Portal Prelude Mod With NVIDIA RTX IO DirectStorage-Like Acceleration Debuts On Steam

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If you're a dedicated fan of the original Portal, then chances are high that you're familiar with Portal: Prelude. For everyone else, Prelude was a popular fan-made mod that serves as an unofficial prequel to the megahit game that introduced us to Aperture Science, the portal gun, and murderous AI GLaDOS.

Well, just as Portal got the RTX Remix treatment in the aptly-named Portal with RTX, so too has Portal: Prelude been given a fresh coat of RTX Remix paint. Portal: Prelude RTX is a re-release of the 2008 mod with updated assets and brand-new ray-traced lighting courtesy of NVIDIA's Omniverse-based RTX Remix toolkit. It was created by modder David "Kralich" Driver-Gomm in cooperation with Nicolas "NykO18" Grevet, the original author.

The lighting in Portal with RTX is even better now than it was when we took this last year.

If you missed our original coverage of Portal with RTX, just know that both that and this release are faithful re-creations of the original games with new assets to support a new renderer that uses "full ray-tracing," otherwise known as path-tracing. Rather than using raster graphics to "fake" the appearance of real lighting, a path-traced renderer physically simulates the behavior of light within the scene in real-time.

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Simply replacing the textures and then slapping the game into a path-traced renderer isn't enough to properly present a "with RTX" release, though. Creating a proper ray-tracing conversion for a game of this vintage requires touching every single asset as well as going over the level data with a fine-toothed comb to add real lights that will cast shadows and spread color around rooms through indirect (bounced) lighting.

The results are breathtaking, to be sure, but nothing we haven't seen before in Portal with RTX. The real interesting story of Portal: Prelude RTX is its first-ever support for RTX IO.

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In case you forgot what RTX IO is—and we wouldn't blame you, as NVIDIA's said nearly nothing about it since announcing its existence with the launch of the GeForce RTX 30 'Ampere' series of GPUs back in September of 2020. RTX IO, perhaps more-properly written RTX I/O, is essentially NVIDIA's own customized form of Microsoft's DirectStorage, much as "RTX" is partially a branded form of DirectX Raytracing (DXR).

NVIDIA published a video demonstrating the difference RTX IO can make

The biggest difference between RTX IO and DirectStorage is that RTX IO doesn't require DirectX to do its thing; it supports Vulkan interfaces, as well. Indeed, Portal: Prelude RTX and other RTX Remix mods use the Vulkan graphics API. The implementation of RTX IO in Portal: Prelude RTX includes not only the BypassIO feature used in Forspoken, but also another feature we haven't actually seen used in DirectStorage yet: GPU decompression.

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Image: Microsoft

We've written about GPU decompression before, but as a quick summary, games normally move data from disk to system memory, decompress it on the CPU, and then send the full-size assets to the GPU to be used. With DirectStorage 1.1 and RTX IO, what happens instead is that the compressed assets are sent directly to the GPU, then decompressed there for usage. This saves on both system memory usage as well as PCIe bandwidth.

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Even better is that, thanks to the GPU-friendly GDeflate algorithm devised by NVIDIA (and contributed freely to the open market for use in DirectStorage), GPUs can also perform the decompression much faster than CPUs. All of this contributes to drastically improved load time—NVIDIA claims "5X Faster Texture Load Time" compared to loading uncompressed textures. Similarly, the company says that a game using RTX IO saves 44% disk space compared to a game without, although we don't know of many games that ship with uncompressed textures.

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If you'd like to play Portal: Prelude RTX when it launches at 3pm ET today, all you need is a GPU that supports Vulkan ray-tracing extensions and a license for the original Portal on Steam. Currently, supported GPUs include all GeForce RTX cards as well as AMD Radeon RX 6000 and RX 7000 GPUs, although NVIDIA says that there is currently a bug on AMD GPUs where volumetrics cause graphical errors; you can work around this by hitting Alt+X and disabling Volumetric Lighting. Just be prepared for shockingly bad performance if you try to play at your monitor's native resolution—better crank up the DLSS if you want playable framerates.