This variant comes from the 2nd Annual World Sudoku Championship instruction booklet (pdf). This competition was held In Prague earlier this year. The idea is to consider the sudoku puzzle also as heights of buildings as seen from the top down. The total counts on each edge (side) represent how many buildings are viewable from that location. Note that taller buildings (higher numbers) hide shorter buildings (lower numbers). Once again the starting numbers are sparse due to the edge clues.