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The OpenGL Pipeline Newsletter - Volume 003

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Windows Vista and OpenGL-the Facts

April 20, 2007

The Khronos OpenGL ARB Working Group has received requests for clarification concerning the performance of OpenGL on Windows Vista. These questions are understandable as Microsoft has dramatically changed the user experience with the Windows Aero compositing desktop and introduced a completely new display driver architecture. This article will discuss how these changes affect OpenGL.

The industry now has a growing body of real-world experience in shipping OpenGL on Windows Vista and the OpenGL ARB wishes to reinforce the positive synergy between OpenGL graphics and Windows Vista in three key areas

  1. Windows Vista fully supports hardware accelerated OpenGL;
  2. OpenGL applications can benefit from Window Vista•s improved graphics resource management;
  3. OpenGL performance on Windows Vista is extremely competitive with the performance on Windows XP.

Windows Vista Fully Supports OpenGL

Hardware-accelerated OpenGL is fully supported on Windows Vista with the Windows Aero compositing desktop user experience - just as with Direct3D.

OpenGL hardware acceleration is handled in exactly the same way in Windows XP and Windows Vista - through an Installable Client Driver (ICD) provided by graphics card manufacturers. Without an OpenGL ICD installed, Windows XP and Windows Vista both revert to rendering OpenGL in software on the CPU rather than using GPU acceleration.

Figure 1 shows the path through which OpenGL and Direct3D applications render their output under Windows Vista. Firstly, the OpenGL or Direct3D user-mode graphics drivers take the data from the application and use the graphics hardware to render one frame of content. That frame is then presented to the Desktop Window Manager (DWM) which composites the frame into the desktop using GPU acceleration. The DWM provides the desktop compositing functionality used by the Windows Aero and Windows Standard user experiences in Windows Vista.

The DWM is a key element of the Windows Vista user experience and one of its main functions is to manage the presentation of the Windows desktop by compositing the outputs of multiple applications to the screen. As a complete 3D application in its own right, the DWM uses GPU memory and resources and places an additional load on the GPU, which impacts application graphics performance by approximately 10% for typical applications. End users should weigh the benefits of the DWM and the Windows Aero user interface against this performance cost. An easy way to experiment with a particular application is to disable the DWM, by selecting the Windows Classic or Windows Basic user interface style.

During full-screen applications, such as games, the DWM is not performing window compositing and therefore performance will not appreciably decrease.

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Figure 1 - OpenGL, Direct3D and the Desktop Window Manager (DWM)

OpenGL Benefits from Improved Resource Management

The new Windows Display Driver Model (WDDM) in Windows Vista provides a firewall and the thunk layer (that handles communication between the graphics hardware/kernel mode graphics driver and the user-mode graphics driver) to increase overall system stability. This architecture differs from Windows XP, where the OpenGL ICD talked directly to the GPU hardware and handled many low-level functions, including memory management.

WDDM affords some implicit benefits for OpenGL applications. For example, graphics resources previously managed by the OpenGL ICD, such as video memory, are now virtualized by the operating system. When multiple applications are running, the OpenGL ICD exposes maximum texture storage capacity to each application and Windows Vista takes care of resource allocation and scheduling, resulting in more efficient use of resources across applications.

OpenGL Performance is Competitive on Windows Vista

Some have suggested that OpenGL performance on Windows Vista is poor compared to Windows XP. This is not the case.

A comparison between the performance on Windows XP and Windows Vista of the SPECviewperf professional OpenGL benchmark, two OpenGL game benchmarks, and two Direct3D game benchmarks are presented below.

Professional OpenGL Application Performance

The SPECviewperf figure below shows that the graphics performance of professional OpenGL applications on Windows Vista is very close to that on Windows XP. SPECviewperf runs in a window, and on Windows Vista the DWM was turned off, by selecting the Windows Classic color scheme, to achieve maximum graphics performance. These numbers are extremely impressive, given that the WDDM is a radical departure from the Display Driver model on Windows XP and the new Windows Vista drivers have not yet benefited from the years of tuning and optimization that the Windows XP OpenGL drivers have undergone. Software developers and graphics card manufacturers have had access to production-ready Windows Vista since November 2006, and graphics performance on Windows Vista will continue to improve over time as the drivers mature. As with any version of Microsoft Windows however, it is important to download the latest drivers from the graphics card vendor - particularly in the early lifetime of a new operating system.

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If DWM were enabled through enabling Windows Aero - performance would drop by about 10%. For many users this tradeoff is worthwhile in order to use the more advanced user interface.

OpenGL Game Performance

The Doom3 (demo1) and Prey (move.demo) figures below show that Windows Vista performance for full-screen OpenGL games is comparable to the performance delivered on Windows XP. As these applications are full screen, DWM is not active and there is no performance drop in Windows Aero.

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Direct3D Game Performance

The diagram below shows how the Direct3D games Half Life 2 Episode One (demo1) and Oblivion (Outdoor, HDR off) perform on Windows XP and Windows Vista. These numbers were obtained on the same system and same driver versions as the OpenGL benchmarks. In all cases the benchmarks were run in full-screen mode.

These figures show that Windows Vista performance for full-screen Direct3D games is comparable to the performance delivered on Windows XP • just as for OpenGL. As these applications are full screen, DWM is not active and there is no performance drop in Windows Aero.

These results confirm that both Direct3D and OpenGL are excellent 3D APIs for Windows Vista.

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Conclusion

OpenGL is fully integrated into the Windows Vista display architecture just like Direct3D -both interfaces are first-class graphics API citizens. OpenGL hardware acceleration is available on Windows Vista through highly optimized drivers provided by the graphics hardware vendor just as on Windows XP. OpenGL applications can benefit from the improved resource management provided by Windows Vista. OpenGL performance on Windows Vista is extremely competitive compared to Windows XP and will rapidly improve as support for this new operating system matures. The enhanced Windows Aero user interface can decrease graphics intensive application performance by about 10% but this performance drop can be eliminated by selecting Windows Classic or Windows Basic user interface style or using full-screen applications. To ensure maximum performance and stability on Windows Vista, users should regularly check for driver upgrades from their graphics hardware supplier.

The third edition of the OpenGL Pipeline Newsletter, an OpenGL ARB publication, has included an article written by NVIDIA for software developers with tips to get the maximum out of their OpenGL application on Windows Vista. This article can be found here: http://www.opengl.org/pipeline/article/vol003_7/

For questions about this article, or about OpenGL in general, please contact Barthold Lichtenbelt at blichtenbelt@nvidia.com, Khronos OpenGL ARB Working Group chair.

References

General overview “What is Aero”

Overview of how DX9, DX10, GDI, OpenGL and the DWM are architected under the WDDM

OpenGL ARB article with tips for ISVs developing OpenGL applications on Windows Vista

Game performance results were obtained on an AMD Athlon 64 FX-62 2.8 GHz system with a NVIDIA GeForce 7900GTX graphics card, with driver version 97.73 for Windows XP, and 101.20 for Windows Vista, running in full-screen mode. The SPECviewperf results were obtained on a Dual Xeon 3.0 GHz system with a NVIDIA Quadro FX 5500 graphics card, with driver version 91.36 for Windows XP, and 100.72 for Windows Vista.

OpenGL is a registered trademark of Silicon Graphics Inc. SPECviewperf® is a registered trademark of the Standard Performance Evaluation Corporation, www.spec.org. All other product names, trademarks, and/or company names are used solely for identification and belong to their respective owners.

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