Zombie Army 4: the making of an 'impossible' Switch conversion

Switch’s ‘impossible ports’ are the new ‘arcade perfect’ – games devised for much more powerful hardware somehow, miraculously, receiving remarkably impressive conversions onto relatively meagre hardware. However, variable performance and low resolution are also hallmarks of these otherwise astonishing technical achievements and it’s here where we need to highlight the work of Rebellion North in delivering some exceptional conversions. Zombie Army 4 is its latest Switch conversion, running at the same 30fps as the PS4 game at a target 1080p resolution and retaining the title’s signature 80-100 strong horde of on-screen zombies. Below, you’ll see our analysis work on this exceptional port, but the bulk of this piece isn’t about the ‘what’, it’s about the ‘how’, with the developer itself sharing its methodology and insights into the conversion process.

At first glance, Zombie Army 4 on Switch is a ringer for the PS4 version and seemingly matching the resolution and frame-rate goes a long way – just as it did for the firm’s excellent Sniper Elite 4 conversion. As you’ll read in the interview, development targets native resolution and 30fps in Switch’s mobile mode, before scaling up to the docked experience. Dynamic resolution is used, with a 918p to 1080p range, dropping to 684p to 720p handheld. Beyond that, the nips and tucks are many and varied, but crucially, not especially noticeable. There are some exceptions though: cutbacks to dynamic shadows and a lack of screen-space ambient occlusion.

The changes from here onwards are more subtle. Geometry quality is dropped to the far distance – with a more aggressive LOD change-over that doesn’t notice in regular play. Effects such as particles and transparencies are also dialled back in quality. Likewise for the texture situation. With only 3.5GB of usable RAM to work with on Switch, memory was a huge constraint for Rebellion’s port of Zombie Army 4. Still, the team found smart solutions. Above all, they were determined to avoid a blanket dropping in res to every texture across the game. Reflections are also adjusted, yet screen-space reflections are maintained on water bodies, backed up by an enhancement to Rebellion North’s existing cube-map tech for Switch.

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