代写CS6114、代写c++，Java编程

日期：2022-04-13 09:51

CS6114 Assignment

Assignment 03

Video Coding

V ideo coding is the process of preparing digital video content for storage or transmission, such as in a data file or  bitstream.  The libx265 H.265 codec (MPEG-4 High Efficiency Video Coding) was designed to achieve

from 25% to 50% better data compression at the same level of video quality,

or substantially improved video quality at the same bit rate than the previous

libx264 H.264 (MPEG-4 Advanced Video Coding) standard for a variety of

different applications. In turn H.264 was designed to achieve good video

quality at substantially lower bit rates than previous MPEG standards.

One application area where H.264 and H.265 are currently in use is the delivery

of streamed content over data networks using HTTP. In this application, the

H.264/H.265 standard is used in conjunction with the MPEG-DASH standard to

describe (MPEG-DASH) and encode media content for delivery using standard

web infrastructure (web server/CDN/ HTTPS/TCP/IP).

MPEG-DASH partitions content into a sequence of short, usually fixed duration

segments. These segments are encoded at a variety of different bitrates, giving

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https://www.vcodex.com/hevc-an-introduction-to-high-efficiency-coding/

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CS6114 Assignment

alternative versions of the content. These alternative segments are time-

aligned so that while the content is being played back by an MPEG-DASH

client, the client can use a bitrate adaptation (ABR) algorithm to select the next

segment to be presented that has the highest bitrate (quality) that can be

downloaded in time for playback without causing stalls or buffering.

Adaptive streaming requires that each segment of these alternative versions of

the content do not exceed (or significantly fall short of) the nominal bitrate for

that version of the content. To achieve this objective the encoding must

employ constrained bitrate encoding techniques.

Rate Control

Rate control is the process used by the encoder in deciding how to allocate

bits to encode each picture. The goal of (lossy) video coding is to reduce the

bitrate while retaining as much quality as possible. Rate control is a crucial

step in determining that tradeoff between size and quality.

CBR and VBR encoding sets a target data rate and a bitrate control technique

is applied by the encoding application to achieve the target bitrate. It can be

difficult to choose an appropriate data rate for constrained connections and

the quality of experience (QoE) for viewers can be impacted if the range of

VBR is too high or in the case of CBR, if the nature of the content varies

greatly. Often constrained VBR between 110%-150% is used, however this

assumes a target bitrate to achieve an acceptable level of quality is known

before the content is encoded.

Not all video content is equally compressible. Low motion and smooth

gradients compress well (few bits for high perceived quality) , whereas high

motion and fine spatial detail are less compressible (more bits to preserve

quality). Often it is easier to specify a target quality and let the encoding

application vary the data rate to achieve this target. However, the data rate

required to achieve the target quality is unknown in advance.

Constant Rate Factor (CRF) encoding specifies a quality level and the

encoding application adjusts the data rate to achieve the target quality. The

result is content with a fixed quality level, but the data rate is unknown in

advance. If quality is the objective this is not a concern, but if the data rate

varies significantly over the duration of the content, it may have implications

for the deliverability.

Capped CRF applies the data rate necessary to achieve a target quality,

together with a maximum data rate to ensure deliverability.

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CS6114 Assignment

In this assignment, you will encode source material using both the libx264

(H.264) and libx265 (H.265) codecs and compare the resulting bitstreams in

terms of coding efficiency and quality. The expected use of the encoded

material is delivery via MPEG-DASH. However, you do not have to create an

MPD file or fragment the representations.

Encoding

In ffmpeg the the Video Buffering Verifier (VBV) enforces that the bitrate is

constrained to a maximum bitrate. This is essential for content that will be

streamed, as it ensures that each segment will not exceed (or substantially fall

short of) the nominal bitrate for that version of the content. VBV can be used

both with 2-pass VBR (use it in both passes), or with CRF encoding (capped

CRF). For example using libx264 (H.264)

ffmpeg -i source.mov

-c:v libx264 -crf 23 -maxrate 500K -bufsize 256K

source-at—0.5M.mp4

Using the above parameters will probably not result in full use of the available

bitrate (i.e the resulting bitrate will not be 500kbps). It can be advantageous to

specify a target average bitrate and allow the maximum rate to exceed this by

a small amount.

