XNA VertexBuffer for Terrain Editor

Question

I'm working on a personal project that, like many XNA projects, started with a terrain displacement map which is used to generate a collection of vertices which are rendered in a Device.DrawIndexedPrimitives() call.

I've updated to a custom VertexDeclaration, but I don't have access to that code right now, so I will post the slightly older, but paradigmatically identical (?) code.

I'm defining a VertexBuffer as:

VertexBuffer = new VertexBuffer(device, VertexPositionNormalTexture.VertexDeclaration, vertices.Length, BufferUsage.WriteOnly);
VertexBuffer.SetData(vertices);

where 'vertices' is defined as:

VertexPositionNormalTexture[] vertices

I've also got two index buffers that are swapped on each Update() iteration. In the Draw() call, I set the GraphicsDevice buffers:

Device.SetVertexBuffer(_buffers.VertexBuffer);
Device.Indices = _buffers.IndexBuffer;

Ignoring what I hope are irrelevant lines of code, I've got a method that checks within a bounding shape to determine whether a vertex is within a certain radius of the mouse cursor and raises or lowers those vertex positions depending upon which key is pressed. My problem is that the VertexBuffer.SetData() is only called once at initialization of the container class.

Modifying the VertexPositionNormalTexture[] array's vertex positions doesn't get reflected to the screen, though the values of the vertex positions are changed. I believe this to be tied to the VertexBuffer.SetData() call, but you can't simply call SetData() with the vertex array after modifying it.

After re-examining how the IndexBuffer is handled (2 buffers, swapped and passed into SetData() at Update() time), I'm thinking this should be the way to handle VertexBuffer manipulations, but does this work? Is there a more appropriate way? I saw another reference to a similar question on here, but the link to source was on MegaUpload, so...

I'll try my VertexBuffer.Swap() idea out, but I have also seen references to DynamicVertexBuffer and wonder what the gain there is? Performance supposedly suffers, but for a terrain editor, I don't see that as being too huge a trade-off if I can manipulate the vertex data dynamically.

I can post more code, but I think this is probably a lack of understanding of how the device buffers are set or data is streamed to them.

EDIT: The solution proposed below is correct. I will post my code shortly.

Answer 1

First: I am assuming you are not adding or subtracting vertices from the terrain. If you aren't, you won't need to alter the indexbuffer at all.

Second: you are correct in recognizing that simply editing your array of vertices will not change what is displayed on screen. A VertexBuffer is entirely separate from the vertices it is created from and does not keep a reference to the original array of them. It is a 'snapshot' of your vertices when you set the data.

I'm not sure about some of what seem to be assumptions you have made. You can, as far as I am aware, call VertexBuffer.SetData() at any time. If you are not changing the number of vertices in your terrain, only their positions, this is good. Simply re-set the data in the buffer every time you change the position of a vertex. [Note: if I am wrong and you can only set the data on a buffer once, then just replace the old instance of the buffer with a new one and set the data on that. I don't think you need to, though, unless you've changed the number of vertices]

Calling SetData is fairly expensive for a large buffer, though. You may consider 'chunking' your terrain into many smaller buffers to avoid the overhead required to set the data upon changing the terrain.

I do not know much about the DynamicVertexBuffer class, but I don't think it's optimal for this situation (even if it sounds like it is). I think it's more used for particle vertices. I could be wrong, though. Definitely research it.

Out of curiosity, why do you need two index buffers? If your vertices are the same, why would you use different indices per frame?

Edit: Your code for creating the VertexBuffer uses BufferUsage.WriteOnly. Good practice is to make the BufferUsage match that of the GraphicsDevice. If you haven't set the BufferUsage of the device, you probably just want to use BufferUsage.None. Try both and check performance differences if you like.