Tightened quad half-extent along one covariance eigen-axis: the visible
ellipse's semi-axis √(R²·λ) plus GSPLAT_FOOTPRINT_PIXEL_MARGIN,
never larger than the legacy (already viewport-clamped) extent. Both
rectangles are centred and share the eigen axes, so the result is their
intersection — a subset of the old quad that still contains every fragment
of it that can pass the discard tests.
Parameters
legacyExtent: number
The pre-existing T·√λ extent after the max-extent clamp (px).
Tightened quad half-extent along one covariance eigen-axis: the visible ellipse's semi-axis √(R²·λ) plus GSPLAT_FOOTPRINT_PIXEL_MARGIN, never larger than the legacy (already viewport-clamped) extent. Both rectangles are centred and share the eigen axes, so the result is their intersection — a subset of the old quad that still contains every fragment of it that can pass the discard tests.