Video Compression

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Video Compression
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Introduction The need of compression

• True color image 24bits per pixel
• Full screen 640x480 required 7 million bit
  (about 7M bit)
• Smooth video clip at least 25 frames/second
• Therefore, 7 million bits x 25 175 million
  bits/second required (about 175 Mbps) !!!
• Also, 1 min video is required about 10.5 G byte
  of data

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Introduction

• Video Compression deals with the compression of
  visual video data.
• Video contains spatial and temporal redundancy.
• Usually reduce the redundancy using lossy
  compression
• Image compression techniques to reduce spatial
  redundancy
• Motion compensation techniques to reduce temporal
  redundancy

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Motion Picture Compression

• Moving pictures are essentially a series of
  static images.
• This being the case many of the techniques for
  static image compression will work for each of
  the individual frames.
• Further compression can be achieved by taking
  into account the nature of moving pictures.

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Moving Picture Compression

• In order to prevent flicker, moving pictures
  often have to refresh at high rates (sometimes at
  least 55Hz).
• The differences between subsequent frames in a
  series will not necessarily be great and so
  techniques for Motion Compensation in order to
  assist the compression.

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Motion Compensation

• Compare two sequential frames in a series, note
  the differences between them, and code only the
  residuals.
• Each compressed frame is rebuilt by adding
  decoded residuals to the prediction based on the
  previous frame.

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Motion Compensation Block Matching

• Partition image into blocks
• Previous frame is searched for each block of the
  current frame.
• If a match is found, the location of the block
  (the position vector) in the previous frame is
  coded.
• A trial predicted frame is built by using blocks
  from the previous frame, displaced by the
  movement vector.

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• Video sequence -gtPicture -gtSlice -gt Macroblock -gt
  Block with 8 pixels in both dimensions

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Motion Compensation
Second Frame
Motion vector
Residual
First Frame
Prediction
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Motion Compensation

• The predicted frame is subtracted from the next
  actual frame, and the residual image is encoded
  for transmission.
• At the receiving end the process is reversed, the
  predicted frame is built from the previous frame,
  and the residual image is decoded and added to
  the predicted frame.

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Motion Compensation Process

• First Frame normally is encoded without motion
  compensation
• This frame is called Intra frame (i-frame)
• The other frames are called P-frame and are
  predicted from the I-frame of P-frame that come
  immediate before it.
• Therefore the output of the prediction schemes
  can be described as IPPPPPP

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Motion Compensation Process

• Frames can also be predicted from future frames.
• It is called B-frame
• The future frames have to be encoded before the
  predicted frame.
• Therefore the encoding order does not necessary
  match the real frame order.
• Such process is predicted from two directions
• Therefore this coding scheme can be describe as
  IBBPBBPBBPBB

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Motion Compensation

• Searching an entire frame for a matching piece
  requires much processing power.
• Searching is therefore restricted to a small
  region surrounding each new block, this is
  defined by an encoding parameter called the
  search range.

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Motion Compensation

• Transmitting long motion sequences by means of
  frame prediction has two problems
• If an error occurs in one Frame, it will
  propagated through future frames, until a change
  obliterates it.
• In many cases video must be editable.
• Should be possible to cut any group of frames
  from a video sequence and use them in another
  sequence.
• With predictive codeing a video sequence must
  begin at the first frame.
• Both problems are solved by periodically sending
  a full frame that is not a prediction (eg. Every
  10 frames). These Frames are called I-Frames.

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MPEG flavours

• MPEG-1
• Designed for data rates of up to 1.5 Mb/s
• Single speed CDROM rate
• T1 data communications.
• MPEG 2
• Designed for higher rates 15Mb/s and higher
  quality performance including HDTV
• MPEG 4 and 7 under development.