Multimedia – Graphics, Sound (Copy)

Cheat Sheet: A Level Computer Science – Image and Sound Representation

1. Bitmap Image Representation

Key Terms

• Pixel (Picture Element): Smallest unit of a digital image; holds a single colour value.
• Image Resolution: Number of pixels in an image (e.g., 1920×1080); higher resolution = more detail.
• Screen Resolution: Number of pixels a screen can display.
• Colour Depth / Bit Depth: Number of bits used to represent the colour of one pixel.
  • Common depths:
    • 1-bit: 2 colours (black & white)
    • 8-bit: 256 colours
    • 24-bit: ~16.7 million colours (True colour)
• File Header: Contains metadata such as resolution, colour depth, and file format.

Bitmap File Size Calculation

Formula:

File size (bits) = width × height × colour depth

• Convert to bytes: divide by 8
• Consider file headers and optional compression for exact file size

Impact of Changing Image Properties

2. Vector Graphics Representation

Key Terms

• Drawing Object: Basic shape/component (e.g., line, circle, rectangle)
• Property: Attributes like position, colour, line thickness, radius, etc.
• Drawing List: List of objects and their properties used to construct the image

Encoding Method

• Stores mathematical instructions for shapes rather than pixel data
• Smaller file size compared to bitmaps for diagrams/logos
• Scalable without loss of quality

Bitmap vs Vector Use Cases

3. Sound Representation and Encoding

Key Terms

• Analogue Sound: Continuous wave – original form of sound
• Digital Sound: Discrete values obtained through sampling
• Sampling: Measuring sound amplitude at regular intervals
• Sampling Rate (Hz): Number of samples per second (e.g., 44,100 Hz)
• Sampling Resolution (bit depth): Number of bits per sample; affects dynamic range

Impact on File Size and Accuracy

Sound File Size Estimation

Formula:

File size (bits) = sampling rate × bit depth × duration (s) × number of channels

• Stereo = 2 channels; Mono = 1 channel
• Convert bits to bytes: divide by 8