu.edu/~zwang/files/papers/msssim.pdf>.

   [PSNRHVS]  Egiazarian, K., Astola, J., Ponomarenko, N., Lukin, V.,
              Battisti, F., and M. Carli, "A New Full-Reference Quality
              Metrics Based on HVS", 2002.

   [SSIM]     Wang, Z., Bovik, A., Sheikh, H., and E. Simoncelli, "Image
              Quality Assessment: From Error Visibility to Structural
              Similarity", 2004,
              <http://www.cns.nyu.edu/pub/eero/wang03-reprint.pdf>.

   [STEAM]    Valve Corporation, "Steam Hardware & Software Survey: June
              2015", June 2015,
              <http://store.steampowered.com/hwsurvey>.









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Authors' Addresses

   Thomas Daede
   Mozilla

   Email: tdaede@mozilla.com


   Jack Moffitt
   Mozilla

   Email: jack@metajack.im







































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