representation of bitmapped graphics
DESCRIPTION
Representation of Bitmapped GraphicsTRANSCRIPT
GraphicsAn image is made up of tiny dots called pixels.
The resolution determines the quality of the picture. The smaller the pixels the finer the detail that can be displayed on the screen.
High Resolution Image Low Resolution Image
Graphics PackagesGraphics packages can be classed as either:
Bitmapped graphics (painting) Vector graphics (drawing)
Bitmapped GraphicsThe image is saved in a two dimensional array using binary numbers to represent the colours in the pixels. Each pixel is represented by the same number of bits.
In this example, two colours have been used (black/white) and so each pixel is represented by one bit.
0 = white, 1 = black
Bitmapped GraphicsIn this example, four colours are used in the graphic and so each pixel is represented using two bits.
00 = white, 01 = blue, 10 = red, 11 = black
Bitmapped GraphicsHow many colours could be represented using 3 bits?
Bitmapped GraphicsHow many colours could be represented using 3 bits?
000
001
010
011
100
101
110
111
8 colours (23) could be represented using 3 bits
Bit DepthThe number of bits used to represent the colour of the pixels is called the bit depth.
Colours Bits
2 1
4 2
8 3
16 4
32 5
64 6
128 7
256 8
65536 16
16777216 24 (True Colour)
Storage Requirements (examples)An image, 5 inches by 7 inches is stored at 600 dots per inch (dpi) using 65536 colours.
How much memory would be required to store this image?
7 inches
5 inches
Storage Requirements (examples)Pixels used to store image = (5 x 600) x (7 x 600)= 12600000
Image size 5 in x 7 in
DPI 600
Colours 65536
Storage Requirements (examples)Pixels used to store image = (5 x 600) x (7 x 600)= 12600000
65536 colours = 16 bits = 2 bytes
Image size 5 in x 7 in
DPI 600
Colours 65536
Storage Requirements (examples)Pixels used to store image = (5 x 600) x (7 x 600)= 12600000
65536 colours = 16 bits = 2 bytes
Amount of memory = 12600000 x 2 bytes
= 25200000 bytes
= 25200000 / 1024 / 1024
= 24.03 Mb
Image size 5 in x 7 in
DPI 600
Colours 65536
Storage Requirements (examples)A 10 inch by 8 inch photograph is stored at 1200 dpi using 256 colours.How much memory would be required to store this image?
10 inches
8 inches
Storage Requirements (examples)Image size 10 in x 8
in
DPI 1200
Colours 256
Pixels used to store image = (10 x 1200) x (8 x 1200)= 115200000
Storage Requirements (examples)Image size 10 in x 8
in
DPI 1200
Colours 256
Pixels used to store image = (10 x 1200) x (8 x 1200)= 115200000
256 colours = 8 bits = 1 byte
Storage Requirements (examples)Image size 10 in x 8
in
DPI 1200
Colours 256
Pixels used to store image = (10 x 1200) x (8 x 1200)= 115200000
256 colours = 8 bits = 1 byte
Amount of memory = 115200000 x 1 byte
= 115200000 bytes
= 115200000 / 1024 / 1024
= 109.86 Mb
Advantages of bitmapped graphicsThe file area is fixed
The A4 page in the previous slide will need 110 Mb of memory irrespective of the size of the graphic on the page.
The graphic can be edited to pixel level
Disadvantages of bitmapped graphicsEnlarging the image causes a loss in quality
An A4 page using 256 colours at 1200 dpi will need 110 Mb of memory
An A4 page using 65,536 colours at 1200 dpi will need 220 Mb of memory
Compression can be used to decrease the size of these files when they are stored on discThe file is printed at the same resolution as it is shown on
the screen
The file size is very large
A graphic created on the screen using 300 dpi cannot be printed at a higher resolution (eg 600 dpi)
Data compressionReduces the size of bit-mapped files to make them faster to download and take up less disk space / memory
There are a variety of different techniques used to compress filesLossless CompressionNone of the original data is lostMethods are found to store the information in a more efficient way e.g. Instead of 250 white pixels, you store 250 times white pixelExample - TIFF
Lossy CompressionThis method involves sacrificing some of the data that our eyes will probably not noticee.g. Instead of using millions of colours we can just use hundreds.Therefore we only use 16 bits rather than 24 bits to represent the colour depthExample - JPEG
CreditsHigher Computing – Data Representation – Bitmapped Graphics
Produced by P. Greene and adapted by R. G. Simpson for the City of Edinburgh Council 2004
Adapted by M. Cunningham 2010
All images licenced under Creative Commons 3.0• Sets of blue waves and surfing boards by Horia Varlan (Flickr)• Black & White Atlanta skyline by Glen Edelson (Flickr)• A Cactus Flower for Capt. Suresh by Koshy Koshy (Flickr)• Sunset over the Danube - Budapest by joiseyshowaa (Flickr)• Edinburgh skyline by rovingI (Flickr)