Computer Graphics: Algorithms, Shading Techniques, and Display Systems

Posted on Jan 11, 2026 in Computers

Fundamentals of Computer Graphics

What is Computer Graphics?

Computer graphics is the field of computer science focused on creating, manipulating, and storing images and visual content using computers. It is a crucial discipline because it enables the visualization of complex data, enhances user interfaces, and drives innovations in entertainment, science, and design.

Importance and Applications

  • Data Visualization: Computer graphics simplifies and visualizes complex data for easier understanding, analysis, and presentation through charts, graphs, and interactive visuals.
  • User Interface: It forms the foundation for user interfaces on nearly every digital device, making it possible to have icons, menus, and other graphical elements on screens.
  • Entertainment: Computer graphics are fundamental to modern entertainment, from special effects in films and television to creating virtual worlds in video games.
  • Scientific and Engineering Design: It allows for the creation of precise models and simulations for fields like medicine, architectural design, and aerospace engineering.

Examples

  • Gaming and Entertainment: The realistic environments and characters in a video game like Fortnite or the special effects in a movie like Avatar are created using computer graphics techniques.
  • Medical Imaging: Doctors use computer graphics to interpret and visualize scans like X-rays, MRIs, and CT scans to help diagnose and plan treatments.
  • Architectural Design: Architects use computer graphics for modeling and visualization.

Core Graphics Algorithms

Midpoint Ellipse Algorithm

The Midpoint Ellipse Algorithm is an efficient, incremental method used in computer graphics to determine the pixels needed to draw an ellipse on a digital screen. It is an extension of the midpoint circle algorithm and relies on the ellipse’s symmetry to plot points.

The core idea is to sample points along the curve by checking the midpoint between potential pixel locations to see which pixel is closer to the actual ellipse boundary using only addition and subtraction operations, avoiding computationally expensive calculations like square roots or multiplication in real-time.

Symmetry

An ellipse has four-way symmetry across its major and minor axes, but for a general ellipse with non-equal radii, it’s divided into two regions per quadrant. The algorithm calculates pixels for only one quadrant, and then mirrors them to the other three.

Decision Parameter

The algorithm uses a “decision parameter” (or initial value $p_{10}$ in region 1 and a subsequent $p_{20}$ in region 2) to decide which of two possible adjacent pixels to illuminate. The sign of this parameter determines if the midpoint between the candidates lies inside or outside the true ellipse boundary.

Incremental Calculation

The decision parameter is updated incrementally at each step using its previous value and the changes in $x$ and $y$ coordinates. This ensures fast execution, as no complex mathematical functions are recalculated in every iteration.

Flood Fill Algorithm (Seed Fill)

The term you are likely referring to is the Flood Fill Algorithm, a fundamental concept in computer graphics and image processing used to fill a connected region with a specific color or pattern. A “fluid filling algorithm” might also refer to specialized algorithms for simulating real-world fluid dynamics in engineering.

Definition and Mechanism

The flood fill algorithm, also known as the seed fill algorithm, is an algorithm that determines the area connected to a given starting point (or “seed”) in a multi-dimensional array (like a grid of pixels in an image) and changes the color of all connected pixels that share a common “target” color to a new “replacement” (fill) color.

The process works like “flooding” an area with paint until it reaches a boundary of a different color. It can be implemented using either a recursive function or an iterative approach with a stack or queue data structure, treating the image as a graph to traverse adjacent pixels.

Relevance and Applications

The flood fill algorithm is a simple yet vital technique with numerous practical applications across various domains:

  • Image Editing Software: The most common example is the “bucket fill” or “paint bucket” tool found in graphic design software like Adobe Photoshop or Microsoft Paint. It allows users to quickly fill an enclosed area with a solid color or pattern.
  • Game Development: It is used in games for tasks such as revealing areas in Minesweeper, generating maps or cave systems, solving mazes, and even simple 2D collision detection by checking connected regions.
  • Image Processing: In image processing and analysis, the algorithm is used for image segmentation—identifying homogeneous regions of an image based on pixel similarity or color.
  • Networking: The general concept of “flooding” is also a non-adaptive routing technique in computer networks where incoming packets are sent out on every outgoing link.

