[racket] place-image winking effect with low y coord

If I record to a gif the effect isn't in the gif, though it does happen
on-screen while it is recording the gif. Here's a video, using a camera
phone. Sorry for the poor quality.

https://drive.google.com/file/d/0BxB9fW-JZz-ST3dfeFNxakRhMnRzcGtPQUtuaWJxMVRQTEFz/view

Pause it around the 1 sec mark.

On Mon, Oct 6, 2014 at 7:10 AM, Matthias Felleisen <matthias at ccs.neu.edu>
wrote:

>
> Sorry, I don't understand what you mean. I have modified the main function
> so that (1) the whole thing is slower and (2) it records the whole thing as
> an animated gif:
>
> (define (main)
>   (big-bang 0                                        ;; initial state
>             (record? #t)                             ;; <---------------
>             (on-tick add-3-to-state 1/10)            ;; when the clock
> ticks, add ...   <-------------
>             (to-draw draw-a-ufo-onto-an-empty-scene) ;; when the state
> changes, draw ...
>             (stop-when state-is-300)))               ;; when the UFO's y
> coordinate is 300, stop.
>
>
>
> The animated gif is here:
>
>  http://www.ccs.neu.edu/home/matthias/Tmp/UFO/i-animated.gif
>
> -- Matthias
>
>
>
> On Oct 5, 2014, at 11:52 PM, Brian Craft <bcboy at thecraftstudio.com> wrote:
>
> > 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 <matthias at 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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