Direct3D is Microsoft's high-speed 3D software solution. It gives you the ability to create high-performance, real-time 3D applications for typical desktop computers. The system was designed for speed and requires little memory.
Direct3D is implemented in two distinctly different modes: Retained Mode, a high-level API in which the application retains the graphics data, and Immediate Mode, a low-level API in which the application explicitly streams the data out to an execute buffer.
Retained Mode
Direct3D Retained Mode API is designed for manipulating 3D objects and managing 3D scenes. Retained Mode makes it easy to add 3D capabilities to existing Windows-based applications or to create new 3D applications. Its built-in geometry engine supports advanced capabilities like key frame animation and frees you from creating object databases and managing the internal structures of objects. In other words, after using a single call to load a predefined 3D object, the application can use simple methods from the API to manipulate the object in the scene in real-time without having to work explicitly with any of the internal geometry.
Retained Mode is built on top of Immediate Mode and is tightly coupled with the DirectDraw™ application programming interface (API). Microsoft will incorporate Retained Mode into a future version of Windows. For more information, see DirectDraw and Introduction to Direct3D Retained-Mode Objects.
Immediate Mode
Direct3D Immediate Mode is Microsoft's low-level 3D API. It allows you to port games and other high-performance multimedia applications to the Windows operating system.
Immediate Mode is a thin layer above real-time 3D accelerator hardware that gives you access to the features of that hardware. It also offers optimal software rendering for some hardware features that are not present. Immediate Mode gives you the flexibility to exploit your own rendering and scene management technologies. It is a device-independent way for applications to communicate with accelerator hardware at a low level, enabling maximum performance.
Unlike Retained Mode, Immediate Mode does not provide a geometry engine; applications that use Immediate Mode must provide their own object and scene management. Therefore, you should be knowledgeable in 3D graphics programming to use Immediate Mode effectively.
Direct3D is based on the OLE Component Object Model (COM), and is tightly integrated with DirectDraw. Microsoft will incorporate Direct3D into a future version of Windows. For more information, see Introduction to Direct3D Immediate-Mode Objects.
Hardware Abstraction and Emulation
The Direct3D API (like the rest of the DirectX API) is built on top of a hardware-abstraction layer (HAL), which insulates you from device-specific dependencies present in the hardware. A companion piece to the Direct3D HAL is the hardware-emulation layer (HEL). The Direct3D HEL provides software-based emulation of features that are not present in hardware. The combination of these hardware abstraction and emulation layers ensures that the services of the API are always available to you.
The Direct3D HAL is tightly integrated with the DirectDraw HAL and the GDI display driver, giving hardware manufacturers a single, consistent interface to Microsoft graphics APIs, and a long-term, unified driver model for accelerated 3D. Hardware manufacturers need to write only a single driver to accelerate Direct3D, DirectDraw, GDI, and OpenGL. Hardware can accelerate all or part of the 3D graphics rendering pipeline, including geometry transformations, 3D clipping and lighting, and rasterization. The Direct3D HAL has been designed to accommodate future graphics accelerators in addition to those available today.