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NVIDIA TXAA Brings Movie CGI Rendering To PC Games
Date: Aug 15, 2012
Author: Joel Hruska
The Secret World & TXAA
Nvidia's Kepler packs a number of efficiency and performance improvements, but one of the GPU's major features has been locked away until now. When the company launched their new GPU core earlier this year, Nvidia debuted a new type of anti-aliasing it dubbed TXAA. Unlike other types of AA, which can often be forced on in the driver and applied to any game, TXAA required specific application support. Funcom's new MMO, The Secret World, was patched last week to add TXAA and we took the game out for a spin to see what it looks like.

What is TXAA?

Click to embiggen

Nvidia is still keeping most of the details of TXAA under wraps, but GeForce.com offers this explanation:
TXAA is a combination of hardware anti-aliasing, a custom anti-aliasing resolve, and a temporal filter. To filter pixels, TXAA uses a contribution of samples both inside and outside of the pixel in conjunction with samples from prior frames, to offer the highest quality filtering possible. In motion, TXAA approaches the quality of other high end, professional anti-aliasing algorithms, though the higher quality filtering used by TXAA does result in a softer image compared to the lower quality filtering of traditional MSAA.
Despite their similar names, TXAA is not a form of FXAA (AMD calls its version MLAA). Fast approXimate Anti-Aliasing creates an pseudo-anti-aliasing effect by applying a post-processing edge-detection filter that blurs line edges. Intel invented and developed early iterations of what it referred to as morphological anti-aliasing, and Nvidia has improved on these early efforts, but FXAA isn't a true form of anti-aliasing.

The "temporal filter Nvidia refers to is a different way of achieving the goals of standard spatial anti-aliasing.TXAA starts with an MSAA implementation, but then averages edge values across multiple frames rather than blowing them up to ultra-high resolutions. That's why Nvidia states that the technology looks better in motion -- static screenshots can't accurately capture the blending that occurs when a character is on the move.

The only anti-aliasing modes available in The Secret World are FXAA (in regular and high quality flavors) and 2x/4x TXAA. It's possible to force certain anti-aliasing methods through the use of Nvidia Inspector, but we stuck to the basic options available in-game. Because the performance hit from enabling FXAA is virtually nil, we skipped testing the standard mode and went straight to HQ. Screenshots below illustrate the three settings.

No AA on the far left, FXAA HQ in the middle, and 4x TXAA on the right. Click to enlarge.

Key jagged spots and areas of interest in this first set of images include:
  • The bike
  • The poster hanging above it
  • The red car in the middle-right of the image
FXAA improves some of these areas more than others. The red car is noticeably smoother, but the bicycle doesn't improve much. Bike spokes are worst-case scenario; thin diagonal lines are difficult to anti-alias effectively. The one major problem that FXAA doesn't address -- we'll show you this momentarily -- is texture flickering when the character is moving.

4x TXAA significantly improves the scene with only a modest frame rate hit. Here's the bicycle in all three modes, at 300% zoom.

No AA on the far left, FXAA HQ in the middle, and 4x TXAA on the right. Click to enlarge

TXAA does introduce a bit of blur, but it's less than what you'd see from a 4x supersampling implementation. Nvidia has stated that one of the benefits of TXAA is that it duplicates how Hollywood films use AA in rendered scenes. The Secret World doesn't exactly jump out and scream Inception, but the new anti-aliasing method is designed to produce more realistic images rather than overly sharp digital scenery.

Nvidia recommends evaluating the technology in motion, so hit next to see a video comparison.
TXAA in Motion
The following two videos give you an apples-to-apples comparison of how each anti-aliasing method looks in-game. We recommend viewing in full-screen and HD mode, as the smaller video player view can obscure some of the differences.

Texture shimmering in both of these cases is pretty bad. It's enough to be distracting during normal game play, particularly on the light poles and the streetcar rails. Thin, sharp lines flicker, and in a cityscape, those sorts of lines are everywhere.

So here's how TXAA resolves it.

If you're comparing to the first videos, check the streetcar rails, the light poles, and the red car.  As we move into the alley, check the bike stands on the right-hand side. 2x TXAA really looks good here; it eliminates most of the shimmering we noticed previously without significantly blurring the scene. The GeForce GTX 670 we tested on had no problem keeping the frame rate around 50fps at 1920X1080 resolution.

The blur that we've mentioned isn't just an byproduct of the AA method -- it's designed to mimic how Hollywood incorporates CGI with real-world footage. Timothy Lottes, one of the TXAA designers, has an extensive blog post on the topic, along with side-by-side shots of how the eye interprets subtle blurring as presenting a more realistic image.

The Not-So-Final Verdict:

With just one game to examine and no way to force TXAA in games that don't explicitly support it, it's hard to call whether the technology will catch or not. Historically, specialized graphics functionality that's only supported by Nvidia or AMD, but not both, hasn't done particularly well.

We suspect TXAA's long-term popularity will hinge on how easy it is to support and whether or not the upcoming crop of game engines are capable of using standard MSAA. Game developers are used to supporting Nvidia-specific AA modes like CSAA. If TXAA requires a similar level of optimization, it may catch on quickly.

We're optimistic about TXAA's long-term potential. Image output is superior to 4xMSAA and the temporal component works well. Nvidia has chosen to keep TXAA as a Kepler-only feature for now, which may slow its market growth, but The Secret World demonstrates that there's a place for this technology. Hopefully next time it appears, we'll be able to compare it directly against MSAA.

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