# Screen-space global illumination

coloured light bleeding between surfaces.

```{raw} html
<a class="example-run-button" href="../demos/features_3d_ssgi.html">▶ Run in browser</a>

```

**Tags:** `3d` `ssgi` `global-illumination` `colour-bleed` `pbr`

Screen-space GI (design D8) casts a half-resolution hemisphere of rays from every
lit surface, marches them through the depth buffer + thin G-buffer, and gathers
the on-screen scene colour they hit: one bounce of indirect diffuse. The result
feeds the pluggable indirect-diffuse ambient hook (design D8), so a brightly lit
red wall throws a red tint across the white floor beside it and the green wall
tints the other side, all on top of the flat ambient (a Cornell-box classic).
The gather is Hi-Z traced (sharing the SSR trace utilities) and temporally
accumulated to denoise, so the bounce converges over a few frames; fast camera
motion trails a little (an accepted tradeoff). A direction that leaves the screen
or finds no near surface gathers nothing, so open floor keeps the flat ambient
and nothing is crushed.


Usage:
    uv run python examples/features/3d/ssgi.py

Controls:
    Space   - Toggle SSGI on/off
    Escape  - Quit

## Source

```{literalinclude} ../../examples/features/3d/ssgi.py
:language: python
:linenos:
```