Does every Entity in an XNA game need it's own BasicEffect instance?

Question

From what I see, the position/matrix is stored in the BasicEffect instance, so it makes sense to me that every object has it's own position matrix, but I'm not 100% sure it's correct to let every Entity have it's own BasicEffect instance, so I'm asking here.

Answer 1

Probably not. But you could...

First of all, you'll note that BasicEffect in fact has 3 matrices: World, View, and Project. World is per-object. But view and project are per-camera - so these should not be duplicated in different places around your program (for reasons of maintainability).

Secondly: You should usually treat states that go onto the GPU as write-only. The upshot of this is that you generally store the information to generate those states (such as effect parameters) yourself - in your own classes. Then, for each object you draw, you generate the appropriate state and set it on the GPU and follow it up with a draw call.

So with a single, shared BasicEffect, what you'll usually do is set the projection matrix once at the start of your game. Then set the view matrix at the start of each frame. And then set the world matrix right before you draw each object.

But - there are scenarios where you might want an Effect for each object. For example: you might have completely different settings for different objects. But at the same time - you still don't want to waste GPU resources by having multiple copes of the same effect program in existence.

To solve this, use the Effect.Clone method (MSDN), which gives you a copy of the Effect object that shares the underlying GPU resources.

(Note, of course, that BasicEffect is an Effect.)

Answer 2

BasicEffect is shader program. So it is resource it must be single( to save memory etc). And parameters are passed same as in function.

Answer 3

You have different WVP matrices for each object. But you can create a single BasicEffect instance to render all the objects. You just have to make sure that before rendering, you assign the correct matrices to the BasicEffect properties.

for every object
{
    basicEffect.World = obj[i].world;
    basicEffect.View = obj[i].view;
    basicEffect.Projection = obj[i].projection;
    GraphicsDevice.SetVertexBuffer(obj[i].vertexBuffer);
    foreach (EffectPass pass in basicEffect.CurrentTechnique.Passes)
    {
        pass.Apply();
        GraphicsDevice.DrawPrimitives(PrimitiveType.TriangleList, 0, 1);
    }
}

Check this