Flashcards in Data representation-graphics Deck (33):
what is the definition of a pixel?
The smallest addressable area or the smallest block of colour in an image
A picture element
What is a bitmap?
When the pixels of an image are mapped to positions in the memory which store the colour of that pixel in binary code
What is the resolution of a VDU screen?
The number of pixels per row x number of pixels per column
What is the resolution of an image?
The number of pixels per centimetre
(can be measured differently for VDUs)
What are three typical resolutions of a VDU screen?
1024 x 798
800 x 600
640 x 480
What is the definition of colour depth?
The number of bits used to represent the colour of a single pixel in a bitmapped image
What is the RGB colour model for changing the colour of a pixel?
red, green and blue are set at different light levels to combine to make a range of different colours
How do you make white and black in the RGB colour model?
white=all colours set to 100%
black=all colours set to 0%
What is the name for a black and white image?
Ina black and white image, how many bits are used per pixel?
What is 12 bit direct colour?
4 bits (16 possible values) are given to each of the red, blue and green components
When is 12 bit direct colour sometimes used?
Devices with limited colour display
(e.g. mobile phones)
What is true colour?
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
What is 32 bit colour?
Essentially true colour with an extre 8 bits
The extra bits are either ignored or used to represent an alpha channel (providing partial transparency)
Why does 32 bit colour use 32 bits?
The word size of most modern PCs is 32 or 64 bit
What is the size of a bitmap in bytes?
The size of the image in pixels multiplied by the colour depth of the image in bytes
How are vector graphics different from bitmaps?
It records information about geometric and other objects which make up an image
What is a graphic vector object?
A graphic component in a vector graphic such as a line or circle
What is a drawing list?
The list of drawing commands used to recreate a vector graphic
How is a line described in a vector graphic?
draws from (x,y) to (x,y)
How is a circle described in a vector graphic?
circle (x,y,r,fill colour,filled, border colour)
How is a rectngle described in vector graphics?
rect (x,y,x,y,fill colour,filled,border colour)
draws from top left to bottom right
Give examples of properties for components in a vector graphic
size, direction, thickness, shading, font size, typeface, fill colour, border colour, mathematical description of curves etc
What happens when a bitmapped image is scaled up?
The number of pixels remains the sam, they just enlarge, leading to them becoming visible and causing distortion
What happens when a vector graphic is scaled up?
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
What are the advantages of vector graphics over bitmaps?
-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
What are the advantages of bitmaps over vector graphics?
Images that have continous areas of colour (e.g in photographs) take up fewer bytes as bitmaps
What is the purpose of data compression?
data is compressed into a smaller number of bytes than it would normally occupy
What is run length encoding?
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
Is run length encoding lossy or lossless?
The original image can be recreated perfectly from the compressed version
What is lossy compression?
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
What is the second way in which lossy compression can be done?
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