Fenris Creations open-sources CORTAO, a screen-space ambient occlusion technique

Fenris Creations has introduced CORTAO (Circular Occluder Ray-Traced Ambient Occlusion) and made it open source through its Carbon engine framework. Original source code and documentation are available, along with a Carbon-optimized version.

Senior Rendering Engineer Daniel Isheden first developed CORTAO as an independent project. It later became a full production tool with the studio's support. The team has used it in EVE Frontier, an upcoming space survival MMO, across open space, large-scale battles, megastructures and enclosed interiors.

The technique takes a different approach to screen-space ambient occlusion. It uses what is currently visible on screen to represent nearby surfaces as circular occluders, then calculates how they block ambient light. It runs in real time and needs no baked scene data, which suits environments that players build and reshape. Testing reportedly shows better visual accuracy at low rendering cost, especially in visually complex scenes.

Isheden compares CORTAO with GTAO, which is the current state of the art. He says CORTAO reaches a level of physical accuracy rivaling brute-force hardware ray tracing while relying only on screen-space information. He also says it treats depth discontinuities more conservatively, avoiding the view-dependent occlusion of GTAO and producing less obvious artifacts when screen-space information is limited.

He adds that CORTAO converges much better, giving higher-quality results at lower sample counts even in scenes where GTAO works well. He describes this as especially important for low-end hardware. Visual comparisons and performance benchmarks are on a separate page linked from the original report. According to the report, no AI was involved in developing CORTAO.