1.2 Multimedia

# Multimedia

- Bitmap Images 
  - Data for a bitmapped image is encoded by assigning a solid colour to each pixel, i.e., through bit patterns. 
  - Bit patterns are generated by considering each row of the grid as a series of binary colour codes which correspond to each pixel’s colour. 
  - These bit patterns are ‘mapped’ onto main memory 
- Defination of Bitmap 
  - Made up of pixels 
  - Each pixel has one colour 
  - Colour of each pixel stored as a binary number 
  - Pixels: • A single square of one colour 
    - The smallest addressable element in an image 
    - Bitmap image also contains the File Header: Data about the bitmap image (e.g. 
    - number of colours, 
    - File format/File type 
    - File size 
    - Dimensions, resolution & colour depth of the image 
    - Date and place when the photo was taken 
    - Time and date when the photo was changed 
    - Camera settings when the photo was taken 
    - type of data compression 
    - )

- Image Resolution: number of pixels that make up an image 
- Pixel density – number of pixels per square centimetre. 
  - If image resolution increases, then image is sharper/more detailed 
- Screen Resolution 
  - Number of pixels which can be viewed horizontally & vertically on the device’s screen 
  - Number of pixels = width × height 
  - E.g. 1680 × 1080 pixels 
- Colour depth/Bit depth: number of bits used to represent the colour of a single pixel 
  - An image with n bits has 2n colours per pixel 
  - E.g. 16-colour bitmap has 4 bits per pixel ∵ 2<sup>4</sup>=16
  - Colour depth↑: colour quality↑ but file size↑ 
  - File Size = Number of Pixels × colour depth 
  - Convert bits to bytes by dividing by 8 if necessary. 
- Applications: scanned images and general computer usage ∵ small file size and can be easily manipulated. 
- Vector Graphics 
  - Made up of geometric shapes which requires attributes 
  - Made up of drawing objects 
  - Drawing objects: a mathematically defined construct (of shapes like rectangle, line, circle, etc.) 
  - Drawing list: a list that stores the commands required to draw each object( 
    - This list contains a command for each object 
    - A command has a list of attributes 
    - Each attribute defines a property of the object 
    - Property including line colour, the thickness of a line, fill colour,The relative position of each object 
  - ) 
  - Properties of each object are the basic geometric data which determine the shape and appearance. 
  - Data is encoded using mathematical formulas to generate properties in order to draw lines & curves to create the image 
  - If object is resized, properties are recalculated. 
  - The benefits of vector graphics: 
    - Needs to be large for the signs without becoming pixelated. 
    - Smaller file size means faster transfer rates are possible. 
    - Smaller file size reduces storage requirements when stored many times (on multiple documents). 
∴ Scalable without losing quality unlike bitmaps 
- Applications: company logos 
- Sound 
  - Analogue data is data values that are continuously changing. 
  - Sound signals are vibrations through a medium. Hence are analogue in nature as there can be an infinite amount of detail for sound. 
  - Analogue signals converted (encoded) to digital signals by sampling: 
    - The amplitude of the sound wave is first determined at set time intervals. 
    - This gives an approximate representation of the sound wave. 
    - The sound wave is then encoded as a series of binary digits. 
- Sampling Rate 
  - Number of samples taken per unit time/per second 
    ###### Increasing the sampling rate
  - improves the accuracy of the sound file because (digital) waveform more closely resembles the analogue waveform quantization errors are reduced increases the amount of detail stored 
  - but increases the file size 
- Sampling Resolution/Bit depth 
  - Number of bits used to encode each sample 
  - Increasing sampling resolution increases accuracy of digitized sound wave but increases the file size 
  - decreases the file size of the sound file because fewer bits are used to store each sample
- Bit Rate: no. of bits for storing 1 second of sound 
- Common features of sound edited software include the ability to 
  - » edit the start/stop times and duration of a sample 
  - » extract and save (or delete) part of a sample 
  - » alter the frequency and amplitude of a sample 
  - » fade in and fade out 
  - » mix and/or merge multiple sound tracks or sources 
  - » combine various sound sources together and alter their properties 
  - » remove ‘noise’ to enhance one sound wave in a multiple of waves (for example, to identify and extract one person’s voice out of a group of people) 
  - » convert between different audio formats.