Chun, ObjectiveTcl is supposed to be called PoeTcl nowadays. (there was another older project named ObjectiveTcl that is quite similar).

PoeTcl basically implements a Ruby-like object system on top of plain Tcl in 100-200 lines of pure Tcl code, no C. The fact that it has been possible to do that in Tcl, has impressed me much.

Laserbeak said something about version 2, but that's because he's confusing GridFlow and DesireData. GridFlow has version 0.8, version number 1 will be skipped, version 2 is in the plans, and it involves migrating away from Ruby and towards Tcl.

negate both of your input pictures using [# inv+ 255], do a multiply using [# *>>8], then negate the result again using [# inv+ 255]. That's what the screen effect is.