Hi Jack,

The problem is that an SDL::Rect stores it's x and y coordinates as
integers.  When you subtract a small value from the coordinate the new
coordinate is truncated, which is why you are able to move in negative
directions.  When you add a small value the same thing happens, but the
truncated value is the same as the value you added to, so no movement.  To
work around this, store the coordinates in variables outside of the rect,
add and subtract using these variables, then update the SDL::Rect


On Sun, Jun 10, 2012 at 7:50 AM, Jack Maney <jma...@adknowledge.com> wrote:

> Everyone,
> First of all, thanks to Tobias Leech for so quickly answering my previous
> question on alpha values.
> Now, I'm trying to understand movement.  I managed to hack together a
> simple, choppy, "movement by teleportation" app in which one can use ASDW
> to move a rectangle around in the screen.  However, inspired by the Pong
> code in the SDL manual, I'm trying to recreate this properly by using
> motion handlers.  The resulting code is at http://pastebin.com/mVHANk7T
> The strange thing about it is that movement works just fine, but only up
> and to the left (ie via A and W) not to the right or down (ie via D and S).
>  I'm at a bit of a loss as to what's going on...why would a negative
> x-velocity and positive y-velocity be fine, but not a positive x-velocity
> or a negative y-velocity?  I know that the correct velocity values are
> being set (by de-commenting the debugging print statement and running the
> code), so it's not an issue of the wrong velocity values being set...  Is
> there something obvious that I'm missing here?
> Thank you for your time,
> Jack

Jeffrey T. Palmer

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