Video Data

Topic 4: Multimedia Technology

What is Video?

• A video is just a collection of bit-mapped images that when played quickly one after another give the illusion of a moving image

• It is a sequence of individual pictures or frames

• The standard rate at which these frames are taken is 25 frames per second (this gives realistic movement)

• Work in the same way as digital cameras• They use the same light sensors, called

CCDs • They have a lower resolution than

cameras• They are designed to capture lots of

images (frames) fast

Input (Capture):Digital Video Camera (Camcorders)

Input (Capture):Digital Video Camera (Camcorders)

Input (Capture): Webcam

• Designed for the purpose of creating videos to be transmitted over the Internet

• They do not need to be high resolution, keeping down the bit rates and price.

• They use low resolution array CCDs and low quality lenses, keeping costs down

Input (Capture): Video Capture Card

• A video capture card is needed to capture videos from analogue sources– video tape players, television

broadcasts and analogue video recorders

• Some modern graphics cards include the ability to capture video

Video Data:Storage of Video data

Topic 4: Multimedia Technology:

AVI (Audio Video Interleave)

• Type of RIFF file (container file)• Uncompressed• AVI ratio set during saving

– The audio is embedded into the video at different intervals.

• Lossy compression cuts out unnecessary parts of a video clip

• Saves each frame of video as a JPEG– These are called ‘i-frames’

• Data that stays the same in following frames is removed

• The next frames only store data on what has changed since last i-frame

MPEG

MPEG

• MPEG-1 – VHS video quality with 353 x 240 pixels and 30 fps frame rate

support• MPEG-2

– The standard for DVD-Video and Digital Television • MPEG-3

– Intended for HDTV but these revisions were incorporated into MPEG-2)

• MPEG-4 – Designed for low-bandwidth networks - e.g. video phones) (Part

used by DivX• MPEG-7

– Builds on the interactive and extra data capabilities of MPEG-4 and is a full multimedia description format

MPEG Standards

Video Quality And File Size

• Colour depth: – Increasing colour depth improves quality and

increases file sizes.• Resolution:

– Increasing resolution improves quality and increases file sizes.

• Frame rate: – Measured in frames per second (FPS). – Increasing frame rate increases file size. Lower

frame rates reduce file size but make video clip ‘jerky’.

Video Quality And File Size

• Video time: – increasing or reducing the time of a video is

the obvious way to affect the file size. – Quality of the display of the clip is not

affected.• Lossy compression:

– Using MP3 compression reduces file sizes without affecting quality.

Calculations

File Size (Bytes) = Frame Size (Bytes) x Frame Rate (fps) xVideo Time (s)

Calculations

Uncompressed AVI

Frame Rate 24 fpsFrame Size 320 x 240 pixelsDuration 9sColour Depth 16 bits per pixelDuration 9sFile Size =

Video Data:Video Editing

Topic 4: Multimedia Technology:

• Each frame is displayed as a thumbnail image.

• Each frame can be individually edited• The audio would also be on an timeline• Some packages provide multiple timelines

Timeline

• Simple plan of final product

• Usually freehand and rough

• Important process for all multimedia applications

• Produced at Analysis stage

Storyboards

• Basically, cutting or removing the parts you don’t want.

• You may want to remove a frame or a whole scene.

Crop

Sequencing

• Once you’ve got your video cropped, edited you have to put it in sequence.

• In other words, put it in the correct order

A B C D

B D C A

Transitions

• Used to ‘join up’ different scenes/frames• Different types are available, for example:

Wipe

Transitions

• Used to ‘join up’ different scenes/frames• Different types are available, for example:

Dissolve

Transitions

• Used to ‘join up’ different scenes/frames• Different types are available, for example:

Box Out

Transitions

• Used to ‘join up’ different scenes/frames• Different types are available, for example:

Fade