Data representation-graphics

Question 1

what is the definition of a pixel?

Answer 1

The smallest addressable area or the smallest block of colour in an image
A picture element

Question 2

What is a bitmap?

Answer 2

When the pixels of an image are mapped to positions in the memory which store the colour of that pixel in binary code

Question 3

What is the resolution of a VDU screen?

Answer 3

The number of pixels per row x number of pixels per column

Question 4

What is the resolution of an image?

Answer 4

The number of pixels per centimetre

can be measured differently for VDUs

Question 5

What are three typical resolutions of a VDU screen?

Answer 5

1024 x 798
800 x 600
640 x 480

Question 6

What is the definition of colour depth?

Answer 6

The number of bits used to represent the colour of a single pixel in a bitmapped image

Question 7

What is the RGB colour model for changing the colour of a pixel?

Answer 7

red, green and blue are set at different light levels to combine to make a range of different colours

Question 8

How do you make white and black in the RGB colour model?

Answer 8

white=all colours set to 100%

black=all colours set to 0%

Question 9

What is the name for a black and white image?

Answer 9

monochrome

Question 10

Ina black and white image, how many bits are used per pixel?

Answer 10

one

1=white, 0=black

Question 11

What is 12 bit direct colour?

Answer 11

4 bits (16 possible values) are given to each of the red, blue and green components

Question 12

When is 12 bit direct colour sometimes used?

Answer 12

Devices with limited colour display

e.g. mobile phones

Question 13

What is true colour?

Answer 13

8 bits (256 possible values) are used to represent each colour component
called true colour because it approaches the level at which the human eye can distinguish colours

Question 14

What is 32 bit colour?

Answer 14

Essentially true colour with an extre 8 bits

The extra bits are either ignored or used to represent an alpha channel (providing partial transparency)

Question 15

Why does 32 bit colour use 32 bits?

Answer 15

The word size of most modern PCs is 32 or 64 bit

Question 16

What is the size of a bitmap in bytes?

Answer 16

The size of the image in pixels multiplied by the colour depth of the image in bytes

Question 17

How are vector graphics different from bitmaps?

Answer 17

It records information about geometric and other objects which make up an image

Question 18

What is a graphic vector object?

Answer 18

A graphic component in a vector graphic such as a line or circle

Question 19

What is a drawing list?

Answer 19

The list of drawing commands used to recreate a vector graphic

Question 20

How is a line described in a vector graphic?

Answer 20

line (x,y,x,y,colour,width)
draws from (x,y) to (x,y)

Question 21

How is a circle described in a vector graphic?

Answer 21

circle (x,y,r,fill colour,filled, border colour)

Question 22

How is a rectngle described in vector graphics?

Answer 22

rect (x,y,x,y,fill colour,filled,border colour)

draws from top left to bottom right

Question 23

Give examples of properties for components in a vector graphic

Answer 23

size, direction, thickness, shading, font size, typeface, fill colour, border colour, mathematical description of curves etc

Question 24

What happens when a bitmapped image is scaled up?

Answer 24

The number of pixels remains the sam, they just enlarge, leading to them becoming visible and causing distortion

Question 25

What happens when a vector graphic is scaled up?

Answer 25

Distortion is avoided
scaling is geometric because the graphic is defined by relative positions of objects in a coordinate system
It has no intrinsic resolution so no resolution is lost

Question 26

What are the advantages of vector graphics over bitmaps?

Answer 26

• geometric images require fewer bytes
• geometric images load faster from secondary storage and download faster over the internet
• vector images scale without distortion
• some formats allow you to search more easily for a particular geometric object to be manipulated

Question 27

What are the advantages of bitmaps over vector graphics?

Answer 27

Images that have continous areas of colour (e.g in photographs) take up fewer bytes as bitmaps

Question 28

What is the purpose of data compression?

Answer 28

data is compressed into a smaller number of bytes than it would normally occupy

Question 29

What is run length encoding?

Answer 29

A data compression technique which takes advantage of the fact that consecutive cells often contain the same bit pattern (colour)
This is reduced to two bytes: number of consecutive cells, colour of cells

Question 30

Is run length encoding lossy or lossless?

Answer 30

lossless

The original image can be recreated perfectly from the compressed version

Question 31

What is lossy compression?

Answer 31

unimportant information (e.g. background scenery) is saved with a lower resolution
This means the original image cannot be recreated exactly from the compressed version but it will be close enough that most humans will not be able to see the difference

Question 32

What is the second way in which lossy compression can be done?

Answer 32

The lower sensitivity of the eye means that the bits per pixel for colours like blue, which we cannot see as well as others, can be reduced

Question 33

For what kind of colour does lossy compression use fewer bits?

Answer 33

lower intensity colours