Reflection Probe Parallax Correction: Project Local Reflections Through a Proxy

Identify the fixed capture viewpoint

A reflection cubemap stores the environment as seen from one capture position. Sampling it only with a reflected direction treats the represented surroundings as infinitely distant. Moving the shaded object then fails to produce the expected positional change of nearby reflected walls. Unity's box-projection explanation illustrates why this is particularly visible indoors. [2]

Parallax correction adds a simple spatial model around the capture. It improves placement when that model resembles the nearby environment; it does not reconstruct a complete scene from the cubemap.

Intersect a proxy and redirect the lookup

For a point inside a box proxy, trace the reflected ray to its exit point on the box. Form a new lookup vector from the cubemap capture position to that intersection. Use this vector to sample the cubemap. Lagarde describes both axis-aligned and oriented-box formulations. [1]

Keep the capture position, proxy geometry and influence region conceptually distinct. The capture need not lie at the proxy's center, and the region used to blend between probes need not be the same shape as the approximated walls. A tool may couple these controls, so verify its actual interpretation.

Worked example: calculate the changed direction

Let a box span x from -5 to 5, y from -4 to 4 and z from -3 to 3, all in metres. The shaded point is (2, 0, 0), the capture is (0, 0, 0) and the ray direction is (1, 1, 0).

Using this unnormalized direction, the ray reaches the x boundary at parameter 3, at position (5, 3, 0). It has not yet reached the y boundary. The corrected lookup vector is therefore (5, 3, 0), rather than the original (1, 1, 0). The cubemap lookup depends on direction, so its magnitude does not specify reflection intensity. This example tests the projection arithmetic only.

Validate placement and transitions

Create a room with clearly marked walls and a moving reflective object. Compare the reflection near the capture and near each wall. Visualize the proxy and its intersection point. For oriented boxes, transform points and directions consistently and recover the intersection in the coordinate space used by the capture.

Handle parallel ray components and points outside the intended proxy domain explicitly. When blending probes per pixel, correct and sample each probe using its own capture and proxy, then combine the lighting with the chosen weights. [1] Inspect overlapping influence regions for jumps or doubled features.

Keep the approximation visible in testing

A box cannot describe furniture, irregular walls or newly moved objects that were absent from the capture. Glossy filtering can hide some placement errors while exposing others. Check several roughnesses and camera paths. Fit room proxies carefully, as the Unity manual recommends, and plan capture updates independently from projection. [2] Measure capture cost, stored data and per-pixel correction together before increasing probe density.

Sources

• [1] Image-based Lighting Approaches and Parallax-corrected Cubemap — Sébastien Lagarde, original developer explanation.
• [2] Troubleshooting Reflections: Box Projection — Unity Technologies, official manual.