CODEWITHSUNDEEP
c#vectorxnagpu
Vaccano
Vaccano

Reputation: 82291

XNA, Vector math and the GPU

I am looking into making a game for Windows Phone and Windows 8 RT. The first iteration of the game will use XNA for the UI.

But since I plan to have other iterations that may not use XNA, I am writing my core game logic in a Portable Class Library.

I have gotten to the part where I am calculating vector math (sprite locations) in the core game logic.

As I was figuring this out, I had a co-worker tell me that I should make sure that I am doing these calculations on the GPU (not the CPU).

So, here is the question, if I use XNA vector libraries to do my vector calculations, are they automatically done on the GPU?

Side Question: If not, should they be done on the GPU? Or is it OK for me to do them in my Portable Class Library and have the CPU run them?

Note: If I need to have XNA do them so that I can use the GPU then it is not hard to inject that functionality into my core logic from XNA. I just want to know if it is something I should really be doing.

Note II: My game is a 2D game. It will be calculating movement of bad guys and projectiles along a vector. (Meaning this is not a huge 3D Game.)

Upvotes: 4

Views: 1583

Answers (2)

sav
sav

Reputation: 2152

So, here is the question, if I use XNA vector libraries to do my vector calculations, are they automatically done on the GPU?

Looking inside the Vector class in XNA using ILSpy reveals that the XNA Vector libraries do not use the graphics card for vector math.

Upvotes: 1

Andrew Russell
Andrew Russell

Reputation: 27195

I think your co-worker is mistaken. Here are just two of the reasons that doing this kind of calculation on the GPU doesn't make sense:

The #1 reason, by a very large margin, is that it's not cheap to get data onto the GPU in the first place. And then it's extremely expensive to get data back from the GPU.

The #2 reason is that the GPU is good for doing parallel calculations - that is - it does the same operation on a large amount of data. The kind of vector operations you will be doing are many different operations, on a small-to-medium amount of data.

So you'd get a huge win if - say - you were doing a particle system on the GPU. It's a large amount of homogeneous data, you perform the same operation on each particle, and all the data can live on the GPU.

Even XNA's built-in SpriteBatch does most of its per-sprite work on the CPU (everything but the final, overall matrix transformation). While it could do per-sprite transforms on the GPU (and I think it used to in XNA 3), it doesn't. This allows it to reduce the amount of data it needs to send the GPU (a performance win), and makes it more flexible - as it leaves the vertex shader free for your own use.

These are great reasons to use the CPU. I'd say if it's good enough for the XNA team - it's good enough for you :)

Now, what I think your co-worker may have meant - rather than the GPU - was to do the vector maths using SIMD instructions (on the CPU).

These give you a performance win. For example - adding a vector usually requires you to add the X component, and then the Y component. Using SIMD allows the CPU to add both components at the same time.

Sadly Microsoft's .NET runtime does not currently make (this kind of) use of SIMD instructions. It's supported in Mono, though.

Upvotes: 5

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