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NVIDIA DLSS version 2.5.1 debuted in the latest Portal RTX patch released last week. After the festivities, it was discovered that this latest version of the software no longer has a built-in sharpening filter. Experienced users have used alternative methods (such as hex editing or DLSS SDK .dll, which however added a watermark) to fix sharpness issues in some games like Red Dead Redemption 2 or God of War. The problems were most noticeable while driving.
The news was confirmed by NVIDIA RTX Unreal Engine Evangelist Richard Cowgill (fatheadlifter on Reddit) with the following posts. NVIDIA appears to be recommending that game developers use NVIDIA Image Scaling sharpening in the future.
Yes, DLSS sharpening is now deprecated in NVIDIA DLSS 2.5.1. Instead, we recommend that developers use NIS (Nvidia image scaler) sharpening. NIS has excellent sharpening technique and can also provide cross-platform hardware fallback without RTX for scaling. The latest version of DLSS simply no longer uses the old DLSS sharpening method. We recommend that developers use NIS sharpening when implementing DLSS. This should result in better image quality.
NVIDIA Image Scaling (NIS) was released in November 2021 as an update to the previous image scaling technology. The new algorithm uses a 6-tap filter, 4-way scaling, and adaptive sharpening filters. Scaling and sharpening are also performed in a single pass, improving performance.
NVIDIA Image Scaling works with GPUs from all manufacturers (including AMD and Intel) and is open source. Game developers interested in adding NIS to their games can download the latest SDK from GitHub.
If you want to check the scaling results available with NVIDIA DLSS 2.5.1, you will either have to download the latest .dll from TechPowerUp and manually embed the file into the game of your choice, or use the handy DLSS Swapper tool from Australian programmer Brad Moore.