# default xna 4.0 (gametime?) donīt works well for 2D physics

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## 4 Replies - 6954 Views - Last Post: 17 September 2012 - 02:08 PM

### #1 EusKoder

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# default xna 4.0 (gametime?) donīt works well for 2D physics

Posted 09 September 2012 - 09:46 AM

I am developing a game using Visual Studio 2010 and XNA 4.0, after advancing to some extent with the project (a platform based 2d platformer msdn starter kit) I got to test it on different computers with different hardware (CPU, graphics, etc.) and I found that the speed of movement object of the game is quite different, I implemented the PSK physics msdn that are based on time,

```            /// <summary>
/// Updates the player's velocity and position based on input, gravity, etc.
/// </summary>
public void ApplyPhysics(GameTime gameTime)
{
float elapsed = (float)gameTime.ElapsedGameTime.TotalSeconds;
Vector2 previousPosition = Position;
// Base velocity is a combination of horizontal movement control and
// acceleration downward due to gravity.
velocity.X += movement * MoveAcceleration * elapsed;
velocity.Y = MathHelper.Clamp(velocity.Y + GravityAcceleration * elapsed, -MaxFallSpeed, MaxFallSpeed);
velocity.Y = DoJump(velocity.Y, gameTime);

// Apply pseudo-drag horizontally.
if (IsOnGround)
velocity.X *= GroundDragFactor;
else
velocity.X *= GroundDragFactor;
//velocity.X *= AirDragFactor;
// Prevent the player from running faster than his top speed.
velocity.X = MathHelper.Clamp(velocity.X, -MaxMoveSpeed, MaxMoveSpeed);
// Apply velocity.
Position += velocity *elapsed;
Position = new Vector2((float)Math.Round(Position.X), (float)Math.Round(Position.Y));

// If the player is now colliding with the level, separate them.
HandleCollisions(gameTime);
// If the collision stopped us from moving, reset the velocity to zero.
if (Position.X == previousPosition.X)
velocity.X = 0;
if (Position.Y == previousPosition.Y)
{
velocity.Y = 0;
jumpTime = 0.0f;
}
}

```

tested eg with a PC (PC1) 2.13GHz Intel Core 2 6400 / ATI Radeon HD 4670 and another one: (pc2) 3.00GHz Intel Pentium D / Intel 82945G Express Chipset Family by displacement difference (moving x axis at supossed (position = velocity * gametime.ElapsedGameTime.TotalSeconds) constant velocity, for example) is 3 seconds in a total of 20 (example: moving pc1 player sprite 6000 pixels in the x-axis at 20 seconds and pc 2 runs the same distance in 17 ). Tested on a 3rd PC: i72700k / Gigabyte GTX 560 TI the results are even worse, after some time after starting the game gets like 3 times slower and showing the number of pixels in each frame moved in a debug window in the game (counting updatespersecond with counter variable for updates cuantity and gametime for counting a second show 63fps), it appears as if the number is always constant ( refreshments lose the Update method?). In this pc if I put the game in fullscreen during the course of the game, the effect of "go slow" is immediate and restore window mode sometimes yield returns to "normal" and sometimes not. Eventually I began to try a new project to test whether the movement is constant in different pc loading only one sprite and its position value in screen printing. Occur The same. I even tried moving a constant amount of pixels explicitly (position + = 5) and different speeds in different pc quantities of pixels moved in x time. I have the game loop as the default (fixedTimeStep=true;SynchronizeWithVerticalRetrace=true;). I've also tried turning off and creating another timestep as discussed in different post (eg http://gafferongames...-your-timestep/ but i canīt achieve the desired result, move the same number of pixels in X seconds on different computers with windows. All pc used for tests use windows 7 enterprise pc1 == x86 the others are x64.

The weirdest thing is that I find information about people describing the same problem and that I wear long nights of searches.

Is This A Good Question/Topic? 0

## Replies To: default xna 4.0 (gametime?) donīt works well for 2D physics

### #2 LiberLogic969

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## Re: default xna 4.0 (gametime?) donīt works well for 2D physics

Posted 10 September 2012 - 01:28 AM

Have you tried setting the Game1.IsFixedTimeStep property to true?

### #3 BBeck

• Here to help.

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## Re: default xna 4.0 (gametime?) donīt works well for 2D physics

Posted 10 September 2012 - 06:19 AM

I know I had some sort of problem with gameTime rounding to seconds. Time between frames is never going to be measured in seconds unless your computer is running so slow that it takes 20 years for it to boot up. I used milliseconds and converted them to seconds, which left a fractional second, but allowed me to work in seconds rather than milliseconds (after all, you probably don't want to work in frames per millisecond).

```gameTime.ElapsedGameTime.Milliseconds/1000f

```

I don't know if that helps. But I know I had a problem with rounding errors without using that.

### #4 lordofduct

• I'm a cheeseburger

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## Re: default xna 4.0 (gametime?) donīt works well for 2D physics

Posted 10 September 2012 - 11:41 AM

Note, the GameTime object stores the elapsed and total time as a TimeSpan:

http://msdn.microsof...me_members.aspx

TimeSpan object store the units broken up in integer values:

http://msdn.microsof...m.timespan.aspx

So the seconds property is a WHOLE number. It represents the current seconds as a part of the timespan. So a Timespan representing 2 mins, 30 secs, 123 milliseconds, would return 30 for seconds from the Seconds property... not 150.123.

If you want ALL the seconds as a fracitonal value you access the TimeSpan.TotalSeconds property, which is a double. There are also TotalMilliseconds, Minutes, Hours, and Days...

So the TotalSeconds property on the other hand would return 150.123 for the previously mentioned TimeSpan.

RTFM...

This post has been edited by lordofduct: 10 September 2012 - 11:43 AM

### #5 EusKoder

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• Posts: 2
• Joined: 09-September 12

## Re: default xna 4.0 (gametime?) donīt works well for 2D physics

Posted 17 September 2012 - 02:08 PM

Problem "solved" the pixel amount difference between computers is because i have a fixedtimestep disabled and refresh rate on one of the tested computers monitor refresh rate is 75hz

If the monitor run at 75hz, in xna with isfixedtimestep=false and vsync actived the game run update 75 times per second.

When the two and more tested pcs are configured at 60hz in all of them works well and at same movement velocity.