FastDoom achieves its speed primarily through optimizing data access patterns. The original Doom wastes cycles retrieving small pieces of data scattered throughout memory. FastDoom restructures data, grouping related elements together (like vertices for a single wall) for contiguous access. This significantly reduces cache misses, allowing the CPU to fetch the necessary information much faster. Further optimizations include precalculating commonly used values, eliminating redundant calculations, and streamlining inner loops, ultimately leading to a dramatic performance boost even on modern hardware.
Fabien Sanglard's blog post, "Why FastDoom Is Fast," delves into the technical intricacies that enable the classic first-person shooter, Doom, to achieve its remarkable speed on older hardware, specifically focusing on the shareware version 1.1. Sanglard's analysis meticulously dissects the game's performance optimization strategies, highlighting the ingenious methods employed by id Software's programmers to maximize the limited resources available at the time.
The core of Doom's speed, as Sanglard explains, lies in its non-reliance on the central processing unit (CPU) for rendering the game world. Instead, Doom leverages the capabilities of the video card, specifically targeting the VGA card's feature set. This delegation of graphical processing allows the CPU to dedicate its cycles to other crucial tasks like game logic, artificial intelligence, and player input processing.
Sanglard elaborates on the ingenious use of binary space partitioning (BSP) trees for level geometry representation and collision detection. This hierarchical structure permits efficient culling of off-screen or occluded areas, dramatically reducing the computational overhead associated with rendering unseen portions of the game world. He meticulously explains how the BSP traversal algorithm efficiently determines visibility, significantly optimizing the rendering pipeline.
Further enhancing performance is Doom's innovative approach to wall texture mapping. Rather than performing complex perspective calculations for each pixel, the game employs an affine texture mapping technique. This simplified method, though resulting in some visual distortions, provides a substantial performance boost compared to perspective-correct texture mapping.
Sanglard also dissects Doom's non-floating-point arithmetic approach. By utilizing fixed-point arithmetic and integer operations, the game avoids the performance penalties associated with floating-point calculations on the hardware of that era. This choice contributes significantly to Doom's speed, especially on systems without dedicated floating-point units.
The blog post meticulously details the game's utilization of lookup tables for various trigonometric and arithmetic functions. Pre-calculating and storing these values allows the game to quickly retrieve results, avoiding real-time computations and further enhancing performance.
Finally, Sanglard's analysis emphasizes the significance of Doom's vertical refresh rate synchronization. By synchronizing the game's rendering with the monitor's refresh rate, the game avoids screen tearing and maintains smooth visual presentation without requiring complex double-buffering techniques. This synchronization, combined with the other optimizations, contributes to Doom's fluid and responsive gameplay experience. In conclusion, Sanglard presents a thorough and insightful explanation of the numerous technical innovations that make Doom a paragon of performance optimization, showcasing the ingenious programming prowess of id Software.
Summary of Comments ( 43 )
https://news.ycombinator.com/item?id=43258709
The Hacker News comments discuss various technical aspects contributing to FastDoom's speed. Several users point to the simplicity of the original Doom rendering engine and its reliance on fixed-point arithmetic as key factors. Some highlight the minimal processing demands placed on the original hardware, comparing it favorably to the more complex graphics pipelines of modern games. Others delve into specific optimizations like precalculated lookup tables for trigonometry and the use of binary space partitioning (BSP) for efficient rendering. The small size of the game's assets and levels are also noted as contributing to its quick loading times and performance. One commenter mentions that Carmack's careful attention to performance, combined with his deep understanding of the hardware, resulted in a game that pushed the limits of what was possible at the time. Another user expresses appreciation for the clean and understandable nature of the original source code, making it a great learning resource for aspiring game developers.
The Hacker News post "Why FastDoom Is Fast" (https://news.ycombinator.com/item?id=43258709) has several comments discussing various aspects of the original article about optimizing Doom's performance.
Many commenters express appreciation for the deep dive into Doom's optimization techniques. They highlight the ingenuity of the original developers in pushing the limits of the hardware at the time. Some commenters share their own experiences working with older hardware and the challenges and satisfactions of squeezing performance out of limited resources.
A recurring theme is the contrast between modern game development and the approaches used in older titles like Doom. Commenters point out how modern game engines often prioritize features and ease of development over performance, sometimes leading to bloat and inefficiency. Doom's lean, hand-optimized code is seen as a refreshing counterpoint to this trend.
Several comments delve into specific optimization techniques mentioned in the article. These include discussions of fixed-point arithmetic, lookup tables for trigonometric functions, and clever use of the CPU's instruction set. Commenters explain the benefits of these techniques in the context of the limited processing power and memory available at the time.
Some comments focus on the broader implications of the article's findings. They discuss how understanding these older techniques can be valuable for modern developers, even though the hardware landscape has changed drastically. Learning from the past can inspire creative solutions to performance challenges in current projects.
A few commenters share anecdotes about playing Doom in its early days and the impact it had on the gaming industry. These comments add a historical context to the technical discussion, reminding readers of the game's legacy and influence.
There's also discussion about the interplay between performance and gameplay. Commenters note how Doom's fast pace and responsive controls were a direct result of its optimized code. This reinforces the idea that technical excellence can directly enhance the player experience.
Finally, some comments provide links to related resources, such as other articles about game optimization and historical accounts of Doom's development. This adds further depth to the conversation and allows readers to explore the topic further. Overall, the comment section offers a rich discussion of Doom's optimization, its historical context, and its relevance to modern game development.