Computer Graphics Systems: Raster vs. Random Scan

Posted on Jun 16, 2026 in Computer Engineering

Display Architectures in Computer Graphics

In computer graphics, architecture displays are fundamentally divided into two categories based on how the electron beam (or display processor) constructs and refreshes the image on the screen: Raster-Scan Systems and Random-Scan (Vector/Stroke/Calligraphic) Systems.

1. Raster-Scan System

A Raster-Scan System is the most common type of display graphics architecture, used in modern TVs, computer monitors, and smartphones.

How it Works

The electron beam sweeps across the screen one row at a time, moving from top to bottom. As the beam moves horizontally across each row (called a scan line), the beam intensity is turned on and off (or varied in color) to create a pattern of illuminated spots.

• Horizontal Retrace: When the beam reaches the end of a scan line, it is turned off (blanked) and rapidly flies back to the left side of the screen to start the next scan line.
• Vertical Retrace: Once the beam reaches the bottom right corner of the entire screen, it blanks out and returns to the top left corner to begin rendering the next frame.

Core Architecture Components

• Pixel: The smallest addressable picture element on a screen matrix.
• Frame Buffer (Refresh Buffer): A dedicated area of memory where the picture definition is stored as a matrix of intensity values for all screen points. The display processor reads these values line-by-line to paint the screen.
• Bit Map vs. Pix Map:
  • If the system has 1 bit per pixel (black and white), the frame buffer is called a bitmap.
  • If the system supports multiple bits per pixel (colors/grayscale), it is called a pixmap.

2. Random-Scan System (Vector Display)

A Random-Scan System operates like a dynamic digital sketchpad. Instead of scanning the entire screen uniformly, the electron beam is directed only to the parts of the screen where a line or component needs to be drawn.

How it Works

If the system needs to draw a triangle, the electron beam moves directly from vertex A to B, then B to C, and back to A. It doesn’t waste time scanning empty spaces on the screen. It is also referred to as a Vector, Stroke-Writing, or Calligraphic display.

Core Architecture Components

• Display Buffer (Refresh Display File): Instead of storing pixel intensity maps, the memory holds a list of component line-drawing commands (e.g., MOVE(X1, Y1), LINE(X2, Y2)).
• Display Controller: Cycles through this command file repeatedly at high speeds (30 to 60 times per second) to keep the lines refreshed on the phosphor screen before they fade.

Key Differences: Raster-Scan vs. Random-Scan

Refresh Rate and Video Memory Calculations

To understand how a Raster-Scan system handles storage mathematically:

1. Memory Depth Formula

The total number of colors N that a system can display simultaneously depends on the number of bits n allocated per pixel in the frame buffer. If a system uses a 24-bit True Color configuration (n = 24), it can display 2²⁴ colors.

2. Frame Buffer Size Calculation

To find the minimum video memory required to run a specific resolution:

Example: Calculate the frame buffer memory needed for a standard 1024 × 768 resolution display with 8-bit color depth.

Graphics Software

Graphics software acts as the software interface between the user/application program and the display hardware. It provides tools and functions to create, manipulate, and render images on a screen.

1. Types of Graphics Software

Graphics software is broadly classified into two categories based on how it functions:

• General Programming Packages: These provide a set of graphics functions that can be used in a high-level programming language (like C, C++, Python, or Java) to build custom visual applications. They handle the low-level math of drawing shapes, handling lighting, and mapping coordinates. Examples: OpenGL, DirectX, Vulkan, and WebGL.
• Special-Purpose Application Packages: These are pre-built, user-friendly applications designed for non-programmers. Users can create and edit visuals directly through a Graphical User Interface (GUI) without writing code. Examples: Adobe Photoshop (for raster graphics), Adobe Illustrator / CorelDRAW (for vector designs), and AutoCAD / Blender (for 3D modeling and animation).

