Ah, Petr, you describe exactly my default method (work on the (,32) image & then _COPYIMAGE(,33). I secretly hoped that Steve would come back with "Oh, dear, dear, you silly idiot: here is a much better method!". As the (,33) is actually on the screen, I suppose the best method would be to do the _MEM manipulations of the screen _RGBA32() values, as Steve has begun to show us in his videos. It's just a pity that at least one of his students has managed to sneak in from the inveterately stupid department!
Quick Addendum: Just tried the _MEM technique with the software image & copy to hardware image each loop. It works. Thanks Steve & Petr.