Watching a movie

1
Watching a movie

• What makes a movie move?
• A movie is really a sequence of still images.
• When still images are presented often enough, we
  perceive smooth motion.
• Cinema films run at 24 frames per second.
• Normal TV runs at 25 or 30 frames per second.
• Most cartoons run at 12 frames per second.

2
Watching a movie

• Still images have to have quite high quality - we
  may look at them for many seconds.
• Movie frames can have quite low quality - we only
  see each frame for a fraction of a second.

3
Digital video

• Video sequences contain large amounts of info
• If a movie is recorded digitally at 320 by 240
  pixels in 24-bit colour at 30 frames per
  second...
• How big will an uncompressed 5 minute video clip
  be?
• duration in secs x frame rate x size in pixels x
  bit resolution / 8
• 5 60 30 320 240 3 about 1.93GB!
• The example uses a data rate of about 7MBps.
• 40xCD has a data rate of less than 6MBps.

4
Video

• More Examples
• 1 sec of 640x480 full colour (24bit) 30
  frames/sec requires 26Mb
• 1 Gbyte hard disc can store 38secs of video!!!
• Fast hard drives can transfer about 7Mb/sec
  therefore they cannot handle the data transfer

5
Size of Digital Video

• Solutions
• Reduce the size, full frame is a luxury we cant
  afford
• Reduce the colour depth, use 256 colours rather
  than 16 million
• Reduce the frame rate, try 15fps
• Use compression techniques
• MPEG (Motion Picture Expert Group)
• AVI (Audio/Video Interleaved)

6
Some digital video formats

• mov Quicktime movies
• a few different compression methods available.
  Often good quality and quite good compression.
• avi Audio-Video Interleaved
• many different compression schemes possible.
  Quality and compression comparable to Quicktime.
• mpg
• MPEG, the Motion Picture Experts Group format.
  PC and TV quality movies. Smaller size in general
• Other formats may not be compatible with
  multimedia editors

7
Video Compression

• Compressors do not store full information about
  each frame
• Key frames are compressed and then only
  differences between frames are stored
• Compression reduces amount of information stored
  per frame which gives better playback performance
• Compression can be time consuming
• Can take 10-15 mins to compress 30secs of video
• Decompression is fast

8
Video Compression

• Frame rate of video sequence and data transfer
  speed of playback system are major factors
• Video playing 15 frames/sec from CD-Rom with
  150Kb/sec transfer rate averages 10Kb data per
  frame
• Therefore 10Kb must be sufficient to hold video
  data and 67msecs (1/15 sec) audio data

9
MP4 players

• 20-40GB
• Hold up to 15,000 songs and up to 300,000 photos
• Watch up to 70-100 hours video on the 2-4" LCD
  screen
• Up to 20 hours battery life
• About 200-300

10
Why use video?

• Content
• Narration
• Visualisation
• Processes
• Reinforcement
• Interface
• Instructions
• Incidental
• Scene setting
• Transitions
• Titles and credits
• Welcome messages

11
Video software

• Capture video and audio from VCR or camera
• Select frame rate, frame size, colour depth and
  audio settings
• Convert from one file format to another, export
• Cut and paste frames
• Simple sound editing
• Add and animate graphics
• Transitions
• Special effects
• Reverse, speed up/ slow down
• Add titles