Killing Floor 2: Graphics Technology & First GameWorks Features Unveiled
Killing Floor 2 is the long-awaited sequel to Tripwire Interactive’s multi-million selling Killing Floor, a co-op first person shooter that redefined ‘horde mode’ with unique gameplay, refreshing humor, and dozens of excellent tweaks and changes that have enabled Killing Floor to outplay and outlast all other contenders to their ‘best in genre’ crown. With tens of thousands of players every day, five and a half years after release, it’s no wonder anticipation for Killing Floor 2 is reaching fever pitch as we fast approach the sequel’s 2015 Early Access release.
In August we announced a technical partnership with Tripwire Interactive, promising the future inclusion of NVIDIA GameWorks technologies that will improve graphics, increase immersion, and make full use of your GeForce GTX GPUs in Killing Floor 2. Today, we can reveal the first fruits of our partnership, in addition to several other advanced technologies that Tripwire Interactive have recently implemented.
NVIDIA HBAO+ Ambient Occlusion
Ambient Occlusion (AO) adds contact shadows where two surfaces or objects meet, and where an object blocks light from reaching another nearby game element. The AO technique used and the quality of the implementation affects the shadowing’s accuracy, and whether new shadows are formed when the level of occlusion is low. Without Ambient Occlusion, scenes look flat and unrealistic, and objects appear as if they are floating.
For Killing Floor 2, Tripwire Interactive have integrated the platform agnostic NVIDIA HBAO+, widely accepted as the best AO technique available, delivering unbeatable results at a comparably low performance impact.
The image below links to an interactive comparison demonstrating HBAO+ On vs. HBAO+ Off. Note the lack of shadows on the theater stairs, and the general improvement in shadowing screen-wide, on and around objects.
The following image shows an AO-only view of a detailed scene, exemplifying the benefits of AO in general. With HBAO+, shadows are more accurate, of varying intensities, and visible even where the level of occlusion is minimal, such as the inside of the chandelier, where even minute pieces of detail of correctly shadowed.
Screen Space Reflections
Many man-made materials can reflect detail, but traditionally we see only water, mirrors, glass, and over overly-shiny surfaces reflect detail in games. Increasing the quality, coverage, and visibility of reflections on all suitable surfaces is therefore key to improving immersion, so for Killing Floor 2 Tripwire Interactive has implemented Screen Space Reflections (SSR), enabling all reflective materials and surfaces to accurately reflect any detail screen-wide.
In the past, achieving this level of reflective detail was difficult because each reflector required the scene to be re-rendered, inflating CPU draw calls and severely impacting performance. As a result, developers switched to less intensive, less realistic techniques with limited reflective properties. Tripwire Interactive’s solution, however, sees SSR run entirely on the GPU, reducing the performance impact greatly. And as the examples here demonstrate, the improvements to image quality are considerable.
Tripwire’s SSR reflections also interact with their innovative Persistent Blood System, which sees all player and enemy blood persist on-screen for the duration of the match. In addition to literally painting the levels red, Tripwire’s blood modifies the shininess of the underlying materials, intensifying reflections as demonstrated in the interactive comparison below.
To accurately render skin, additional techniques and technologies are required. In games and the real world, light bounces from most objects and surfaces, which we can accurately and easily render with any number of technologies. In comparison, light striking skin is absorbed and diffused, with some rays being emitted, though at a lower intensity. Without a suitable technology simulating this effect, character skin is uniformly lit, detracting from the quality of the picture.
In Killing Floor 2, players are surrounded by friends and enemies at all time, making accurate skin rendering a must. The solution for Tripwire Interactive: Jorge Jiminez’s Screen Space Sub-Surface Scattering (SSSSS), an improved version of Sub-Surface Scattering (SSS) seen in several recent games.
Compared to traditional SSS, SSSSS only shades skin visible from the player’s perspective, which is particularly important for Killing Floor 2, where hundreds of enemies can attack simultaneously. Without this SSS addition, the performance cost would be crippling. SSSSS also introduces level of detail features, ensuring Subsurface Scattering on a distant enemy is rendered at a lower quality with a correspondingly low performance impact. Again, key for a game with hundreds of characters visible across a level.
The interactive comparison below highlights the benefits of Subsurface Scattering on two of Killing Floor 2’s ‘zeds’ in a room with high-intensity lighting.
Bloom lighting has been a standard part of real-time post processing for many years, adding flares of light and added visual intensity around bright lights and brightly lit objects and surfaces. Typically, Bloom is implemented by filtering out dark pixels and blurring bright ones, but for Killing Floor 2 Tripwire Interactive has improved upon this technique by downsampling and blurring bright pixels six times, and adding a weighted average of the blurred images to the final color output. This result is a higher-quality and wider bloom effect for little additional cost, versus performing a single blur pass.
Artists can additional tune the bloom intensity and other values to achieve a variety of aesthetic looks, varying the appearance of locations and effects. This approach, similar to the technique used by Martin Mittring in Epic Games’ Elemental Demo, can be examined in the interactive comparison below.
Killing Floor 2 uses a variable-intensity, screen-space motion blur technique as part of its post processing system to simulate blur caused by fast-moving objects and camera motion. Adapted from the NVIDIA Motion Blur D3D Advanced Sample, freely available from the NVIDIA GameWorks website, Tripwire Interactive’s solution features per-pixel motion information, blur filters, and other technologies to improve the smoothness and quality of motion blur from frame to frame, giving the player a heightened sense of speed when fast-moving enemies attack.
Exaggerated examples below demonstrate the object-based blur cast on and around the enemies as they attack.
Depth of Field
Depth of Field simulates the blurring of out-of-focus regions of an image due to the aperture and focal length of a virtual camera. In Killing Floor 2, the effect is seen when the player aims a weapon in iron-sights. Objects in the near field are blurred based on the depth of the focal object, and different blur settings are used for the world and the player’s first-person weapon to avoid over-blurring the weapon.
Behind the scenes, a constant time box filter based on a Summed Area Table is used to implement depth of field blur, enabling Tripwire to vary the size of the blur pixel-to-pixel at a fixed and predictable performance cost. Furthermore, it enables Tripwire to realistically blur the Screen Space Reflections highlighted earlier, simulating the appearance of reflections in reality, and tackling aliasing that may be visible.
To demonstrate the above, here’s a screenshot of Killing Floor 2’s night vision effect, which also features color shift, contrast enhancement, film grain, and other techniques.
There’s plenty more to come from Killing Floor 2, including additional graphical enhancements (NVIDIA PhysX FleX, anyone?), yet to be announced gameplay features, and of course, the 2015 Early Access release on Steam. For further details stay tuned to GeForce.com and Tripwire Interactive’s Killing Floor 2 website.