While commercial engines like Unity and Unreal dominate, a niche still exists for custom game engines. VGInsights data shows a decline in custom engine usage, yet games built with them accounted for 43% of units sold in 2024. A custom engine is a reusable software framework that handles interactions between game code and the operating system, managing the game loop, rendering, and assets.

The decision to build a custom engine, rather than using off-the-shelf solutions, involves defining what constitutes "custom." This can range from using libraries like SDL3 with Assimp to building a renderer from scratch. Historically, many successful indie games, such as Bastion, FTL: Faster Than Light, and Stardew Valley, utilized custom engines.

Reasons for building a custom engine include deep learning about game engine architecture, which can offer insights into proprietary engines used in AAA development. It allows for optimization for specific game needs, as exemplified by Noel Berry's C# engine for 2D games. While not a path to general-purpose engine competition, it fosters a unique understanding and control over the development process.