Global Illumination Models (Ray Tracing, Radiosity)

Computer graphics has transformed the way we visualize and simulate virtual worlds. One of the fundamental aspects of creating realistic and believable images is the rendering of global illumination. Global illumination models, such as ray tracing and radiosity, play a crucial role in simulating the complex behavior of light in computer-generated scenes. In this article, we will explore the key concepts and techniques behind these two powerful global illumination models.

Ray Tracing

Ray tracing is a widely used technique in computer graphics that simulates the path of light as it interacts with objects in a scene. It aims to produce highly realistic images by considering how light rays bounce, reflect, and refract off surfaces before reaching the camera.

The basic principle behind ray tracing involves casting rays from the viewer's eye through each pixel of the image and tracing their path through the scene. These rays can be categorized into three types: primary rays, secondary rays, and shadow rays.

  • Primary Rays: These rays originate from the viewer's eye and pass through each pixel of the image plane. They help determine the visibility of objects in the scene and identify the closest point of intersection.

  • Secondary Rays: Once a primary ray intersects with an object, secondary rays are generated to simulate the reflection and refraction of light. These rays follow the laws of reflection and refraction and bounce off surfaces accordingly.

  • Shadow Rays: Shadow rays are cast from points on surfaces towards light sources to determine whether they are obstructed by other objects. By assessing the visibility of each light source, shadow rays contribute to the computation of accurate shadows in the final image.

Ray tracing can accurately capture complex lighting effects, such as reflections, refractions, and shadows, leading to visually appealing and realistic images. However, it can be a computationally expensive technique, as it often requires tracing millions of rays to create a single frame.


Radiosity is another powerful global illumination model that focuses on the computation of indirect light within a scene. Unlike ray tracing, which calculates the path of individual rays, radiosity aims to simulate the full interaction of light between surfaces by solving a set of equations.

The concept behind radiosity is based on the principle of energy transfer. It considers the exchange of light energy between surfaces, taking into account factors such as surface reflectivity, surface area, and light source power. By dividing the scene into small patches, radiosity calculates the amount of light energy emitted by each patch and distributes it among adjacent patches using a system of linear equations.

Radiosity can provide highly realistic renderings, especially for scenes with diffuse and non-glossy surfaces. It effectively captures the softness of indirect lighting and can produce smooth color bleeding effects. However, radiosity may struggle with scenes that involve highly reflective or refractive materials, as it primarily focuses on diffuse interactions.

Combining Ray Tracing and Radiosity

While ray tracing and radiosity have their strengths and weaknesses, they are often combined to create more accurate and visually impressive renderings. This hybrid approach, known as ray-tracing radiosity, takes advantage of the benefits provided by both techniques.

By combining ray tracing and radiosity, artists and designers can achieve accurate global illumination effects while maintaining realistic reflections and refractions. Ray tracing helps handle complex lighting phenomena, such as glossy reflections, while radiosity contributes to the overall illumination and light bouncing in the scene.

Modern rendering engines often employ this hybrid approach, allowing for efficient and dynamic simulations of global illumination in real-time applications like video games and virtual reality experiences.

In conclusion, global illumination models such as ray tracing and radiosity play vital roles in computer graphics by simulating the behavior of light in virtual scenes. While ray tracing focuses on tracing the path of individual rays to capture complex lighting effects, radiosity computes the energy transfer between surfaces to simulate indirect lighting accurately. By combining these two techniques, artists and designers can create visually stunning and realistic renderings.

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