Ok thanks, I understand better now.
However what you wrote does suggest that, where appropriate, there is a big speedup, because the query, are you there? is a quick one, which half-the time will save you any further query.
Think about sky drums, this is where we put a tree-line/city skyline/mountain ridge image mapped onto a cylinder. Maybe its a hdr, but the point is the upperhalf of the cylinder will be invisible… no need to read the hdr.