clip-distances

Writing to clip_distances in a vertex shader makes the rasterizer clip primitives against your own planes - no fragment-shader discard, no wasted fragment work, and watertight edges. Classic uses are water reflections (clip below the surface), portal rendering, CAD section views, and mirrored geometry.

The discard-based fallback works everywhere but costs fragment invocations and can produce shading artifacts along the clip edge, so engines prefer the real thing where available. Hardware support is broad on desktop and newer mobile GPUs.