Vertex Merging

A checkbox in the Bidirectional panel. It adds the caustics seen through glass or water that Bidirectional alone cannot render: the floor of a pool through its surface, light gathering inside a glass.

• Slightly soft at first, sharpening with samples, at a higher cost per sample. Off by default.
• With it on, Light Paths defaults to 0.5; higher gives cleaner caustics.
• Merge Radius, in pixels, default 2. Lower is sharper at the cost of noise; higher reduces noise in faint caustics spread over a large area.

Files saved with the old VCM method open as Bidirectional with Vertex Merging on.