Tech Focus: Low cost stereo 3D

Despite take-up of 3DTVs being rather low, the video games industry's commitment to the new format has been surprisingly robust. While we can assume that there has been extensive evangelism for the new display tech from Sony that will have filtered down to publishers and developers, it's interesting to see that a great many cross-format games have supported stereoscopy on both PlayStation 3 and Xbox 360 - Call of Duty: Black Ops being the most high-profile example.

It's all the more surprising bearing in mind the development challenges that supporting 3D represents. Fully implemented stereoscopic 3D can run smoothly on PC with very few issues even on a mid-range graphics card: there have been several generational leaps in the power of AMD and NVIDIA's GPUs in the years since the Xbox 360 and PlayStation 3 launched and in a market dominated by console conversions, it's an innovative use for the extra processing power beyond higher frame-rates and resolution.

However, neither Microsoft nor Sony's hardware was ever designed to handle 3D, and while the quality of the stereoscopic 3D games seen thus far has often been very high, there have been compromises in the quality of the visuals, and often, by extension, the gameplay too. Black Ops looks cool in 3D, but the precise controls offered by 60Hz feedback just isn't there any more.

There are two key challenges in working with 3D. First of all, true HDMI 1.4 stereoscopic 3D sees pixel throughput double - a clear challenge for PS3's RSX chip. Secondly, geometry needs to be processed twice to produce discrete views for each eye.

Guerrilla Games' Killzone 3 is a good example of how cutting edge software is adapted to support stereo 3D: resolution per eye is cut in half, effectively solving the fill-rate issue, while more aggressive LODs are employed to minimise the geometry issues. However, even with these compromises, frame-rate can still drop significantly against the traditional 2D game in certain areas.

Some developers are experimenting with new techniques based on the process of reprojection. Here, a traditional 2D image is created and information from the depth buffer is used to extrapolate out the second view. It's a technique that Sony demonstrated at GDC last year, and according to developer Zipper Interactive, an iteration of this technique is being used in the upcoming SOCOM 4.

But it's possible to get a good look at reprojection techniques in play on a fully fledged cross-platform AAA title in the here and now. Ninja Theory's Enslaved: Odyssey to the West added TriOviz 3D to the game in its recent Pigsy's Perfect Ten DLC update. While Darkworks' 3D-enabling middleware has already been deployed on Rocksteady's tremendous Batman: Arkham Asylum, this is the first time we've seen the tech operating on HDMI 1.4 3DTVs. Not only that, but TriOviz has also been integrated into Unreal Engine 3, making it an inexpensive upgrade for UE3 titles currently in development.

"We had seen the TriOviz tech long before it appeared in UE3 and decided that we had to have it," says Mike Ball, co-founder and chief technical director of Enslaved developer Ninja Theory.

"As soon as we saw it we were blown away that here was a practical way to introduce 3D into our game. We knew that the beautiful environments that we had built in Enslaved would shine even more when viewed with 3D depth."

TriOviz is noteworthy because of its claims about performance, citing extremely low processing times. This means that - in theory - developers can add 3D without having to reduce resolution or pare back geometry.

As Darkworks' product manager Alexis Arragon told Digital Foundry when Arkham Asylum shipped:

"The native integration of TriOviz for Games into a game eats up approximately 2ms each frame. That's below a 10 per cent hit in a 30FPS game. For more technologically-demanding games, there are memory and CPU optimisations to implement to go below the 2ms barrier. One of our prospects successfully integrated our solution in a 60FPS game. Because our SDK works like a shader, there are multiple ways to optimise performances when using TriOviz for Games."

"We had considered a full stereoscopic 3D solution but we had long dismissed this as being impractical for the renderer setup that we have," says Ninja Theory's Mike Ball.

"One of the benefits we found with TriOviz is that it has a fixed performance cost that we were able to easily accept by removing the motion blur post processing whilst 3D is active."

Darkworks reckon that an existing game can support TriOviz with around two to three weeks' development time, and cite a memory overhead of between 2MB and 10MB to include the tech.

"To generate the appropriate back buffer containing the combined left and right eye view there are some intermediary buffers that are required," confirms Ball.

"These can change size depending on platform and output format."

Getting the best possible PlayStation 3 solution also added to the perennial RAM headache. Put simply, more pixels being rendered requires more memory to store them.

"We had to spend some time scrambling to find the additional memory needed to support HDMI 1.4 version on PS3 which is always a little difficult due to the split memory architecture. It wasn't easy but it was certainly worth it to support the full resolution HDMI 1.4 3D," says Ball, who also corroborates Darkworks' claims about how quickly TriOviz can be integrated.

"The initial integration was very short, in the 2-3 week period but we also had to make a number of modifications to support the new addition of the HDMI 1.4 modes so this caused additional work for us. Now that the technology has settled down and an Unreal integration is available, the addition of TriOviz technology should be pretty simple."

While the notion of full resolution 3D with a small performance overhead might sound too good to be true (and PS3 performance analysis suggests the overhead of HDMI 1.4 is significant), there are disadvantages to the stereo reprojection technique.

First of all, as the source of the image is 2D in nature, camera separation potential is quite limited compared to a full stereoscopic 3D solution - and that's due to the lack of available visual information. Imagine walking up to the edge of a wall that obscures your left eye, while your right eye can see beyond it. With TriOviz there would be no discrete view for the right eye - you'd see nothing instead.