SLR 12

Bitmap Images

• Comprised of pixels - smallest identifiable part of an image

• Each pixel is assigned a binary value - represents its colour

• Resolution - total pixels in an image - width x height

• PPI (pixels per inch) refers to the density of the pixels - 72 is standard

• DPI (dots per inch) relates to printing

• Colour depth can be calculated using 2^n where n is the number of bits

• RGB system uses 24 bits - 16,777,216 colours - what the eye can see

• 32 bits are used to store transparency - alpha channel

• File size = resolution x colour depth

Metadata:

• Data about data:

  • colour depth

  • date and time of creation

  • width and height in pixels

• The reason image file size may be different to the value calculated - could also be due to compression

Vector Images

• Stores geometric object not pixels

• Stores the properties of these shapes - drawing list:

  • size of angles

  • fill colour

  • start point and end point (co-ordinates)

  • line colour

  • line thickness

• Able to recreate exact copies of the object

• Mathematically calculated - can be resized without pixelation

• Each shape needs different properties - circle would need centre point and radius

• Bitmaps are better for photographs - more natural as the photo may not be made up of lines/ shapes

• Stored as an SGV file

• Can layer images without losing what is stored underneath - bitmap will replace the original pixels

Character Sets

• Each letter has a binary value associated - you press the key and the electrical signals that represent this letter are transmitted to display the binary value

• A character set is all the characters a computer can represent

• ASCII = American Standardised Code for Information Interchange - 7 bits - extended ASCII uses 8 bits

• 8 bit ASCII is not used much - 127 allows us to store most letters, numbers and symbols

ASCII:

• 0-31 = Controls

  • Null = 0

  • CR (enter) = 13

  • ESC = 27

• 48-57 = Numbers - cannot be used for arithmetic - binary is used instead

• 65-90 = Capital letters - 65 = A

• 91-123 = Lowercase letters - 91 = a

• All other values are used for symbols

Unicode:

• Uses UTF-8 - continuation of columns and headers to represent values

• Compatible with almost all devices

• Backwards compatible

Error Checking

• Checks each bit is correct

Parity Bits:

• Odd/ even way to check data

• A byte of data will be sent with a parity bit - the next byte will have an odd or even number of 1s

• In an odd parity, the spare bit will be used to make the number of 1s odd

• In an even parity, the spare bit will make the number of 1s even

• A 0 is used to keep it the same, a 1 is used to change it

Majority Voting:

• No parity bit

• Data is sent 3 times and the computer uses the majority rule to check the data

• More data needs to be sent and data may be sent in error 3 times

Check Sum:

• Mathematical algorithm ensures the checksums match

  • 70 + 204 + 12 = 286

• There could still be an error

  • 74 + 200 + 12 = 286

Check Digit:

• Examples include barcodes and ISBNs

• Often used with a large number

• If it matches the final digit, there are no errors