Previous article in the series: A Quick Overview of MSAA
Despite having the flexibility to implement a custom resolve for MSAA, the “standard” box filter resolve is still commonly used in games. While the box filter works well enough, it has some characteristics that can be considered undesirable for a reconstruction filter. In particular, the box function has a discontinuity at its edge that results in an infinite frequency response (the frequency domain equivalent of a box function is the sinc function). This causes it to introduce postaliasing when used as a reconstruction filter, since the filter is unable to isolate the original copy of a signal’s spectrum. The primary advantage offered by such a resolve is that it’s cheap from a performance point of view, since only subsamples within a single pixel need to be considered when computing a resolved pixel value.
The question we now want to answer is “can we do better?” Offline renderers such as Pixar’s PRMan support a variety of filter types for antialiasing, so it stands to reason that we should at least explore the possibilities for real-time graphics. If we decide to forego the “standard” resolve offered by ResolveSubresource and instead perform our own resolve using a pixel or compute shader that that accesses the raw multisampled texture data, we are pretty much free to implement whatever reconstruction filter we’d like. So there is certainly no concern over lack of flexibility. Performance, however, is still an issue. Every GPU that I’ve run my code on will perform worse with a custom resolve, even when using a simple box filter with results that exactly match a standard resolve. Currently the performance delta seems to be worse on AMD hardware as opposed to Nvidia hardware. On top of that, there’s additional costs for the increased texture samples required for wider filter kernels. Separable filtering can be used to reduce the number of samples required for wide filters, however you must have special considerations with the rotated grid sample patterns used by MSAA. Unfortunately I haven’t solved these problems yet, so for this sample I’m just going to focus on quality without too much regard for performance. Hopefully in a future article I can revisit this, and address the performance issues.
At this point I feel that I should bring up TXAA. If you’re not familiar, TXAA is a library-supported antialiasing technique introduced for recent Nvidia Kepler-based GPU’s. There’s no public documentation as to exactly how it works, but Timothy Lottes has mentioned a few details here and there on his blog. From the info he’s given, it seems safe to assume that the MSAA resolve used by TXAA is something other than a box filter, and is using a filter width wider than a pixel. Based on these assumptions, you should be able to produce similar results with the framework that I’ve set up.
The sample application renders my trusty tank scene in HDR to an fp16 render target with either 1x, 2x, 4x, or 8x MSAA enabled. Once the scene is rendered, a pixel shader is used to resolve the MSAA render target to a non-MSAA render target with one of ten available reconstruction filters. I implemented the following filters:
|Smoothstep (Hermite spline)|
I started out by implementing some of the filters supported by PRMan, and using similar parameters for controlling the filtering. However I ended up deviating from the PRMan setup in order to make things more intuitive (in my opinion, at least). All filters except for the sinc filter were implemented such that their “natural range” of non-zero values were in the [-0.5, 0.5] range. This deviates from the canonical filter widths for several of these filters, notably the cubic filters (which are normally defined for the [-2, 2] range). I then used a “filter width” parameter to inversely scale the inputs to the filtering functions. So for a filter width of 1.0, the filters all have a width equal to the size of a single resolved pixel. The one exception is the sinc filter, where I used the filter width to window the function rather than scaling the input value. I should also note that I implemented all of the filters as radial filters where the input is the screen-space distance from the output pixel center to the sample position. Typically filters for image scaling are used in separable passes where 1D filters are passed the X or Y sample distance. Because of this my “Box” filter is actually disc-shaped, but it produces very similar results. In fact for a filter width of 1.0 the results are identical to a “standard” box filter resolve. The “Triangle” filter uses a standard triangle function,which can be considered a “cone” function when used as a radial filter. “Gaussian” uses a standard Gaussian function with a configurable sigma parameter, with the result windowed to [-0.5, 0.5]. The “Smoothstep” filter simply uses the smoothstep intrinsic available in HLSL, which implements a cubic hermite spline. The “Generalized Cubic” filter is an implementation of the cubic function suggested by Mitchell and Netravali in their paper, with the B and C parameters being tweakable by the user. The “B-spline”, “Mitchell” and “Catmull-Rom” filters use this same function except with fixed values for B and C. “Sinc” is the standard sinc function, windowed to [-FilterWidth, FilterWidth] as mentioned previously.