Applying VBV to CRF encoding, requires determining the CRF value that on

average, results in the maximum bitrate, but does not exceed it. If the encode

always exceeds the maximum bitrate, the CRF is too low. However, if the

bitrate does not always hit the maximum, extra quality may be gained by

lowering the CRF value. A value of +6 halves the bitrate (in general, but this is

content dependent). The buffer size (bufsize) parameter determines how

strict ffmpeg is at checking the variability of the bitrate. For streaming you will

want to ensure that the bitrate for each segment conforms closely to the

nominal bitrate.

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CS6114 Assignment

Steps

The source content is a Quicktime movie containing a video stream. The video

stream is encoded using libx264 (H.264) using intra-frame compression only

(i.e. all I-pictures). The content is a mixture of different styles of content.

Use ffmpeg and the libx264 codec to encode two H.264 video

bitstream representations, at nominal bitrates of 0.5Mbps and 2Mbps

using GOP lengths of 50 and 100.

Use ffmpeg and the libx265 codec to encode two H.265 video

bitstream representations, at nominal bitrates of 0.5Mbps and 2Mbps

using GOP lengths of 50 and 100.

You will generate eight encodings

H.264 0.5Mbps using GOP length of 50

H.264 0.5Mbps using a GOP length of 100

H.264 2Mbps using GOP length of 50

H.264 2Mbps using a GOP length of 100

H.265 0.5Mbps using GOP length of 50

H.265 0.5Mbps using a GOP length of 100

H.265 2Mbps using GOP length of 50

H.265 2Mbps using a GOP length of 100

You must investigate

What CRF value and constrained bitrate parameters are needed to

achieve these target nominal bitrates for each combination of codec

and GOP length?

Are these values the same for H.264 and H.265?

Which codec is better able to achieve the nominal bitrate?

The difference in encoding times.

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CS6114 Assignment

Use the supplied Jupyter notebook to investigate

The bitrate of each GOP in each encoding. Does each conform to the

target nominal bitrate? Which encoding is the most consistent? Explain

your findings.

The quality of the encodings. Are there differences in quality between

the encodings and within the encodings? Are these differences

practical (i.e. noticeable)?

You may choose to augment the Jupyter notebook to generate additional

visualisations or analyses of the results.

Questions

Using these results write a report (maximum 2000 words) addressing the

following questions

What differences are there between H.264 and H.265 encoding?

Is the quality of the content consistent for the different codecs and

encodings?

Does changing the GOP length affect the quality of the coded content?

What GOP length would you recommend to give the best balance

between quality (coding efficiency) and adaptability (shorter GOP

lengths) for each of these codecs?

Which metric (PSNR or SSIM) is a better measure of quality? Do you

notice any visual difference?

Is there a relationship between picture type, size and quality? Does this

relationship hold across different bitrates and codecs?

Be sure to include any other observations that you noted and the results and

rationale for any additional analysis you performed.

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CS6114 Assignment

Deliverables

An archive (zip file) containing the encoded bitstreams, any data files (e.g. CSV

files), Jupyter notebooks and your report. Do not include the source material

provided to you.

Resources

The following resources are available on Canvas or externally

A Jupyter notebook that compares videos and produces a CSV file

containing a measure (the PSNR and SSIM) of the frame-by-frame

difference between the reference movie (the original) and an encoded

video

This Jupyter notebook also demonstrates how frame-level information

from ffprobe can be combined with the quality measures

Encoding for MPEG-DASH tutorials https://blog.streamroot.io/encode-

multi-bitrate-videos-mpeg-dash-mse-based-media-players/