What are the reasons for not using path tracing instead of rasterization? Is the main factor the higher computational cost or are there other factors involved?

 Path tracing and rasterization are two different techniques used in computer graphics to render images, each with its own set of advantages and disadvantages. While path tracing provides highly realistic images by accurately simulating the behavior of light, it is not always the preferred choice over rasterization due to several factors beyond just the higher computational cost. Here are the primary reasons:

1. Higher Computational Cost

- Performance: Path tracing is computationally intensive because it involves simulating numerous light rays and their interactions with surfaces. Each pixel may require thousands of rays to achieve noise-free images, leading to significant processing time and power consumption.

- Real-time Rendering: Achieving real-time performance with path tracing is challenging, making it unsuitable for applications like video games and interactive simulations where frame rates of 60 FPS or higher are essential.

2. Hardware Requirements

- GPU Power: Path tracing requires powerful GPUs with substantial parallel processing capabilities, such as those found in high-end gaming or professional graphics cards.

- Cost: The need for expensive hardware can be prohibitive for many users and applications, especially those that aim to run on a wide range of devices, including older or less powerful systems.

3. Development Complexity

- Algorithm Complexity: Implementing an efficient path tracer is more complex compared to rasterization. It requires a deep understanding of advanced graphics concepts and optimizations like importance sampling, denoising techniques, and acceleration structures (e.g., BVH, KD-trees).

- Resource Management: Path tracing involves managing large amounts of data related to light paths, which increases the complexity of memory management and optimization efforts.

4. Noise and Convergence

- Noise: Path tracing produces noisy images at lower sample counts, requiring additional techniques like denoising filters to achieve acceptable image quality.

- Convergence Time: High-quality path traced images need a large number of samples per pixel to converge to a noise-free state, leading to long rendering times.

5. Suitability for Different Applications

- Real-Time Applications: Rasterization is well-suited for applications requiring real-time interaction, such as video games and virtual reality, where speed is crucial.

- Simple Scenes: For scenes that do not require complex lighting and shading effects, rasterization can produce acceptable results much more efficiently.

6. Existing Ecosystem and Tooling

- Established Pipelines: The graphics industry has a well-established pipeline for rasterization with mature tools, libraries, and frameworks (like DirectX and OpenGL).

- Compatibility: Many existing applications and engines are built around rasterization, making it easier to integrate and maintain within current workflows.

Summary

While path tracing delivers superior image quality through physically accurate lighting simulations, its higher computational cost, complex implementation, and unsuitability for real-time applications make rasterization the preferred choice for many scenarios. Rasterization strikes a balance between performance and visual quality, making it more practical for a broader range of applications, especially those where real-time performance and lower hardware requirements are critical.