What are WDDM and WGF? Windows Vista in Gamer’s Eyes

An important and very interesting innovation in Windows Vista is the new graphics driver model which is called Windows Display Driver Model (WDDM, it had earlier been known as Longhorn Display Driver Model or LDDM). WDDM became necessary as it is closely related to the new Desktop Window Manager within whose framework the Aero interface works. From a technical point of view, DWM resembles the Quartz Compositor from Mac OS X. In DWM, application windows are drawn not directly on the screen as in previous versions of Windows, but are rendered into off-screen buffers which are then combined by the manager and output to the display device, hence the term desktop composition. Each window is an independent area for rendering. This allows using various graphics effects of the Aero interface, but requires support on the graphics driver side.

Although Windows Vista supports both driver models, the new user interface can only be used with a WDDM driver that supports the new WGF 1.0 API (and, later on, WGF 2.0). The abbreviation WGF spells out as Windows Graphics Foundation. This is the name of new APIs that replace DirectX. WGF 1.0 is based on a modified and enhanced Direct3D 9 API, so it is sometimes referred to as Direct3D 9.L. Working together with DWM, which architecturally sits on top of WGF 1.0 and WDDM-compatible drivers, this API runs the Windows Vista interface on an integrated or discrete DirectX 9-compatible graphics processor installed in your system. WGF 1.0 doesn’t bring about some great improvements, although simplifies the work of application developers to some extent. As for OpenGL support, there is no news. This API is supported either by using a driver provided by the GPU developer or by translating OpenGL calls into WGF 1.0 calls.

The WGF 2.0 API is sometimes referred to as DirectX 10 and is the next step in the evolution of the Windows Graphics Foundation. For this API to function properly, it must be supported by a WDDM driver as well as by graphics hardware with necessary functionality, namely graphics cards with a unified graphics pipeline, e.g. cards based on the ATI R600 GPU. Such graphics cards will be able to execute version 4.0 shaders and work with a new type of shaders, geometry shaders. These and some other features will make them compatible with WGF 2.0. In other words, it is WGF 2.0 that can be regarded as a truly new API, providing new capabilities, improving graphics quality in games and achieving a new level of realism. It is from WGF 2.0 that gamers will profit the most.

Another important feature of WGF 2.0 or Direct3D 10 is the lack of the so-called “capability bits”. Instead, the new standard defines a uniform list of minimum functionality for the graphics card to support. A graphics subsystem must support all the features from that list to be touted as Direct3D 10-compatible. Otherwise, there is no compatibility. This guarantees unification among next-generation GPUs and allows game developers to use WGF 2.0 without taking into consideration the architectural peculiarities of a particular GPU irrespective of its manufacturer, AMD/ATI Technologies, Intel or Nvidia Corporation.

We will see if games that make use of the new API will be able to run under WGF 1.0 only after such games, drivers, etc. are released. This is unlikely, however, considering the hardware requirements of WGF 2.0, but still it’s yet too early to discuss the advantages of the new API in earnest. Let’s get back to WDDM for now.

The WDDM architecture, which is the foundation for the Desktop Window Manager and, accordingly, for the Aero interface, is based on three key features:

• WDDM scheduling
• Graphics memory virtualization
• Shared use of DirectX surfaces