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Glossary/GPU/Global illumination

Global illumination

Global illumination is lighting that accounts for light bouncing between surfaces, so a red wall tints a nearby white floor rather than leaving it flat.

Also known as GI, indirect lighting

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Direct lighting only considers light arriving straight from a source. Global illumination adds indirect light: every bounce off every surface. It is what makes an interior look inhabited rather than like objects floating in a void, and it is where most of the perceived realism in a rendered scene comes from.

Computing it exactly requires solving an integral over all incoming directions at every point, which is what path tracing does by sampling. Real-time engines instead use approximations: baked lightmaps, irradiance probes, voxel cone tracing, screen-space GI, or a restricted ray-traced GI pass. Each trades accuracy for a frame budget.

GI is the reason path-traced workloads scale so brutally with scene complexity. Indirect bounces touch geometry the camera never sees, so acceleration-structure traversal dominates. SystemCheck's path-tracing stage exercises this indirect-light path, which is why its results correlate better with ray-tracing hardware capability than with raw raster performance.

In a game's settings menu, global illumination often appears as a quality level or as a separate ray-traced option. It can be one of the most demanding settings, and also one of the most visible, because it changes how every room and shadow looks. If frame rate matters more than lighting quality, lowering it is a common trade; if a game offers a screen-space or probe-based mode, it usually costs less than a full ray-traced one.

Related terms

Path tracingPath tracing is ray tracing that follows light along many random bounce paths per pixel, converging on a physically accurate image as samples accumulate.Read moreRay tracingRay tracing computes lighting by following rays from the camera into the scene to find what each pixel actually sees and how light reaches it.Read moreRasterizationRasterization is the rendering technique that projects triangles onto the screen grid and shades the covered pixels, and it drives nearly all real-time graphics.Read moreShader throughputShader throughput is how much arithmetic a GPU's programmable cores complete per second, and it limits performance in shading-heavy modern rendering.Read more
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