The Graphics APIs are responsible for connecting high-level code to drivers (one kind of low-level software which controls the external hardware). The quality and efficiency of Graphics APIs are vital to building advanced graphic environments and applications. We could classify these APIs into two kinds: platform-specific APIs and cross-platform APIs.

Platform-specific APIs are usually the most efficient APIs on the platform. They conventionally are developed along with the system development process so that they can interact with the operating system seamlessly. In addition, hardware manufacturers are willing to support them with the highest priority. Nowadays, DirectX series are the first-class APIs on Windows, which power most existing games. Metal is the current official API on macOS; however, things become far more interesting on Linux.

Cross-platform APIs are those which are capable of running on multiple platforms. A fact is that the cross-platform APIs are almost all designed for Linux at first and they are in fact the first-class APIs on Linux. However, because of the "open source" root of Linux, the APIs are able to run on other platforms, requiring minimal effort from system developers. OpenGL and Vulkan are the prominent implementations built by the Khronos Group. OpenGL is older, less efficient on large applications while the abstraction is higher and easier to use. Vulkan is more advanced with modern graphic features while the abstraction is more sophisticated and takes a lot of work to integrate in high-level applications.

Insight: Currently, the axiom is that a Graphics API cannot be both the most efficient and cross-platform. We may expect Vulkan to be the game- changer.