XNA - Update vertices property stored in a VertexBuffer

Question

I've a class that creates a cube using VertexPositionColor and these vertices are stored in a VertexBuffer. Now i would like to dynamically change the color of my vertices. In my class i have a reference to a VertexPositionColor array wich contain all my vertices. I've wrote a SetColor function wich accept a Color in parameter and update each Color property of my vertices. But when i call it, it doesn't work. This is normal because vertices are stored in VertexBuffer and is not updated.

So, how can i update vertices property and keep using VertexBuffer?

Answer 1

To change a vertex buffer it can't be active:

   GraphicsDevice.SetVertexBuffer(null);

To transfer vertex data to the gpu memory:

   VertexBuffer.SetData<VertexPositionColor>(VertexData);

To active the vertex buffer again:

   GraphicsDevice.SetVertexBuffer( VertexBuffer );

Note that if your color change affects every vertex with one unique color, you may consider to pass to a shader a single color as a parameter, and do not modify vertex.

Answer 2

The only way to change the contents of a VertexBuffer in XNA is by calling VertexBuffer.SetData().

When you need to update your vertex data often, it is recommended you use a DynamicVertexBuffer which streams vertices to the GPU.

Your best bet is to do some profiling using either:

• a DynamicVertexBuffer (stored in VRAM) and DrawPrimitives()
• a simple array (stored in RAM) and DrawUserPrimitives()

And see which one is faster for your specific use case.

Answer 3

I would do this using two vertex buffers. The first is static and contains the data that won't change, the second dynamic and containing the data that will change. That way you get to minimize the impact of data transfers. I'm not certain what the XNA equivalent of the relevant D3D terminology is, but GraphicsDevice.SetVertexBuffers looks to be what you need.