To visualize the filtering function, I added a real-time 1D plot of the currently-selected filter function using the current filter width. I also added a plot of the 1D fourier transform of the filter function (calculated with the help if the awesomely easy-to-integrate Kiss FFT library), so that you can also visualize the frequency response of the selected filter type. This can be useful for estimating the amount of postaliasing produced by a filter, as well as the attenuation of frequencies below the Nyquist rate (which results in blurring).
After the resolve is performed, the result is fed into a standard post-processing chain. This phase includes average luminance calculation for auto-exposure, bloom, and HDR tone mapping. I added an option to tone map subsamples in a manner similar to Humus’s sample, so that the results can be compared to resolve prior to tone mapping. When this option is activated, the bloom and auto-exposure passes work with non-resolved MSAA textures since the output of the resolve no longer contains linear HDR values. Note that the resolve is still performed prior to post-processing, since I wanted to keep the resolve separate from the post-processing phase so that it was more visible. In production it would most likely be done after all post-processing, however you would still need the same considerations regarding working with non-resolved MSAA data.
Here’s a full list of all options that I implemented:
MSAA Mode – the number of MSAA samples to use for the primary render target (1x, 2x, 4x, or 8x)
Filter Type – the filtering function to use in the resolve step (supports all of the filters listed above)
Use Standard Resolve – when enabled, a “standard” box filter resolve is performed using ResolveSubresource
Tone Map Subsamples – when enabled, tone mapping is applied before the subsamples are resolved
Enable FXAA – enables or disables FXAA with high-quality PC settings
Render Triangle – renders a plain red triangle in the center of the screen
Bloom Exposure – an exposure (in log2 space) applied to HDR values in order to create the bloom source values
Bloom Magnitude – a multiplier for the bloom value that’s combined with the tone mapped result
Auto-Exposure Key Value – key value for controlling auto-exposure
Adaptation Rate – rate at which exposure is adapted over time
Roughness – roughness used for material specular calculations
Filter Size – the radius of the filter kernel (in pixels) used during the resolve step
Gaussian Sigma – the sigma parameter for the Gaussian function, used by the Gaussian filter mode
Cubic B – the “B” parameter to Mitchell’s generalized cubic function, used by the Generalized Cubic filter mode
Cubic C – the “C” parameter to Mitchell’s generalized cubic function, used by the Generalized Cubic filter mode
Magnification – magnification level for the final output (magnification is performed with point filtering)
Triangle Rotation Speed – the speed at which the red triangle (enabled by Render Triangle) is rotated
The following table contains links to 1280×720 screenshots from my sample application using various filter types and filter widths. All screenshots have use 4xMSAA, and perform the resolve in linear HDR space (bloom and tone mapping are performed after):
This table contains similar screenshots, except that the tone mapping is performed prior to the resolve:
As we take a close look at the images, the results shouldn’t be too surprising. For the most part, wider filter kernels tend to reduce aliasing while smaller filters preserve more high-frequency detail. Personally I find that the cubic spline filters with no negative lobes (smoothstep and B-Spline) will produce the best results, with the best balance between aliasing and blurring occurring around the 2.0-3.0 range. Here is a magnified image showing the results of 4xMSAA with a standard 1-pixel-wide box filter, followed by the same image with a 3-pixel-wide B-spline filter:
4xMSAA with a “standard” 1-pixel-wide box filter, followed by 4xMSAA with a 3-pixel-wide B-spline filter
The aliasing is pretty significantly reduced on geometry edges with the B-spline filter, particularly edges with higher contrast. Here’s another pair of images that are magnified even further, so that you can see the edge quality:
Highly magnified images showing 4xMSAA with a 1-pixel-wide box filter, followed by a 3-pixel-wide B-spline filter
