Screen-space reflections (SSR)

A polished floor often reflects a baked cubemap, so a rolling ball never shows up in it. Screen-space reflections march rays through the depth buffer and reuse the frame already drawn.

The problem

Reflection probes are captured once, from one point. Anything that moves, and anything the probe didn't see, is missing from the reflection. The depth buffer and the colour buffer of the current frame already describe everything on screen.

How it works

The source is a Unity URP renderer feature. Its PBR Accumulation mode runs three passes per frame:

1. Hit: for each glossy pixel, rebuild the world position from depth and pick one reflection direction from the GGX distribution of its roughness. Rougher surfaces scatter the rays wider.
2. March the ray in world space. The step starts at 0.2 m and grows by 1.33x each step, up to 64 steps. Once the ray goes behind the depth buffer, it halves the step and walks back (binary search). A hit counts if the ray is less than the thickness (0.25 m, growing with the step) behind the surface.
3. Resolve: read the colour at the hit point, weight it by a Fresnel term and the surface's reflectivity, fade it near the screen edge and on surfaces below a smoothness of 0.6.
4. Temporal denoise: reproject last frame's result, clamp it to the colour range of the pixel and its 4 neighbours, and blend in 75% of it. The blend drops as the camera moves.

What I hit

The source also has a cheaper mirror-ray mode. I ported the PBR path because the floor has two finishes: mirror-polished tiles (smoothness 0.95) and every third tile honed (0.72). On the honed tiles one ray per pixel is plain noise until the temporal pass averages it.

Rays that leave the screen find nothing. In the tilt-down shot the top of the hall leaves the frame and its reflection is cut off. The magenta view marks every glossy pixel whose ray missed.

Unity provides motion vectors. I compute camera-only motion from depth and the previous view-projection, so the moving ball's own motion is not tracked; the colour clamp limits the smear.

My build

three.js (r187), WebGL2 fragment passes. A G-buffer pass writes normal, smoothness and specular colour; then hit, resolve and temporal passes at 1920 x 1080. Settings are the source's High preset and defaults. Scene: a 64 m hall with 20 pillars, ceiling light panels, neon strips, a glowing ball and a cube.

Trade-offs

• Correct reflections of moving objects at the cost of a few full-screen passes
• Contact reflections line up exactly with the geometry
• Only what is on screen can be reflected; off-screen and hidden surfaces fall back to nothing here
• One ray per pixel needs the temporal pass, which lags on fast motion
• Thin objects can be missed by the thickness test
• Rendered on a software GPU, so no timings

When to use it

Wet streets, polished floors and water, where the reflected objects are usually on screen too. Pair it with probes for the parts that leave the frame.