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Sponsored Feature: Dynamic Resolution Rendering


October 21, 2011 Article Start Previous Page 3 of 3
 

The Effect of Motion Blur

Motion blur smears pixels and reduces observed aliasing effectively, hence a lower resolution can be used when the camera is moving. However, the sample does not exploit this in its resolution control algorithm. The following screenshots show how reducing the resolution to 0.71x the back buffer results in higher performance, but roughly the same image. Combined with varying motion blur sample rates, this could be a way to reduce artifacts from undersampling with large camera motions whilst maintaining a consistent performance.


Figure 9:
Motion blur with dynamic resolution off


Figure 10:
Motion blur with dynamic resolution on at 0.71x resolution. Note the decreased frame time yet similar quality end result

Supersampling

Supersampling is a simple technique where the render target used to render the scene is larger than the back buffer. This technique is largely ignored by the current real-time rendering community-multi sampled anti-aliasing and other anti-aliasing techniques have replaced its usage due to their better memory consumption and performance.

Using dynamic resolution significantly reduces the performance impact of adding supersampling, as the actual resolution used can be dynamically adjusted. There is a small performance impact to enabling supersampling, mainly due to the extra cost of clearing the larger buffers. The sample code implements a 2x resolution render target when supersampling is enabled, but good quality results are observed for relatively small increases in resolution over the back buffer resolution, so a smaller render target could be used if memory were at a premium. Memory is less of an issue on processor graphics platforms, as the GPU has access to a relatively large proportion of the system memory, all of which is accessible at full performance.

Once dynamic resolution rendering methods are integrated, using supersampling is trivial. We encourage developers to consider this, since it can be beneficial for smaller screen sizes and future hardware which could have sufficient performance to run the game at more than its maximum quality.

Render Target Clearing

Since dynamic resolution rendering does not always use the entire render targets surface, it can be beneficial to clear only the required portion. The sample implements a pixel shader clear, and on the Intel® HD Graphics 3000-based system tested, the performance of a pixel shader clear was greater than that of a standard clear when the dynamic ratio was less than 0.71x for a 1280x720 back buffer. In many cases, it may not be necessary to clear the render targets, as these get overwritten fully every frame.

Depth buffers should still be cleared completely with the standard clear methods, since these may implement hierarchical depth. Some multi-sampled render targets may also use compression, so should be cleared normally.

Performance Scaling

The sample code scales well with resolution, despite the heavy vertex processing load due to the large highly detailed scene with no level of detail and only very simple culling performed. This gives the chosen control method significant leverage to maintain frame rate at the desired level.

Most games use level-of-detail mechanisms to control the vertex load. If these are linked to the approximate size of the object in pixels, the resulting performance scaling will be greater.


Figure 11:
Dynamic Resolution Performance at 1280x720

Resolution Control

The sample implements a resolution control method in addition to allowing manual control. The code is in the file DynamicResolutionRendering.cpp, in the function ControlResolution. The desired performance can be selected between the refresh rate (usually 60Hz or 60FPS) and half the refresh rate (usually 30FPS).

The control scheme is basic: a resolution scale delta is calculated proportionally to the dimensionless difference in the desired frame time and the current frame time.

Where S' is the new resolution scale ratio, S is the current resolution scale ratio, is the scale delta, k a rate of change constant, T the desired frame time, and t the current frame time.

The current frame time uses an average of the GPU inner frame time excluding the present calculated using Microsoft DirectX* queries, and the frame time calculated from the interval between frames in the normal way. The GPU inner frame time is required when vertical sync is enabled, as in this situation the frame time is capped to the sync rate, yet we need to know if the actual rendering time is shorter than that. Averaging with the actual frame rate helps to take into account the present along with some CPU frame workloads. If the actual frame time is significantly larger than the GPU inner frame time, this is ignored, as these are usually due to CPU side spikes such as going from windowed to fullscreen.

Potential Improvements

The following list is by no means complete, but merely some of the features which the author believes would naturally extend the current work:

  • Combine the dynamic resolution scene rendering with a similar method for shadow maps.
  • Use this technique with a separate control mechanism for particle systems, allowing enhanced quality when only a few small particles are being rendered and improved performance when the fill rate increases.
  • The technique is compatible with other anti-aliasing techniques that can also be applied along with temporal anti-aliasing.
  • Temporal anti-aliasing can use an improved weighted sum dependent on the distance to the pixel center of the current and previous frames, rather than just a summed blend. A velocity-dependent offset read, such as that used in the CryENGINE* 3 [Crytek 2010], could also be used.
  • Some games may benefit from running higher quality anti-aliasing techniques over a smaller area of the image, such as for the main character or on RTS units highlighted by the mouse.

Conclusion

Dynamic resolution rendering gives developers the tools needed to improve overall quality with minimal user intervention, especially when combined with temporal anti-aliasing. Given the large range of performance in the PC GPU market, we encourage developers to use this technique as one of their methods of achieving the desired frame rate for their game.

References

[Sigg 2005] Christian Sigg, Martin Hadwiger, "Fast Third Order Filtering", GPU Gems 2. Addison-Wesley, 2005.

[Crytek 2010] HPG 2010 "Future graphics in games", CevatYerli& Anton Kaplanyan. http://www.crytek.com/cryengine/presentations

[GDC Vault 2011] http://www.gdcvault.com/play/1014646/-SPONSORED-Dynamic-Resolution-Rendering

[Intel GDC 2011] http://software.intel.com/en-us/articles/intelgdc2011/

[Andreev 2011] http://www.gdcvault.com/play/1014550/Anti-aliasing-from-a-Different [PPT 4.6MB]


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