Question

In openGL we could create some polygons and connect them as a group by the function
‘pushMatrix()’ and then we could rotate them and move them as one object..

Is there a way to do it with xna? if i have 3 polygons and i want to rotate and move them all together as a group how can i do that?

EDIT:
‫I am using Primitives Shapes to build a Skeleton of a basketball player.

The game will only be a shoot out game to the basket, which means the player

will only have to move his Arm.

I need a full control over the Arm parts, and in order to do that, I need to move

the Arm which is built from Primitive shapes Harmonicaly. In order to do that,

I’ve tried Implementing the MatrixStack for performing matrix Transformations but with

no success. Any suggestions?‬

Answer 1

I will answer this in basic terms, as I can’t quite gleen from your question how well versed you are with XNA or graphics development in general. I’m not even sure where your problem is; is it the code, the structure or how XNA works compared to OpenGL?

The short answer is that there is no matrix stack built in.

What you do in OpenGL and XNA/DX is the very same thing when working with matrices. What you do with pushMatrix is actually only preserving the matrix (transformation) state on a stack for convenience.

Connecting objects as a group is merely semantics, you don’t actually connect them as a group in any real way. What you’re doing is setting a render state which is used by the GPU to transform and draw vertices for every draw call thereafter until that state is once again changed. This can be done in XNA/DX in the same way as in OpenGL.

Depending on what you’re using to draw your objects, there are different ways of applying transformations. From your description I’m guessing you’re using DrawPrimitives (or something like that) on the GraphicsDevice object, but whichever you’re using, it’ll use whatever transformation has been previously applied, normally on the Effect. The simplest of these is the BasicEffect, which has three members you’d be interested in:

• World
• View
• Projection

If you use the BasicEffect, you merely apply your transform using a matrix in the World member. Anything that you draw after having applied your transforms to your current effect will use those transforms. If you’re using a custom Effect, you do something quite like it except for how you set the matrix on the effect (using the parameters collection). Have a look at:

• http://msdn.microsoft.com/en-us/library/bb203926(v=xnagamestudio.40).aspx
• http://msdn.microsoft.com/en-us/library/bb203872(v=xnagamestudio.40).aspx

If what you’re after is an actual transform stack, you’ll have to implement one yourself, although this is quite simple. Something like this:

Stack<Matrix> matrixStack = new Stack<Matrix>();
...
matrixStack.Push( armMatrix );
...
basicEffect.World = matrixStack.Peek();
foreach (EffectPass pass in basicEffect.CurrentTechnique.Passes)
{
    pass.Apply();

    graphics.GraphicsDevice.DrawPrimitives(...);
}
basicEffect.End();
...
matrixStack.Pop();