Internet DRAFT - draft-daede-netvc-testing
draft-daede-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
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This Internet-Draft will expire on April 21, 2016.
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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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