The same effect occurs in racket/share/pkgs/realm/chapter5/ufo-source.rkt. When the state is increasing from 0, the image is not moving down the screen. It stays in one place, but is initially clipped to 0 horizontal lines. Then 1 line, then 2, and only when the state is 1/2 the image height does it start moving down the screen.
Also, if I call (draw-a-ufo-onto-an-empty-scene 0) in the interactions window, I once again get a different result: it renders the lower half of the image. This is the same behavior I see with my code. Maybe a platform issue? (version) reports 6.0. I'm running it on ubuntu. On Sun, Oct 5, 2014 at 6:13 PM, Matthias Felleisen <matth...@ccs.neu.edu> wrote: > > Can you send the full program please? Thanks -- Matthias > > > > On Oct 5, 2014, at 8:52 PM, Brian Craft wrote: > > > Hello -- Newbie here. I'm going through "Realm of Racket". I'm seeing an > odd effect when using this to-draw handler in big-bang: > > > > (define (draw-a-ufo-onto-an-empty-scene game) > > (let ([pos (game-pos game)]) > > (place-image IMAGE-of-UFO (round (vec-x pos)) (round (vec-y pos)) > > (empty-scene WIDTH HEIGHT)))) > > > > vec is (struct vec (x y)) > > > > As the y coordinate approaches zero, IMAGE-of-UFO gets clipped on the > top and bottom, looking a bit like it's winking out of existence, rather > than sliding off the canvas. This doesn't happen on any other edge. It's > only as y approaches zero. It starts when the edge of the image meets the > edge of the scene. That is, if the image is 20px high, and y starts at 30, > and decreases by 1 on each tick, it will move up the canvas for 30..10, but > from 10 on the image will not change position, but will instead be clipped > on the top & bottom, until it disappears. > > > > This does not happen in the interactions window. If I call the above > function in the interactions window, it renders as expected, with the image > moving smoothly off the edge. > > > > > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users > >
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