Here’s another set of images showing the results of a wider filter kernel on high-frequency details from normal maps and specular lighting:
4xMSAA with a 1-pixel-wide box filter, with a 2-pixel-wide B-spline filter, and with a 3-pixel-wide B-spline filter
As you can see in the images, a 2-pixel-wide B-spline filter is actually pretty good in terms of not attenuating details that are close to a pixel in size. A wider filter reduces aliasing even further, but I feel that filter width of 2.0 is still an improvement over the quality offered by a “standard” resolve. So it’s probably a pretty good place to start if you want better quality, but you prefer a sharper output image. The other cubic filters with negative lobes (such as Catmull-Rom and Mitchell) will also produce a sharper result, however the negative lobe can produce undesirable artifacts if they’re too strong. This is especially true when filtering HDR values with high intensity, since they can have a strong effect on neighboring pixels. For this reason I think that Mitchell is a better option over Catmull-Rom, since Mitchell’s negative lobes are bit less pronounced. The sinc filter is almost totally unappealing for an MSAA resolve, since the ringing artifacts that it produces are very prominent. Here are three images comparing a 4-pixel-wide Catmull-Rom filter, a 4-pixel-wide Mitchell filter, and a 6-pixel-wide sinc filter:
All of the above images used 4xMSAA, but a wider filter kernel can also work well for 2xMSAA. Here’s some close-ups showing “normal” 2xMSAA vs. 2xMSAA with a B-spline filter vs. 4xMSAA with a B-spline filter:
2xMSAA with standard resolve, 2xMSAA with a 3-pixel-wide B-spline filter, and 4xMSAA with a 3-pixel-wide B-spline filter
To wrap things up, here a two close-ups showing the results of a wide B-spine filter applied before tone mapping, and the same image with tone mapping applied prior to filtering:
4xMSAA with a 3-pixel wide B-spline filter applied before tone mapping, and after tone mapping
These results are little interesting since they illustrate the differences in the two different approaches. In the first image the filtering is performed with HDR values, so you get similar effects to applying DOF or motion blur in HDR where bright values can dominate their local neighborhood. The second image shows quite a different result, where the darker geometry actually ends up looking “thicker” against the bright blue sky. In general I don’t find that it produces a substantial improvement when you’re already using a wider filter kernel, or at least not enough to justify the extra effort and performance required to make it work with an HDR post-processing pipeline. However it does tend to play nicer with cubic filters that have negative lobes, since you’re not filtering HDR values with arbitrary intensity.
There are clearly a lot of options available to you if you choose to implement a custom MSAA resolve. I think there’s some good opportunities here to do an even better job at reducing aliasing in games, and personally I’m of the opinion that it’s worth reducing the appearance of tiny pixel-wide details if it results in an overall cleaner image. Either way I don’t think that a box filter is the best choice no matter what your tastes are.
If you want to download the sample app with source code, it is available on my codeplex page. Feel free to download it and perform your own experiments, although I’d appreciate if you’d share your results and opinions!
Some quick notes about the sample code: I decided to use the newer DirectX headers and libraries from the Windows 8 SDK, so you’ll need to have it installed if you want to compile the project. I haven’t fully migrated to VS 2012 yet, so I’ve left the project in VS 2010 format so that it can be opened by either. I also overhauled a lot of my sample framework code, which includes a shader caching system that uses the totally-awesome MurmurHash to detect when a shader needs to be re-compiled. Using the new SDK also entailed ditching D3DX, so I’ve replaced the texture and mesh loading functionality that I was using with some of my own code combined with some code lifted from DirectXTK. One major downside I should mention is that I’m only supporting x64 for now, due to some annoyances with the SDK and redistributing D3DCompiler DLL’s. If anybody is still stuck on 32-bit Windows and really wants to run the sample, let me know and I’ll try to find some time to get it working.