During the panel “Assassin’s Creed 4: Road to Next-gen Graphics” held at the Game Developers Conference, Ubisoft Montreal 3D programmer Bart Wronski gave some quite interesting information on what it took to launch Assassin’s Creed IV: Black Flag on next generation consoles.
One of the most interesting parts of the quite technical presentation was the detailing of the performance cost in milliseconds of advanced next-gen effects like Volumetric Fog and Screenspace Reflections on the Xbox One.
Volumetric fog is the simulation of various atmospheric phenomena including fog, mist, haze, “god rays,” light shafts, dusty/wet air and volumetric shadows. It basically includes all those effects created by the scattering of light.
Turns out that the cost for the effect is “surprisingly small” taking about 1.1 ms in total in Assassin’s Creed IV, with the following breakdown.
Interestingly enough calculating it at double the resolution didn’t multiply the cost by two, but raised it only to 1.6 ms.
While unfortunately a comparative table wasn’t provided for the PS4, according to Wronski, Sony’s new console can handle the calculations asynchronously, using the asynchronous compute feature of its GPU (the ability to perform complex calculations on the graphics chips instead of executing them on the CPU).
On the other hand Screenspace Reflections is a technique that uses screen-space only information and ray-marching/tracing through the depth buffer in order to calculate reflections. It’s been used in Killzone: Shadow Fall as well, just to bring an example.
On a fully reflective scene with glossy floor and objects the performance cost is about 2 ms (Wronski explicitly mentions this is for both new generation consoles, probably meaning that performance in this case is comparable), while on an average scene it drops to 1 ms, broken down in the table below:
Wronski also explained that both PS4 and Xbox One have advanced GPU architectures, providing several custom extensions and capabilities that aren’t available on PC, but they’re based on the AMD GCN architecture, which makes optimization more complex that in old generation consoles.