Representating Digital Media (Representing Video)

Analog TV of the past century

Today

Virtual Reality

Video: Structure and Properties

- Comprised of frames of still images (combined with audio)

- When viewed in rapid succession, the images give the appearance of motion
- Typical frame rates are 24-30
frames per second to 60 fps
-Example: 3600 frames per
hour at 30 fps

- Aspect Ratio: 16:9 typically
- (Display width divided by
height of the display)

Video Compression TESTABLE

- Typically little change between successive video frames

-Use techniques to remove that redundant info

Spatial (intra-frame_ techniques

Use information from within the same frame to reduce file size

Temporal (inter-frame) techniques

- Use data from nearby frames (before or after) to reduce the file size

- Key frames (i-frames):
-Typically compressed using only spatial techniques (like JPEG)
- Can be reproduced independently

P-frames and B-frames

other frames encoded by saving differences between ti and previous key-frame (P=Predictive, B=Bidirectional)

If a key-frame is lost or skipped the displayed video can be distorted

Bitrate vs Size

If the bitrate is too low, the video becomes pixelated

Types of BItrate

1. Constant bitrate (CBR):
2.Variable Bitrate (VBR):

Constant bitrate (CBR):

same bit rate throughout the video

Variable Bitrate (VBR):

Allows higher bit rates during fast motion scenes, and lower bitrates when there is little motion

Example Codes: Should Know These

1.HuffYUV (Huffman, YUV colour space)
2. MPEG-2
3. H.264
4. H.265 (HEVC)

HuffYUV (Huffman, YUV colour space)

Lossless, Huffman encoding, compression ratios around 47% (don't need to memorise the number of CR)

MPEG-2

Lossy, temporal and spatial compression, Huffman encoding

Compression ratios around 3.3% (don't need to memorise the number of CR)

Highly dependant on bitrate and other encoding options

H.264

Lossy, temporal and spatial compression, frame prediction

Compression ratios around 1.6% (don't need to memorise the number of CR)
- Highly dependent on bitrate and other encoding options

Perceived quality higher than MPEG-2 at same bitrate

H.265 (HEVC)

Successor of H.264: further 50% reduction of size, much more complex!

Often drains battery faster

Differences between video codec and video file containers

File containers:

Combine a video stream and an audio stream into one file (or compressed video stream and compressed audio stream)

Does not necessarily indicate the codecs used

Some allow for additional data:

Some allow for additional data:

Additional video stream representing different viewing angles

Multiple language audio tracks

Optional subtitles

Containers are indicated by?

the file extension


.avi (audio Video Interleave): very old

.mp4 (MPEG-4): supports multiple audio/video streams and subtitles, but limited codecs

.mkv (matroska): like mp4, but supports unlimited streams and unlimited codecs