Screen Space Global Illumination (SSGI)

Overview

Screen Space Global Illumination (SSGI) is a real-time technique that simulates indirect lighting effects using screen-space data.
It analyzes depth, normals, and scene color from the current framebuffer to estimate how light bounces between surfaces, adding ambient occlusion and color bleeding effects to create more realistic lighting.

Unlike traditional Image-Based Lighting (IBL) alone, which provides static environment lighting, SSGI dynamically captures light bouncing from visible geometry in the scene. This means a red wall can cast a reddish tint onto nearby white surfaces, or bright objects can illuminate their surroundings — all computed in real time from what's visible on screen.

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Advantages

• Dynamic indirect lighting: Captures light bouncing between surfaces based on their actual colors and positions in the current frame
• No pre-baking required: Works with fully dynamic scenes without needing lightmap baking or precomputation
• Ambient occlusion: Naturally darkens areas where geometry blocks ambient light
• Color bleeding: Surfaces pick up color tints from nearby objects, enhancing realism
• Performance: Relatively efficient compared to full ray-traced global illumination

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Limitations

• Screen-space only: Can only use information visible in the current frame — objects outside the view or behind the camera cannot contribute lighting
• Missing geometry: Occluded or off-screen surfaces don't affect the result, which can cause lighting to "pop" as the camera moves
• Limited ray distance: Traces are limited to a maximum distance to maintain performance, so large-scale indirect lighting may be incomplete
• Noise and artifacts: The technique uses sampling and approximations, requiring temporal and spatial filtering to reduce noise
• Performance cost: Still computationally expensive compared to static IBL, requiring careful tuning for different hardware

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Enabling SSGI

Both the camera and the IBL environment must opt in:

scene.env.light.type = 'ibl';
camera.SSGI = true;

The camera must use HDR, SSGI intensity must be greater than zero, and the IBL must provide both radiance and irradiance data. The device must also support renderable half-float textures, at least two draw buffers, and a 16-byte-per-sample color-attachment budget.

Backend differences:

• WebGPU and WebGL2: Use motion vectors, Hi-Z acceleration, historical scene color and full temporal filtering for higher quality
• WebGL1: Has neither motion vectors nor Hi-Z, so it falls back to linear ray marching with spatial filtering only, single-bounce hits and same-pixel history validation - expect more noise and less indirect light

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Quality Presets

Preset	Resolution	Rays/pixel	Max steps	Denoise passes
quality	Full	2	64	3
balanced	Half	1	48	2
performance	Half	1	24	1
custom	Configurable	1–4	1–256	0–5

camera.ssgiQualityPreset = 'quality';

The custom preset exposes trace resolution, samples per pixel (SPP), maximum steps and denoise passes independently:

camera.ssgiQualityPreset = 'custom';
camera.ssgiHalfResolution = true;
camera.ssgiRaysPerPixel = 2;
camera.ssgiMaxSteps = 48;
camera.ssgiDenoisePasses = 2;

Changing any custom setting through the API automatically switches the preset to custom.

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Controls

Property	Default	Purpose
ssgiIntensity	0.7	Strength of screen-space indirect lighting relative to IBL. Controls how much SSGI affects the final image
ssgiSkyOcclusion	1	How much occluding geometry blocks environment light. 1 is full physical occlusion; lower values reduce darkening
ssgiMaxDistance	32	Maximum view-space trace distance. Larger values capture more distant bounces but cost more performance
ssgiThickness	0.5	Depth intersection thickness for ray hits
ssgiStride	1	Pixel stride for linear ray marching (WebGL1 only)
ssgiMaxRayIntensity	10	Clamps overly bright samples to reduce firefly artifacts
ssgiTemporal	true	Enables temporal accumulation to reduce noise
ssgiTemporalWeight	0.94	Maximum history weight for temporal filtering after stabilization
ssgiDepthReject	0.5	Depth threshold for rejecting invalid temporal reprojection
ssgiNormalReject	0.75	Normal similarity threshold for temporal reprojection

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Performance Tips

• Start with the balanced preset and adjust from there
• Use half resolution (ssgiHalfResolution = true) for significant performance gains with minimal visual loss
• SSGI requests the Hi-Z pyramid itself on WebGPU and WebGL2, so ray tracing is already accelerated there
• Reduce ssgiRaysPerPixel if you have strong temporal filtering
• Lower ssgiMaxSteps in scenes with simple geometry or limited depth complexity