Metro Exodus DLC takes its ray tracing to the next level

October 13, 2019
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Metro Exodus: The Two Colonels is the first piece of DLC released by 4A Games and for PC users at least, an already state-of-the-art, forward-looking rendering implementation is pushed on to the next level – one of the best ray tracing implementations seen so far has a remarkable new feature. 4A’s recently released expansion trades the wide sandbox levels of the original campaign for a more traditional Metro adventure, full of cramped corridors, tense scripted sequences and that bittersweet tone the game is known for. This return to a more familiar Metro experience sets the scene for 4A’s latest transformative graphics update – the addition of emissive lighting ray-traced lighting.

It’s exciting stuff, and the comparison gallery below demonstrates just how much more effective the new lighting is – but before we delve into specifics, let’s briefly recap how RT works in the base game. Metro Exodus implements ray tracing through global illumination, where light emanates from the sun and the sky, bouncing once to light the game world more naturally than traditional rasterised techniques. This makes a huge difference to outdoor areas, as simulating rather than emulating creates new shadows, lights and colours that don’t appear when ray tracing is turned off. However, indoor scenes – where the global illumination of the sun and sky aren’t simulated – only see the benefit of screen-space ambient occlusion (SSAO) replaced by ray traced ambient occlusion. This is a nice change to make, but it’s a more of a subtle tweak to the already extremely dark tunnels of the metro compared to the radical transformation RT brings elsewhere.

In The Two Colonels, indoor scenes finally get their killer RT feature with ray-traced emissive lighting. Put simply, emissives are simply textures that are tagged as being of one or more colours which maintain that colour regardless of lighting conditions. For example, you might see these in sci-fi games for objects that glow. These textures are normally paired with point or spot lights placed inside or nearby, so that the emissive texture looks like it is lighting nearby objects. However, these two systems are disconnected in the game world, so the illusion can be broken by the emissive texture changing colour, growing in size or disappearing altogether when that change is not reflected in the paired point light. Metro Exodus solves this by using ray tracing for its emissive surfaces, so that light emanating from such a surface always matches the surface’s shape and colour – no longer an illusion, this is now a physically-grounded simulation.

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