Umpire has to assume if it hits him on the full and in front that it is going on to hit the stumps.
Not quite - this is a common misconception amongst badly trained umpires and ignorant players.
The umpire should assume if the ball hits the batsman on the full that it would have
continued on its present trajectory and that it would not have deviated due to spin/seam/extra bounce. The rule does not mean that a ball that had it kissed the turf would have headed down leg should be given out because it struck the batsman on the full, it merely means that the umpire should discount everything but the trajectory of the ball. This is most obviously a factor in two situations:
- where you have a bowler who is spinning the ball sharply - here the perceived spin might have been seen (prior to the clarification of the laws) as a factor mitigating in favour of the batsman, but no more.
- where the batsman is struck on the full a long way down the wicket. Here a batsman struck after the ball has pitched would normally get the benefit of the doubt, even though no such concept exists in the laws, because the bounce of the ball comes into play - but with a ball striking on the full, the umpire has merely to be satisfied that the ball struck him in line and would have continued to hit the stumps
Of course, the umpire in the original post may be of the badly trained variety, because it is difficult for a player well down the track to be hit in line by a ball that would have gone on to hit, but hey ho...