focalLengthPixels

Focal length in pixels — viewportHeight / (2·tan(fov / 2)) — the projection scale that keeps a world-space point/splat radius mapping to a constant on-screen size with distance. Shares the same 2·tan(fov / 2) denominator as compute. Falls back to 1 for a degenerate (non-finite or non-positive) viewport / FOV so callers never divide by zero.