Bilinear sample of a row-major grid that ignores hole/invalid samples: only
the valid corners contribute (re-normalised by their weights). Sampling on or
just outside a footprint outline therefore returns the true edge value instead
of blending in a far-away fallback. When all four corners are invalid the
nearest valid sample (within a small search radius) is used, falling back to
fallback only if none is found nearby.
Bilinear sample of a row-major grid that ignores hole/invalid samples: only the valid corners contribute (re-normalised by their weights). Sampling on or just outside a footprint outline therefore returns the true edge value instead of blending in a far-away
fallback. When all four corners are invalid the nearest valid sample (within a small search radius) is used, falling back tofallbackonly if none is found nearby.