SSGI
Screen-space global illumination fakes one bounce of indirect light by tracing rays through what's already drawn on screen.
Rather than computing how light bounces through the whole scene, SSGI marches rays across the depth and colour buffers and gathers light from visible surfaces. It's cheap because the data already sits in the framebuffer — but it shares screen space's perennial flaw: anything off the edge of the frame, or hidden behind something nearer, simply isn't there to bounce off.
It's used as a light-touch layer or as a fallback to fuller solutions like Lumen, the same way SSAO and SSR approximate ambient occlusion and reflections in screen space.