Fenris Creations Releases Open-Source CORTAO Rendering Technique

Fenris Creations on November 12, 2026 released CORTAO, an open-source screen-space ambient occlusion (SSAO) technique for real-time games. Created by Senior Rendering Engineer Daniel Isheden, it is available through the Carbon engine framework, with an implementation optimized and tested in the in-development space survival MMO EVE Frontier.

CORTAO stands for Circular Occluder Ray-Traced Ambient Occlusion. Ambient occlusion adds subtle darkening where nearby geometry blocks environmental light, helping ground objects and making the shape and detail of complex scenes easier to read. Fenris says the technique improves the accuracy and stability of that effect while keeping rendering costs low, reducing dark halos and shading shifts as the camera moves.

The technique needs no baked scene data, so it can respond to environments that players continually build and reshape. Its Carbon implementation was refined against EVE Frontier’s open space, online battles, megastructures and enclosed interiors. The project began as Isheden’s independent research before Fenris provided the production environment used to test and develop it at scale.

Isheden described the visual problem behind the research. “Small errors in shading can make an otherwise convincing scene feel wrong, especially when they become visible as the camera moves,” he said. His goal was to improve accuracy within the rendering budgets available to games.

How CORTAO Builds Ambient Occlusion

EVE Frontier’s technical breakdown explains the difference from horizon-based SSAO. CORTAO samples nearby depth information, discards samples outside the potential occlusion volume, and turns the remainder into circular proxy occluders that approximate the local 3D neighborhood around a pixel. A precomputed lookup table evaluates ray intersections with those proxies, and the percentage of rays left unoccluded determines the ambient-light level.

It supports as many as 32 ray directions and can produce useful results from four depth samples per pixel. Those counts govern separate jobs: depth samples determine how many occluders are constructed, while rays measure visibility against them. Developers scale performance primarily by changing the depth-sample count.

The GPU implementation has three compute shader passes. A prepass packs linear depth and normal data into a mipmapped buffer, the main pass traces against the sampled occluders, and an optional filter prepares the result for temporal anti-aliasing or AI upscalers. CORTAO can also output bent normals, which describe the average unoccluded direction around a pixel for global-illumination systems. It does not include a temporal filtering solution itself.

Fenris’s Benchmark Results

Fenris’s benchmarks compared CORTAO with GTAO, a horizon-based SSAO technique. In a recreated San Miguel cafe scene, the company reported that four-sample CORTAO achieved lower mean squared error against a ray-traced reference than every GTAO configuration it tested. Its simpler-scene test, which favors horizon-based rendering, produced 82% lower error at equivalent execution time. When the techniques were configured for equivalent error in that scene, CORTAO was 249% faster.

In Fenris’s tests, the largest gains came in scenes with high depth complexity, particularly with a larger search radius. CORTAO maintained a more consistent occlusion level across the tested sample counts and produced more conservative occlusion around depth discontinuities, with fewer heavy halos and view-dependent artifacts. The overall result held across the resolutions and GPU vendors the company tested.

Integration Costs and Open-Source Code

CORTAO is designed to replace an engine’s existing SSAO stage using the same class of screen-space inputs and minimal CPU involvement. The principal documented memory trade-off is the required 32-bit linear depth buffer, which leaves room to pack normal information beside depth. Fenris estimates that moving from a 16-bit setup adds approximately 5.3 MB of aliasable VRAM at 1080p, 9.3 MB at 1440p, and 21.1 MB at 2160p.

Two MIT-licensed implementations are available. The Carbon CORTAO repository supplies HLSL shader headers and lists DirectX 11 and DirectX 12 support, using a maximum screen-space radius to handle the EVE universe’s extreme depth ranges. Isheden’s general reference repository natively supports HLSL and GLSL and lists testing with DirectX 11, DirectX 12, OpenGL 4.3 or higher, and Vulkan 1.0 or higher.

The release builds on Fenris’s July 1, 2026 announcement that Carbon had become fully open source, spanning more than two dozen modules. That release included the Trinity graphics module and the Destiny physics-simulation and pathfinding technology, both part of the framework behind EVE Online and EVE Frontier.

The open repositories let outside graphics teams test whether Fenris’s results carry into their own scenes. The added VRAM and need for external temporal filtering are the clearest documented integration costs, making testing within complete rendering pipelines the next measure of CORTAO’s usefulness beyond EVE Frontier.