3D Rendering Techniques

Polygon Shading Methods

The three main polygon rendering methods are Flat, Gouraud, and Phong shading. Phong shading is the most popular for applications that require high realism because it provides the most accurate and realistic shading, including specular highlights, even though it is the most computationally expensive.

Flat Shading (Constant Intensity Shading)

  • Mechanism: Assigns a single color or intensity to an entire polygon.
  • Pros: Very fast to render, as it’s the simplest method.
  • Cons: Creates a faceted, blocky appearance because it doesn’t account for the smoothness of the surface, making it the least realistic.

Gouraud Shading

  • Mechanism: Interpolates the color or intensity of the vertices across the surface of each polygon.
  • Pros: Produces a much smoother appearance than flat shading by reducing the faceted look. It smooths polygons and is still relatively fast.
  • Cons: Can result in less accurate highlights and artifacts like “Mach bands” (unnatural streaks) on curved surfaces.

Phong Shading

  • Mechanism: Interpolates the surface normal vector across each polygon and then calculates the lighting at each pixel.
  • Pros: Generates the most realistic results with smoother transitions and more accurate specular highlights.
  • Cons: Requires significantly more computation, making it slower than flat or Gouraud shading.

Why Phong Shading is Most Popular

Highest Realism: Phong shading provides the highest level of visual fidelity, which is crucial for applications like video games, visual effects, and professional 3D rendering where realism is a priority. It is the only method that can accurately represent specular highlights.

Display Technologies

Plasma Displays

Plasma displays use small, individual cells containing an ionized gas (plasma) to produce light and images. Each pixel in a plasma display is composed of three sub-pixels (red, green, and blue).

Working Mechanism

  1. Electrode Activation: An electric current is applied to horizontal and vertical electrodes at a specific pixel location.
  2. Ionization: The electric field generated by the electrodes energizes the noble gas mixture (typically neon and xenon) within the cell, ionizing the gas and turning it into a plasma state.
  3. UV Light Emission: As the ions and electrons collide and then return to a lower energy state, they release energy in the form of invisible ultraviolet (UV) light photons.
  4. Visible Light Production: These UV photons strike the phosphor material coated on the inside of the cell walls. The phosphors become excited and emit visible red, green, or blue light, depending on the sub-pixel’s coating.
  5. Color Mixing: By varying the intensity of the electric current (pulse-width modulation) for each red, green, and blue sub-pixel, a full spectrum of colors can be produced for the overall pixel, creating the complete image on the screen.

Comparing CRT Color Methods

Beam Penetration Method

The Beam Penetration Method uses a single electron gun and works with two phosphor layers, usually red and green. It produces color by controlling the speed of the electron beam, which penetrates one of the phosphor layers to generate different colors. Because of this mechanism, its color range is limited to four basic colors, and the image quality is generally lower. Beam penetration displays are mostly used in random-scan monitors, and they are cheaper.

Shadow Mask Method

In contrast, the Shadow Mask Method uses three electron guns—one each for red, green, and blue—along with three sets of phosphor dots. Colors are produced by combining the intensities of the three separate beams, allowing the system to generate millions of colors with higher and more realistic image quality. Shadow mask technology is mainly used in raster-scan systems such as TVs, and it tends to be more expensive due to its advanced structure and better output.

Key Differences

The main difference is that the beam penetration method uses a single electron gun and two phosphor layers (red and green) to produce a limited number of colors by varying the electron beam’s speed, while the shadow mask method uses three electron guns (for red, green, and blue) and a shadow mask to precisely align beams with corresponding phosphor dots, creating millions of colors and higher image quality. The shadow mask method is more commonly used in modern displays like raster-scan systems (TVs), while the beam penetration method was used in simpler, random-scan monitors.