Topic 1 - Common Questions & exam technique

State one difference between kibibyte and kilobyte

• Kibi is 1024 bytes AND kilo is 1000 bytes

• Kibi is binary prefix AND kilo is denary prefix

Why might actual file size of an image be larger than calculated size

• File contains metadata

• File will have a header

“Why a value in a register can’t be interpreted as BCD” 

• Denary value for each nibble is greater than 9

“Benefits of using BCD to represent values”

• Easy for digital equipment to use BCD to display output information

• Can represent monetary (financial) values exactly

• Easy to convert from BCD to denary

• Less complex to encode and decode for programmers

“One practical use of BCD and why BCD is used”

• Digital clocks

• Visual displays only need to show individual digits

• Conversion between BCD and denary is easier

Binary subtraction Method:

• x - y => x + (-y)

• Convert -y to two’s complement

• Add x and y

Overflow errors

• Answer too big, can’t be represented by current/given number of bits

“How a word is represented by an ASCII character set”

• Each character has a unique binary code

• Character in the word replaced by the code

• Code stored in the order it is in the word

“Define a character set”

• A group of characters with unique binary numbers

• *All the characters that the computer can use has a unique number

“Similarity between ASCII and Unicode”

• Both represent each character with a unique code

“Define a pixel”

• Single square of 1 color

• Smallest addressable element of a bitmap image

“Define file header”

• Data about a bitmap image 

• Color depth, file type, compression type, dimensions, image resolution

“Bit depth and how it affects image”

• Closer to original image, increases range of colors

“Impact of increasing image resolution to quality of bitmap graphic”

• Less pixelated

• More pixels can be stored

“Define a drawing list”

• A list that stores the commands required to draw each object

“Describe the contents of a vector graphic list”

• List of objects in the drawing

• List of commands for all objects in an image

• Properties of each object eg. Fill color

“Impact of changing sample resolution”

• Increase = More bits per sample, more amplitudes represented, digital sound wave closer to original analogue wave

• Decrease = Less bits per sample, less amplitudes can be represented, digital sound wave further/less accurate to original analogue sound wave

“Lossless compression method & how it works”

• Run-length encoding

• Replace sequences of the same color pixel

• … with color code and number of identical pixels

“Reasons to compress file”

• Reduce file size

• Faster to transmit/download

• Less bandwidth required to transmit

• (Original file too large for email attachment

“Lossless compression - sound

• Reduce amplitude to only range used

• … limited amplitudes = fever bits per sample

• Run-length encoding

• … where consecutive sounds are the same, record the binary value of the sound and the number of times it repeats

• Record the changes instead of the actual sounds

“Lossy methods”

• Reduce bit depth

• … reduces number of bits per color/pixel, meaning each pixel has fewer bits

• Reduce color palette/number of colors

• … fewer colors = fewer bits needed to store each color

• Reduce image resolution

• … fewer pixels per unit = less binary to store

“Lossy NOT on text file”

• None of original data can be deleted

• The text file would be corrupted

“Why lossless may not work on bitmap image”

• RLE stored color and number of times it occurs consecutively

• Image may not have sequences of same color

• Would need to store each color and then the number 1 (as it isn't repeated) which adds data

• Eg. Red-Green-Blue would be Red 1 Green 1 Blue 1

Binary Coded Decimal (BCD)

• A method of restricting the number of possible values so that each group of 4 bits (nibble) corresponds to the denary numbers 0-9 (doesn’t recognise combinations past 9/1001, used in clocks, as each digit only requires 0-9. ***Make sure to split into nibbles

ASCII & Extended ASCII

• Normal ASCII uses 7 bits to store each character, giving 2^7 = 128 options

• Extended ASCII uses 8 bits to store each character, giving 2^8 = 256 options

Unicode

• Unicode uses 16 bits to store each character, giving 2^16 = 65,536 options

Color/Bit depth

• The number of bits used to represent each color

Image resolution

• The number of pixels wide multiplied by the number of pixels high

*Drawing object

• A component created using a formula

*Property

• Defines one characteristic of a component

Analogue data

• Data values that are continuously changing

*Sample rate

• Number of samples taken per second

Vectors vs Bitmap

• Bitmap files are usually bigger than vector image sizes, as they need to store data for each pixel, whereas vectors are smaller as they just store instructions.

• When bitmap images are enlarged the pixels get bigger and this results in pixelation, whereas in vectors new values are recalculated, which does not cause pixelation

*Compression

• To reduce the file size

Lossless compression

• A method of compressing a file that does not remove any detail from the original file.

*Lossy compression

• A method of compressing files that permanently removes some of the detail from the original file.

“Characteristics of Unicode character set”

• 16 bits per character

• Represents 2^16 characters

• Represents every language and other characters such as emojis

“Effect of decreasing bit depth on image and image file

Image:

• Fewer shades of color available

• Image does not match original as data is lost

File:

• Fewer bits used to store each pixel

• Less data stored, therefore file size reduced