Noob problems. Need input :( (FPE 3.0 + Silverlight)

Topics: Developer Forum, User Forum
Sep 28, 2010 at 5:48 PM

Video of my problem :

- Using FPE 3.0

- Using Silverlight for Windows Phone 7.

- NOT using the "Physics Helper" for Silverlight

- I'm creating my rectangle shapes like this: 


            PolygonShape rectangleShape = new PolygonShape();

            float halfWidth = (float)(((this.AssociatedObject.Width) / PIXELS_TO_METERS_RATIO) / 2.0f);
            float halfHeight = (float)(((this.AssociatedObject.Height) / PIXELS_TO_METERS_RATIO) / 2.0f);

            rectangleShape.SetAsBox(halfWidth, halfHeight);

            Fixture rectangleFixture = body.CreateFixture(rectangleShape, 1.0f);


- I'm updating the rectangles on each step like this:


            CompositeTransform pixelTransform = new CompositeTransform();

            pixelTransform.Rotation = MathHelper.ToDegrees(body.Rotation);
            pixelTransform.TranslateX = body.Position.X * PIXELS_TO_METERS_RATIO;
            pixelTransform.TranslateY = body.Position.Y * PIXELS_TO_METERS_RATIO;
            this.AssociatedObject.SetValue(Rectangle.RenderTransformProperty, pixelTransform);

Any idea what could be causing this?


Sep 29, 2010 at 1:38 PM

I think you are drawing the corner of your objects at the body position.  You need to offset the position for the center of your object.

I.e., I had a drawing method that looked something like:

            this.rotationTransf.Angle = this.GameObject.Rotation;
            this.translTransf.X = this.GameObject.Position.X -;
            this.translTransf.Y = this.GameObject.Position.Y -;

The center was just a Point relative to the top left of the graphic.

Oct 5, 2010 at 2:33 PM

This kind of makes sense but i'm having other noob problems which I don't understand.

ex: box2d says we should have a negative gravity (b2Vec2 gravity(0.0f, -10.0f)) which makes sense but whenever I set a negative gravity, my objects fall towards the sky and I don't know why. I'm about to have a panic attack trying to find the common ground between FPE, Box2d, Silverlight and Windows Phone 7.


Oct 5, 2010 at 3:39 PM
Edited Oct 5, 2010 at 3:54 PM

Then just use a positive value for gravity? Something to do with the example setting up their coordinate system differently to match physics textbooks. Makes more sense (to me) to leave -Y as up and Y+ as down to keep things familiar. (see

As an alternative to what ben33333 mentioned.... you can just pass in an Origin parameter into your spritebatch.draw function and it will offset the drawing for you(See )