Strata Source

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Strata Source (more informally known as Strata, and formerly known as the Chaos Engine) is a third-party Source engine branch using licenced code. It is a modified version of the CS:GO engine branch that combines the features of both CS:GO and the Portal 2 engine branch, along with many improvements that bring Source close to modern standards. This engine branch was first released publicly with the launch of Portal: Revolution, and is still in active development.

Features

Example from Portal: Revolution showing the intricate environments capable with Strata Source

Feature list from the Strata Source Wiki
New since the CS:GO engine branch:

Native DirectX 11 renderer: Support for DX11 by default
DXVK support on Linux for DirectX 11
64-bit support: Supports 64-bit on all platforms; 32-bit and previous OS support ^[Clarify] has been dropped
PBR shaders

Introduces a new PBR shader that attempts to create realistic reactions to light

Ported Portal 2 engine branch features: Including but not limited to World Portals and Portal gun
Backwards compatibility with BSP v19-21, as well as BSP v25 support
Backwards compatibility with Source 2013 (and older) branch models: The model format found in games such as is now supported.
WEBM video support, replacing older Bink videos: Increased framerate, lower file sizes, and easier to author
New v7.6 VTF version: Adds support for the higher-quality ATI1N, ATI2N, BC6H, and BC7 texture compression formats, as well as lossless deflate compression on top of existing image format, for reduced size. Better support for 4K textures.
Many improvements to Linux support
Fully cross-platform engine tools and utils
Steam Audio
Many improvements to the Hammer level editor, named Strata Hammer
Many code quality improvements (Refactors, cleanup, and C++20)
Sentry crash report integration