Tell Psychtoolbox, that no further drawing commands will be issued before the
next SCREEN('
Flip') command. This is a hint to the PTB that allows to optimize
drawing in some occasions. If you provide the same value for dontclear that
you're going to pass to the following
Flip command, you can further improve
performance. You can time the execution of all drawing commands between the most
recent
Flip and this command by setting the optional flag sync=1. In that case,
telapsed is the elapsed time at drawing completion. Don't set the sync - Flag
for real experiments, it will degrade performance! Don't issue this command
multiple times between a
Flip, it will degrade performance!