2. Coordinate Representations

When creating an image in software, the coordinates must pass through different stages before they appear on your screen:

1. Model/Object Coordinates (Xm, Ym): The local coordinate system used to define a single distinct object (e.g., designing a single tire for a car model).
2. World Coordinates (Xw, Yw): The global coordinate system where all independent models are placed together to form a complete scene (e.g., placing the entire car onto a simulated racing track road).
3. Device / Screen Coordinates (Xd, Yd): The final integer pixel coordinates specific to the physical display device monitor resolution (e.g., mapping the scene to a 1920 × 1080 display grid).

Input and Output Devices for Graphics

Interactive computer graphics systems require specialized hardware to capture human intent (input) and render the digital result accurately (output).

1. Input Devices

Input devices capture spatial data, orientation, or movement from the user and translate it into coordinates for the graphics processor.

• Keyboard: Primarily used for entering commands, numeric parameters, or text labels within design software.
• Mouse: The most common locator device. It uses an optical sensor on its base to record relative movement across a flat surface, translating it into cursor movement on screen.
• Trackball and Spaceball:
  • A trackball is essentially an inverted mouse; you roll a spherical ball with your fingers while the base stays stationary (great for tight desk spaces).
  • A spaceball doesn’t roll; it senses pressure applied to a fixed ball to allow 3D rotation and translation in CAD/3D modeling applications.
• Joystick: Uses a pivoting stick to report deflection angle and direction. Frequently used in flight simulators and gaming.
• Scanner: Uses an internal light source and charge-coupled devices (CCD) to capture physical drawings or photographs and convert them into digital bitmaps.
• Light Pen: A handheld, photosensitive stylus pointed directly at a CRT screen. When the electron beam passes beneath the pen, it detects the burst of light and sends a timing signal to the computer to log the exact screen coordinate.

Graphics Tablets (Digitizers)

A Graphics Tablet (also known as a Digitizer or Drawing Tablet) is a highly accurate, specialized input device used to convert freehand strokes, drawings, or schematic tracing into digital vector coordinates.

How it Works

1. The Grid: The surface of the tablet contains a dense embedded matrix of microscopic radio-frequency electronic coils.
2. The Stylus: The handheld pen (stylus) contains an integrated transmitter.
3. Electromagnetic Resonance (EMR): When the pen approaches or touches the surface grid, electromagnetic signals pass between the coil matrix and the pen tip.
4. Coordinate Logging: The tablet calculates the exact X and Y location based on which coils receive the strongest signal from the pen.

Key Capabilities in Graphics

• Pressure Sensitivity: Modern styluses don’t just register where you draw; they measure how hard you press down. The tablet sends pressure values to software, allowing digital brushes to dynamically change thickness, opacity, or color blend—mimicking real charcoal or paint.
• Tilt and Angle Detection: Advanced tablets track the angle of the pen, allowing artists to shade surfaces realistically just by tilting the stylus sideways.
• Absolute Mapping: Unlike a mouse (which tracks relative movement), a tablet surface is mapped absolutely to your screen monitor. Touching the top-right corner of the tablet instantly teleports your digital cursor to the top-right corner of your screen.

2. Output Devices

Output devices receive processed binary or vector data from the graphics framework and project it visually.

• Monitors / Displays: Flat panels (LCD, LED, OLED) or historic CRTs that render the pixel matrices stored within the system’s frame buffer.
• Printers: Used to produce hard copies of digital graphics.
  • Impact (Dot-matrix): Prints via physical pins striking an inked ribbon (low quality, legacy tech).
  • Non-Impact (Inkjet/Laser): Inkjets spray microscopic droplets of liquid ink, while lasers use static electricity to bind toner powder to paper. Ideal for high-quality image rendering.
• Plotters: Specialized vector output hardware that uses mechanical pens to draw continuous, high-precision lines directly onto massive paper sheets. Widely deployed in blueprints, architectural schematics, and engineering CAD outputs.