Ghoscher's Blog

Introduction:

In the following three or four articles I’m gonna discuss XNA picking related issues including:
1- Ray picking.
2- RTS style selection box (two approaches).
3- Projection and why you need it.

I’m not an XNA professional so I might make some mistakes, please if you think any thing is wrong tell me and I’ll be happy to respond 😀

What is picking?

Well.. in short, picking is something you’ll need if you want to know what 3D object the mouse cursor is over! in other words picking lets you “pick” a 3D object with your mouse, if you have ever played a 3D RTS (real time startegy) game like C&C Generals, selecting a unit with you mouse is called picking!

In a 2D game knowing what’s the mouse is over – in most cases – is simple, just check if the mouse is inside a rectangle or a circle for example, in a 3D world it’s a little trickier!.

In a previous post I visualized how picking 3D objects works in XNA. (no need to check the old post, every thing you want is here…)
Today I’m gonna explain how the operation works, the official picking sample is great, and much of my explanation is taken from there…
Picking will be explained step by step in addition to some other picking issues…

1-Ray Picking:

A ray goes from a specified point towards a specified direction ( it doesn’t have an ending point).
XNA provides a Ray struct with some good intersection detection methods, you can check ray intersection against bounding volumes (BoundingBox, BoundingSphere, BoundingFrustum) and Planes.
The main idea of picking is to create a ray that goes from a point in the near plane of the camera frustum towards another point in the far plane of the camera frustum, these two points are specified using the mouse screen coordinates (mouse x and y), once we have that ray we can check for intersections with 3D objects using there bounding spheres for example..

Click on image to see full size

Whenever you draw a 3D object to your 2D screen it has to be projected using the matrices  world, view and projection, multiplying the 3D position of a vertex with these three matrices produces the 2D position of the vertex in the viewport. this operation is called projection, transforming from world coordinates to screen coordinates.

When picking we want to do the opposite : transform from screen coordinates (mouse position on the screen) to world coordinates, This operation Read more…