With a traditional graphics API, multi-GPU arrays like AMD CrossFire™ are typically utilized with a rendering method called “alternate-frame rendering” (AFR). AFR renders odd frames on the first GPU, and even frames on the second GPU. Parallelizing a game’s workload across two GPUs working in tandem has obvious performance benefits.

As AFR requires frames to be rendered in advance, this approach can occasionally suffer from some issues:
Large queue depths can reduce the responsiveness of the user’s mouse input
The game’s design might not accommodate a queue sufficient for good mGPU scaling
Predicted frames in the queue may not be useful to the current state of the user’s movement or camera

Thankfully, AFR is not the only approach to multi-GPU. Mantle empowers game developers with full control of a multi-GPU array and the ability to create or implement unique mGPU solutions that fit the needs of the game engine.

In Civilization: Beyond Earth, Firaxis designed a “split-frame rendering” (SFR) subsystem. SFR divides each frame of a scene into proportional sections, and assigns a rendering slice to each GPU in AMD CrossFire™ configuration.2 The “master” GPU quickly receives the work of each GPU and composites the final scene for the user to see on his or her monitor.

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As you can probably surmise, SFR requires high parallelization, efficient inter-GPU communication, and reliable delivery of slices to the master GPU. AMD Radeon™ graphics cards running Mantle are uniquely equipped to meet those requirements.

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As Mantle rises to meet the parallelization requirements of SFR, Mantle also supercharges Beyond Earth’s ability to utilize a gamer’s multi-core CPU.

In computer graphics, a “command buffer” is a type of memory buffer containing instructions (or “commands”) that the GPU will execute to carry out required rendering workloads. Feeding the GPU with a continuous, uninterrupted flow of commands is essential to keeping the whole graphics card at high utilization. High utilization can yield higher framerates and/or higher image quality, depending on the focus of the game developer.

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Mantle is remarkable in its ability to spread a game engine’s command buffer submissions across multiple CPU cores, ultimately allowing for a wider stream of graphics work to be processed and queued to the GPU.

In the case of Sid Meier’s Civilization: Beyond Earth, you’ll see later in this blog that this wide communication lane to the AMD Radeon™ GPU is used to sustain higher overall framerates when empires get large and detailed in the late game.

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Aliasing, the nasty “jaggies” on the edges of 3D objects in a PC game, is the bane of gamers everywhere. Aliasing is produced when a sharp edge is rendered to a monitor, which doesn’t offer sufficiently high pixels per square inch to properly express a smooth line.

There are many types of anti-aliasing designed to combat this unwanted phenomenon, and the majority of them fall into a category known as “multisample anti-aliasing” or MSAA. As the name implies, MSAA relies on “samples,” which is a graphics card’s test for whether or not a pixel on your monitor is occupied by one or more objects from the game world. If a pixel is covered by more than one triangle then the final contents/color of that pixel will be a blend of the information covering that pixel to produce a smoother edge.

Games and GPUs can cooperate to increase the number of samples being taken with each pixel, and these samples may test for color or coverage. Higher coverage sampling improves the accuracy of detecting whether or not an object occupies the pixel; higher color sampling improves the blending between samples confirmed to be occupied. Gamers increase the sample rate by choosing 2x, 4x or 8x MSAA, causing every pixel to be tested for color and coverage in two, four or eight locations.

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Like MSAA, AMD’s Enhanced Quality Anti-Aliasing (EQAA) also comes in 2x, 4x and 8x sampling modes, but each EQAA mode takes twice as many coverage samples as MSAA. Increased coverage testing allows the GPU to more accurately detect objects within a pixel, potentially allowing EQAA to detect and smooth a hard edge that might have been missed with fewer samples. Coverage samples are computationally cheaper than color samples, so EQAA proves to be a good compromise between quality and performance.

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Civilization: Beyond Earth automatically enables EQAA in Mantle (and DirectX®!) on supporting AMD Radeon™ GPUs when the user chooses to enable the in-game anti-aliasing options.

Customers with older GPUs that lack hardware support for Mantle can still take advantage of EQAA through the AMD Catalyst™ graphics driver. Simply enable 2x, 4x or 8xMSAA in the options menu of your favorite game (if supported), and ensure you have “enhance application settings” selected in the 3D Application Settings tab of AMD Catalyst™ Control Center.

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SiDeards73 said:

Interesting read, i personally dont understand any of the AA stuff so it was good to read something that makes me half understand whats going on

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SiDeards73 said:

Interesting read, i personally dont understand any of the AA stuff so it was good to read something that makes me half understand whats going on

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AMDMatt said:

Glad you enjoyed it. If people aren't interested in the tech or discussing it then this is not the thread for them.

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AMDMatt said:

Glad you enjoyed it. If people aren't interested in the tech or discussing it then this is not the thread for them.

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tommybhoy said:

+1

Interesting informative reading.

I read the TR article last night on SFR, also really interested in what potentially could be a game changer with dual setups if it transpires to achieve it's aim.

Problem being(for me), Civ has zero interest to myself and hoping SEv3 has a tasty Mantle CrossFire technique like they hinted too.

Thanks Matt, enjoyed the article.

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AMDMatt said:

I'm looking forward to seeing what Rebellion do with multi gpu under Mantle in Sniper Elite V3. Will they go the AFR route or the SFR route? Perhaps even a third method. Mantle already provides frame variance consistency that DirectX can't compete with currently.

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David Bisset said:

I'm assuming, though I could be wildly wrong, that SFR made sense for this scenario because of the nature of the game and the large number of discrete graphical segments while most FPS games involve objects that cross large segments of the screen, lighting effects that again cross the screen etc. which I'd imagine would cause them to lose a lot of benefits for SFR.
However, I'm not a graphics dev so could be talking out of my rear end!

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andybird123 said:

Without side by side screenshots showing the "advantage", any discussion of AA techniques is just hyperbole

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mmj_uk said:

I was thinking more that this is a convenient way to distract from the fact that Mantle offers nothing to Civilization, even with AMD's "superior" AA technique disabled Mantle is still performing disappointingly.

But yes, some comparison screenshots which highlight the actual difference in game would be nice, the way that it is being marketed I expected to be blown away.

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SiDeards73 said:

Shame they dont let you customise the benchmark options so everyone can select the exact same settings like many other benchmarks to get a true representation of whats what

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pgi947 said:

Some good info...what I'm not 'getting' if you will, is why EQAA is now suddenly a big deal? It's been around in cats since early 2011 and the nvidia varient, CSAA has been about since late 2006. Also it's been forcible via drivers since their respective release dates. So just about any game can use it yet some are getting excited because it's enabled in game, a civ game at that?

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