Graphics and compression

Question 1

Define pixel

Answer 1

Smallest addressable area of an image

Question 2

How do you calculate resolution of VDU screen and an image

Answer 2

VDU screen: number of pixels per row*number of pixels per column An image: Number of pixels per inch/centimetre

Question 3

Define colour depth

Answer 3

Number of bits used to represent a colour of a single pixel

Question 4

How do you calculate image size

Answer 4

Image size(in bits) = bits per colour * number of pixels

Question 5

Define metadata

Answer 5

Information about data stored in same file as image data

Including: date, colour depth, location, etc

Question 6

How do vector graphics work

Answer 6

They are co-ordinates of objects making up pictures stored as mathematical data. Points described relative from origin.
The objects are comprised of paths, can be scaled and group together.

Question 7

What path features does each vector have

Answer 7

Start and end point coordinates
Length
Thickness
Colour

Question 8

Two types of compression

Answer 8

Lossless compression and lossy compression

Question 9

Define lossless compression and lossy compression

Answer 9

Lossless compression: File can be uncompressed, and o.g quality of image can be restored

Lossy compression: Some o.g image quality lost forever. So when decompress you cannot get it back

Question 10

How does lossy JPEG work

Answer 10

Storing at reduced resolution
Reduced number of shades to one as not noticeable to human eye

Question 11

How does lossless dictionary-based work

Answer 11

Contains token numbers that represent letters
Example:
Storing “Station nation creation”
tokens: “tion” = 1, “sta”=2, “na”=3, “crea”=4
so can be stored as ‘21 31 41’ which uses less data

Question 12

How does lossless run length encoding work

Answer 12

RLE (image based) takes account that images have long runs of pixel with same colour. So if 3 or more consecutive of same colour, run of cells is encoded into 2 bytes

Question 13

Advantages and disadvantages of RLE

Answer 13

Advantages: Algorithm easy to implement and CPU does not require much horsepower
Computer generated colour images give fair compression ratios

Disadvantages: RLE only effective when file has lots of repetitive data

Question 14

How does RLE work for monochrome data

Answer 14

Always start with number of whites, then alternate between black and white