Why does my sprite jitter when it arrives at its destination?

Question

I have just started looking into XNA for WP7 - and I am definitely the sort of person who likes to find answers to my questions without begging for help!

I searched here to find an answer but I found it hard to word my question, so apologies if it has been asked.

Basically I have a sprite, which I am moving to and from set locations on the screen. I have defined these locations and set up the methods to detect the input on the screen (I only want to be able to swipe left and right to select the location).

The sprite moves to the correct location when I swipe, but sometimes when it arrives at its location, it jitters very quickly backwards and forwards. My guess is that because of the maths used to calculate the vectors, it never actually hits the exact location, so keeps moving.

My theory was to set up some sort of boundary, where if the sprite was detected within x amount of pixels of the actual location, the sprite is snapped to the correct location. Would this be a good way of doing something? I imagine it isn't the most elegant solution as it will have that snapping motion towards the end, or am I just over-exaggerating what that will look like in my mind?

can anyone suggest a way of getting around this jumpy sprite? Code that i think is relevant below :)

Initialising:

player = new Player();
playerMoveSpeed = 12.0f;
player.Position = positionsSet.getLocation(1);

I defined my 4 locations like this:

Vector2 position1 = new Vector2(10.0f, 30.0f);
Vector2 position2 = new Vector2(210.0f, 30.0f);

My calculating the movement happens like this and is where I believe the stuttering to occur:

Vector2 direction = goTo - player.Position;
direction.Normalize();
player.Position = player.Position + direction * playerMoveSpeed;

Oh, and to get there I call this method (the gametime parameter is something I was trying, its not actually being used at the moment :) ) :

if(player.Position != goTo)
moveToLocation(goTo,gameTime);

Any advice would be massively appreciated guys and girls, I think I am getting there, but it is just annoying me now :)

Answer 1

Vector2 uses floating point numbers, so when you calculate the difference between where you want to go and where you are (goTo - player.Position) it will almost never be exactly zero. This leads to some jitter, which is likely worse at some locations.

If you can't switch to integer-based positions (i.e. Point) you will end up having to cap the movement as you have found out. One way of doing this is to compare length of the direction vector before you normalize it and if it is under a set value simply go to the target position. If your margin of error before snapping is small enough you will not notice the snapping at all.

Answer 2

The following code has the problem that you could go far beyond goTo:

Vector2 direction = goTo - player.Position;
direction.Normalize();
player.Position = player.Position + direction * playerMoveSpeed;

You need to stop moving when you reach your destination. I suggest rewriting it this way:

Vector2 direction = goTo - player.Position;
if (playerMoveSpeed > direction.Length())
{
    player.Position = goTo;
}
else
{
    direction.Normalize();
    player.Position = player.Position + direction * playerMoveSpeed;
}

Also, you will soon find out that your game is not framerate-independent. This is already obvious because you are adding a speed to a position, which is inconsistent: somewehere you will need to multiply that speed by a duration. See this thread on MSDN which is very closely related to your current code.