DX11 Perspective Matrix Jittering Temporal AA
Temporal anti-aliasing algorithms are algorithms where you use samples from previously rendered frames in order to perform a temporal form of supersampling. There are different ways to implement temporal AA, and over the weekend, I wrote an implementation of the simple perspective matrix jittering temporal AA. This type of temporal AA is mentioned in the Beyond3D forums here (http://forum.beyond3d.com/showthread.php?t=46241).
CodePlex link for my program’s source code and binary are provided at the end of the article.
Halo: Reach Temporal AA
http://www.eurogamer.net/articles/digitalfoundry-halo-reach-tech-analysis-article (AA is covered on Page 3.)
Digital Foundry recently published a tech analysis article on Halo: Reach. Based on frame capture analysis and the behavior of the anti-aliasing, they speculated that the anti-aliasing used in Halo: Reach is some kind of temporal anti-aliasing.
CryEngine3 Temporal AA
At Siggraph 2010′s “Advances in Real-Time Rendering in 3D Graphics and Games” course, Crytek presented a talk “CryENGINE 3: Reaching the Speed of Light” in which they gave some details about their anti-aliasing implementation. Their anti-aliasing is a hybrid solution with 2 different AA algorithms applied to near and distant objects. For near objects, an edge-based post process AA is applied. For distant objects, temporal AA (the temporal reprojection with cache miss approach) is applied.
The below papers were given as references for temporal reprojection in Crytek’s talk.
Accelerating Real-Time Shading with Reverse Reprojection Caching (ACM SIGGRAPH Symposium on Graphics Hardware 2007)
Spatio-Temporal Upsampling on the GPU (I3D 2010)
Comments On the Screen Captures
(2010/10/02 added comment)
Texture filtering was not properly set up when I took the above pics. If texture filtering is properly enabled, the aliasing inside the textures becomes gone. With mipmapping-enabled, the effects of temporal AA on texture aliasing becomes minimal.
My implementation of temporal AA blends together 2 samples and should be roughly equivalent to a 2 sample SSAA in terms of quality. For comparison, I also implemented hardware MSAA 2x and hardware MSAA 4x. In the above screenshots, you can see that for long edges, MSAA 2x and temporal AA 2x are around the same quality. MSAA 4x results in much cleaner long edges than both temporal AA and MSAA 2x.
If you look at the inside of the textures, you will notice that temporal AA 2x results in much cleaner straight lines and less overall aliasing compared to both MSAA 2x and MSAA 4x. Temporal AA is a form of supersampling and is able to reduce pixel shader aliasing on surfaces and not just jaggies on edges. However, for the above picture, if texture mipmapping had been properly enabled, the hardware texture filter would have taken care of the aliasing.
In my Japanese blog, I posted screenshots comparing the different AA algorithm’s results on a grid-patterned texture. Temporal AA works really well for this type of high frequency texture. While neither MSAA 2x nor MSAA 4x are able to reduce the aliasing much, temporal AA successfully reduces the moire pattern and the aliasing.
MSAA uses multiple depth samples, but only a single pixel shader sample, while temporal AA, like SSAA, uses multiple pixel shader samples. This means that temporal AA effectively operates at a high resolution compared to screen size and allows the algorithm to reduce aliasing on detailed polygon surfaces requiring more info than a pixel can hold.
The perspective matrix jittering temporal AA algorithm is cheap and works great on still pictures. However, it has problems once the camera starts moving (especially in sideways motions) because the difference between the previous frame’s pixels and the current frame’s pixels becomes large and as a result when the 2 frames are blended, the resulting screen is blurry with slight halo traces of the last frame.
Chapter 10 of the OpenGL Redbook explains the OpenGL accumulation buffer AA algorithm.
Found out that the Redbook explains the OpenGL accumulation buffer AA algorithm from this Michigan State University’s CSE 872 Advanced Computer Graphics class’s tutorial.
Chapter 10 of the OpenGL Redbook has an explanation on using perspective matrix jittering in order to perform anti-aliasing through the accumulation buffer. In the Redbook example, they re-render the scene n-sample times, each time with the perspective matrix jittered by a small sub-pixel amount, and blend the result into the OpenGL accumulation buffer. This subpixel n-sample blending is equivalent to SSAA and gets you an anti-aliased image. The technique requires re-drawing the scene n-sample times within a single frame and is prohibitively costly for most real-time applications.
My temporal AA implementation is a 2-sample perspective matrix jittered AA with one sample taken from the previously rendered frame and one sample from the current frame. The implementation requires an extra rendertarget the same size as the framebuffer in order to store the previous frame’s color buffer (assuming the current frame’s rendertarget is already necessary and so free). Even frames and odd frames are renderered with a different subpixel jitter value and are blended together in a fullscreen post process pass. Same as the OpenGL accumulation buffer AA, the temporal blending of 2 subpixel jittered color buffers should give similiar results to SSAA 2x. The only tricky part about the implementation for me was figuring out how to create a perspective projection matrix that jitters the rendered image by a screenspace subpixel amount. The function that performs this calculation is shown below.
Based on the OpenGL Red Book Chapter 10.
D3DXMATRIX TemporalAA::JitteredFrustum(float left, float right, float bottom,
float top, float fNear, float fFar, float pixdx,
float pixdy, float eyedx, float eyedy, float focus, const CameraInfo& cameraInfo) const
float xwsize, ywsize;
float dx, dy;
xwsize = right - left;
ywsize = top - bottom;
// translate the screen space jitter distances into near clipping plane distances
dx = -(pixdx*xwsize/cameraInfo.fBackBufferW +
dy = -(pixdy*ywsize/cameraInfo.fBackBufferH +
D3DXMatrixPerspectiveOffCenterLH( &mPerspective, left + dx, right + dx, bottom + dy, top + dy, fNear, fFar );
Perspective Matrix Jittering Temporal AA Demo
The background model used in the demo is the “powerplant” sdkmesh model included with the DirectX June 2010 SDK. It’s used in the DX11 Cascaded Shadowmaps demo.
In my demo, the frame sometimes jumps when you are moving the camera. I use the DXUT CFirstPersonCamera and same thing occurs for the Cascaded Shadowmaps demo and is not an artifact of temporal AA.
Source Code & Binary
The perspective matrix jittering temporal AA is easy to implement, cheap and provides anti-aliasing qualities similiar to SSAA 2x. However, the current implementation suffers from distracting screen blurring once the camera starts moving. I haven’t really read up on temporal reprojection yet, but it might be interesting to implement AA using it and see how the results compare to this implementation.