Translating screen coordinates to sprite coordinates in XNA

Question

I have a sprite object in XNA.
It has a size, position and rotation.
How to translate a point from the screen coordinates to the sprite coordinates ?
Thanks,
SW

Answer 1

One solution is to hit test against the sprite's original, unrotated bounding box. So given the 2D screen vector (x,y):

1. translate the 2D vector into local sprite space: (x,y) - (spritex,spritey)
2. apply inverse sprite rotation
3. perform hit testing against bounding box

The hit test can of course be made more accurate by taking into account the sprite shape.

Answer 2

I think it may be as simple as using the Contains method on Rectangle, the rectangle being the bounding box of your sprite. I've implemented drag-and-drop this way in XNA; I believe Contains tests based on x and y being screen coordinates.

Answer 3

You might find the following XNA picking sample useful:

http://creators.xna.com/en-us/sample/picking

Answer 4

You need to calculate the transform matrix for your sprite, invert that (so the transform now goes from world space -> local space) and transform the mouse position by the inverted matrix.

Matrix transform = Matrix.CreateScale(scale) * Matrix.CreateRotationZ(rotation) * Matrix.CreateTranslation(translation);

Matrix inverseTransform = Matrix.Invert(transform);
Vector3 transformedMousePosition = Vector3.Transform(mousePosition, inverseTransform);